hartmann2013rg

hartmann2013rg
ENHANCED CHEMICAL WEATHERING AS A
GEOENGINEERING STRATEGY TO REDUCE
ATMOSPHERIC CARBON DIOXIDE, SUPPLY
NUTRIENTS, AND MITIGATE OCEAN
ACIDIFICATION
Jens Hartmann,1 A. Joshua West,2 Phil Renforth,3 Peter Köhler,4 Christina L. De La Rocha,5
Dieter A. Wolf-Gladrow,4 Hans H. Dürr,6 and Jürgen Scheffran7
Received 10 May 2012; revised 15 November 2012; accepted 24 January 2013; published 23 May 2013.
[1] Chemical weathering is an integral part of both the rock
and carbon cycles and is being affected by changes in land
use, particularly as a result of agricultural practices such as
tilling, mineral fertilization, or liming to adjust soil pH. These
human activities have already altered the terrestrial chemical cycles and land-ocean flux of major elements, although the extent
remains difficult to quantify. When deployed on a grand scale,
Enhanced Weathering (a form of mineral fertilization), the application of finely ground minerals over the land surface, could be
used to remove CO2 from the atmosphere. The release of cations
during the dissolution of such silicate minerals would convert
dissolved CO2 to bicarbonate, increasing the alkalinity and pH
of natural waters. Some products of mineral dissolution would
precipitate in soils or be taken up by ecosystems, but a significant portion would be transported to the coastal zone and the
open ocean, where the increase in alkalinity would partially
counteract “ocean acidification” associated with the current
marked increase in atmospheric CO2. Other elements released
during this mineral dissolution, like Si, P, or K, could stimulate
biological productivity, further helping to remove CO2 from the
atmosphere. On land, the terrestrial carbon pool would likely
increase in response to Enhanced Weathering in areas where
ecosystem growth rates are currently limited by one of the nutrients that would be released during mineral dissolution. In the
ocean, the biological carbon pumps (which export organic matter and CaCO3 to the deep ocean) may be altered by the resulting
influx of nutrients and alkalinity to the ocean. This review
merges current interdisciplinary knowledge about Enhanced
Weathering, the processes involved, and the applicability as well
as some of the consequences and risks of applying the method.
Citation: Hartmann, J., A. J. West, P. Renforth, P. Köhler, C. L. De La Rocha, D. A. Wolf-Gladrow, H. H. Dürr,
and J. Scheffran (2013), Enhanced chemical weathering as a geoengineering strategy to reduce atmospheric carbon
dioxide, supply nutrients, and mitigate ocean acidification, Rev. Geophys., 51, 113–149, doi:10.1002/rog.20004.
1.
1
Institute for Biogeochemistry and Marine Chemistry, KlimaCampus,
Universität Hamburg, Hamburg, Germany.
2
Department of Earth Sciences, University of Southern California, Los
Angeles, California, USA.
3
Department of Earth Sciences, University of Oxford, Oxford, UK.
4
Alfred Wegener Institute, Helmholtz Centre for Polar and Marine
Research (AWI), Bremerhaven, Germany.
5
UMR CNRS 6539, Institut Universitaire Européen de la Mer, Université
de Bretagne Occidentale, Technôpole Brest-Iroise, Place Nicholas Copernic,
Plouzané, France.
6
Ecohydrology Research Group, Department of Earth and Environmental
Sciences, University of Waterloo, Waterloo, Ontario, Canada.
7
Institute of Geography, KlimaCampus, Universität Hamburg,
Hamburg, Germany.
Corresponding author: J. Hartmann, Institute for Biogeochemistry and
Marine Chemistry, KlimaCampus, Universität Hamburg, Bundesstrasse 55,
Hamburg 20146, Germany. ([email protected])
©2013. American Geophysical Union. All Rights Reserved.
8755-1209/13/10.002/rog.20004
INTRODUCTION
[2] Global biogeochemical cycles have shaped the Earth’s
climate and surface environment since the earliest days of
the planet. A profound case in point is the consumption of
CO2 during the chemical weathering of silicate rocks that
has regulated the global carbon cycle and in so doing Earth’s
climate over several eons [Arvidson et al., 2006; Berner,
2004; Kempe and Degens, 1985; Walker et al., 1981].
Today, when human perturbation of the global carbon cycle
is putting social and economic stability at risk [IPCC, 2007],
these weathering processes that have operated naturally
over billions of years might be harnessed to mitigate this
perturbation by accelerating the removal of CO2 from the
atmosphere. This idea of “Enhanced Weathering” by the
application of powdered minerals to the land or ocean
Reviews of Geophysics, 51/ 2013
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HARTMANN ET AL.: ENHANCED WEATHERING
surface to facilitate accelerated dissolution is one of several
geoengineering methods gaining increasing attention as a
means for avoiding potentially devastating environmental
change associated with anthropogenic greenhouse gas
(GHG) emissions. Enhanced Weathering techniques are
already applied at restricted scales, well below that would
be considered as “geoengineering”, through the application of
minerals to adjust soil pH or nutrient supply (e.g., phosphorous,
potassium, or silica) in agricultural landscapes [van Straaten,
2002], but the potential of its wider application to avoid
climate change and the understanding of the consequences
for global biogeochemical cycles and ecosystems is only
beginning to be explored.
[3] Enhanced Weathering has a number of potential
advantages over other proposed geoengineering schemes as
a method for avoiding or decelerating climate change,
although much remains to be understood about how
effectively it would work and what the consequences, risks,
and side effects might be. Enhanced chemical weathering
would help remove CO2 from the atmosphere by accelerating the natural geological processes that transfer carbon
and other elements from the rock and atmospheric reservoirs into the biosphere and ocean over time. As such, it
would not, for example, require long-term storage of an
enormous mass of CO2 in the difficult-to-contain and potential hazardous form of a gas. And, as a side effect, it would
ameliorate some of the effects of ocean acidification.
However, it would alter biogeochemical cycling on local
to global scales and the extent of this alteration and any
secondary effects resulting from this alteration are not yet
well constrained.
[4] The purpose of this publication, which considers
recent work on both the Enhanced Weathering of natural
silicates, in which crushed rocks or minerals are applied
to the land surface or to aquatic systems, and of artificially
produced minerals, is to stimulate discussion and further
research on Enhanced Weathering. This paper begins by
describing how Enhanced Weathering fits into the broader
context of proposals for geoengineering and of stewardship
of global biogeochemical cycles. It then briefly reviews
the role of weathering in global biogeochemical cycles,
introduces the use of Enhancing Weathering for CO2
sequestration, discusses how these may affect biogeochemical
cycles across a range of spatial scales (from the local or plot
scale to the global scale), and explores the theoretical
limitations of Enhanced Weathering as a carbon sequestration
method. Finally, issues about managing Enhanced Weathering
schemes are discussed (see Appendix A). Through this, the paper aims to identify and summarize the key unknowns where
targeted research could make the most significant contributions to improving our understanding of the potential effectiveness and risks of Enhanced Weathering.
1.1. Enhanced Weathering as a Geoengineering
Technology
[5] The CO2 emission scenarios investigated by the IPCC
suggest an impending global warming of more than the 2 K
suggested by the United Nations Framework Convention on
Climate Change in Copenhagen in 2009 as a tolerable
threshold [Joshi et al., 2011; Rogelj et al., 2011]. The most
straightforward way to remain below this target would be
to emit less CO2. This would require emission reductions
of as much as 30%–85% if compared to current emissions
by the year 2050 [Solomon et al., 2009], something which
currently seems to be unlikely to occur. In a recent examination of emissions scenarios [Meinshausen et al., 2011], the
only scenario that falls short of a 2 K temperature increase
is one that utilizes carbon dioxide removal (CDR) from the
atmosphere [Friedlingstein et al., 2011]. Geoengineering,
i.e., controlled and purposeful engineering at the scale of
the Earth system, if well enough understood before it is
deployed, may become necessary to hold global change
within acceptable limits, which themselves need to be better
understood and agreed upon.
[6] Recent proposals for geoengineering of the Earth’s
climate fall into the categories of (a) methods for CDR
(introduced above; including schemes that increase oceanic
and terrestrial biomass, draw CO2 directly out of the air, or
enhance weathering) and (b) solar radiation management
(SRM) techniques, which attempt to alter the planetary
energy balance by diminishing the planet’s absorption of
incoming solar radiation in order to optimize climate
[Crutzen, 2006; UK Royal Society, 2009]. Generally speaking,
SRM involves an artificial increase in extraterrestrial,
atmospheric, or surface albedo, leading to a higher reflectivity
of the Earth and therefore to a loss of incoming solar radiation
(ideas include space-based reflectors, cloud seeding, surface
albedo manipulation by modification of either human
settlements or man-grown vegetation, and the injection of
aerosols into the stratosphere). While SRM might help
to prevent excessive global warming, ignoring the effects
of changes in precipitation, temperature, and sunlight
on plants, it leaves the carbon cycle largely untouched in
the first instance. CDR methods, on the other hand, would
reduce atmospheric CO2 and therefore work toward curing
the root cause of the global warming problem. Additionally,
some CDR methods, including Enhanced Weathering,
would lessen ocean acidification, the “other CO2 problem”
[Doney et al., 2009].
[7] A more quantitative assessment of the potential of
various geoengineering approaches was put forward by
Lenton and Vaughan [2009], although they ignored
Enhanced Weathering. They concluded that only “stratospheric
aerosol injection, albedo enhancement of marine stratocumulus
clouds or sunshades in space have the potential to cool the
climate back toward its pre-industrial state,” though strong
mitigation together with CDR techniques may be able to reduce
CO2 down to preanthropogenic levels by the year 2100.
[8] Since then, a variety of modeling studies have
analyzed specific geoengineering approaches in greater
detail, focusing mainly on SRM [e.g., Ferraro et al.,
2011; Irvine et al., 2010; Keith, 2010; Ricke et al., 2010]
but sometimes also considering CDR [e.g., Köhler et al.,
2010; Oschlies et al., 2010]. Recent research discusses not
only the potential of each approach in terms of mitigating
global warming, but increasingly considers both positive
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HARTMANN ET AL.: ENHANCED WEATHERING
and negative effects, especially in the case of SRM,
such as precipitation changes and impacts of SRM on
crop yields [Hegerl and Solomon, 2009; Pongratz
et al., 2012; Robock et al., 2009]. It also considers
how geoengineering could be used against global sea
level rise [Irvine et al., 2011; Moore et al., 2010] and
how feedbacks between climate, vegetation, and surface
albedo vary over different time periods and potential vegetation disturbance [e.g., O’Halloran et al., 2012]. As yet
there is no synthesis that considers the potential of Enhanced
Weathering, as well as what the range of side-effects might
be. This is the purpose of the present review.
[9] In addition to the growing discussion of the science
of geoengineering, there is an ongoing debate on the
policies and politics of geoengineering [e.g., Blackstock
and Long, 2010; Keith et al., 2010; Robock et al.,
2010]. The need to test the theoretical predictions of
modeling studies with field experiments presents the
problem that the safety and effectiveness of many
geoengineering approaches can only be sufficiently tested at
very large or even global scales. Understanding the ethical
and regulatory context is critical for advancing research in this
field, and details on the political and legal aspects specific to
Enhanced Weathering are discussed in Appendix A.
1.2. Chemical Weathering and Global Cycles of C and
Si: The Basic Concepts
[10] The basic understanding of how silicate weathering
acts to draw down atmospheric CO2 has been discussed
at least since the work of Ebelmen [1845]. Several years
thereafter, one of the first compilations of the geochemical
composition of rocks and the fluvial chemical fluxes that
result from chemical weathering was presented by Roth
[1878, 1879, 1893]. In general, the dissolution of silicate
minerals (Figure 1) consumes CO2 because it releases
cations such as Ca2+ and Mg2+ into solution, thereby
increasing total alkalinity [Wolf-Gladrow et al., 2007] (for
the definition of total alkalinity and its influence on the
carbonate system, see section 2.1), drawing CO2 into
solution to form carbonate ions (CO2!
3 ) and bicarbonate
(HCO!
3 ). At the typical pH values of rivers, around pH 7,
most of the dissolved inorganic carbon (DIC) exists as
bicarbonate. The cations (Ca2+, Mg2+, Na+, and K+)
released by chemical weathering are transported via rivers
to the ocean. Over geological time scales, these cations
either (i) lead to the precipitation of minerals, such as CaCO3,
which sequester carbon in mineral form; (ii) exchange with
other elements in submarine basalts; (iii) are involved in
chemical reactions during the diagenesis and alteration of
sedimentary minerals on the seafloor; or (iv) are precipitated
in form of evaporites [Arvidson et al., 2006; Edmond et al.,
1979; Elderfield and Schultz, 1996; Garrels and Mackenzie,
1971; Mackenzie and Garrels, 1966; Vondamm et al., 1985;
Wheat and Mottl, 2000]. Over the shorter time scales of
decades to centuries that are most relevant to the use of
Enhanced Weathering for CO2 sequestration, the released
cations either remain in solution, thereby increasing the
alkalinity of surface waters and sequestering carbon in
Figure 1. Simplified equations describing reactions for the dissolution of simple carbonate and silicate
minerals by different acids, illustrating the “consumption” of CO2 during weathering by carbonic acid,
as well as the contrasting role of strong acids such as HNO3, which may derive from the application
of nitrogen fertilizers. Carbonate weathering by nitric acid can be a net source of CO2 to the atmosphere
[after Hartmann and Kempe, 2008].
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HARTMANN ET AL.: ENHANCED WEATHERING
aqueous form (as discussed at greater length in section 2) or
are stored, at least temporarily, in terrestrial carbonate minerals, e.g., pedogenic carbonate [Dart et al., 2007;
Manning, 2008; Ryskov et al., 2008] or adsorbed onto clay
minerals and organic matter.
[11] The effect of carbonate weathering on atmospheric
CO2 is slightly different than that of silicate weathering.
Carbonate mineral precipitation releases some of the
drawn-down CO2 back to the atmosphere (Figure 1).
Carbonate weathering by carbonic acids (or organic acids
derived from CO2) can be a transient CO2 sink when solutes
are transported to the marine system, providing Ca2+ remains
in solution together with bicarbonate ions, but once carbonate
reprecipitates, there will be no net effect on atmospheric
CO2. When carbonate weathering is driven by strong acids such
as HNO3 or H2SO4, common anthropogenic “pollutants,” it
may not act as a sink of CO2 at all but in fact could act as
source of CO2 to the atmosphere (Figure 1) [Calmels et al.,
2007; Perrin et al., 2008; Semhi et al., 2000]. In some
natural environments, this process can be driven by the
oxidation of pyrite.
[12] The total magnitude of natural weathering-associated
carbon fluxes is small compared to other fluxes in the
modern carbon cycle (Figure 2), particularly if recent net
influx of CO2 to the ocean and biosphere (which is elevated
due to the notable increase in atmospheric CO2 concentrations
over the last few decades) is taken into account [Peters
et al., 2012]. The net carbon flux from land to the ocean
via rivers is ~0.8 Gt C a!1, and 0.4 Gt C a!1 of this flux is
in the form of dissolved inorganic carbon (DIC) [IPCC,
2007; Ludwig et al., 1996, 1998]. Reported global CO2
consumption fluxes by chemical weathering range from
0.22 to 0.29 Gt C a!1 [Gaillardet et al., 1999; Hartmann
et al., 2009]. This is smaller than the fluxes between other
reservoirs, e.g., 10 Gt C a!1 are emitted to the atmosphere
through anthropogenic activities [Peters et al., 2012]
(Figure 2). Note that the emissions of CO2 from limnic
systems and the land-ocean transition zone are still poorly
constrained and are not included in current Earth System
models or global carbon budgets (cf. the budget approaches
in Aufdenkampe et al. [2011], IPCC [2007], Peters et al.
[2012]). Despite its small magnitude, the flux of DIC
transported by rivers is thought to be important in the
transfer of CO2 out of the atmosphere over periods of time
covering the glacial–interglacial cycles (100,000 years) or
longer [Pagani et al., 2009; Zeebe and Caldeira, 2008].
[13] In addition to driving a direct drawdown of CO2 and
increase in alkalinity, silicate weathering releases dissolved
silicon (DSi), a portion of which is eventually transferred to
the ocean [Dürr et al., 2011; Laruelle et al., 2009; Treguer
et al., 1995]. Dissolved silicon is an important nutrient for
diatoms, which produce a silicified cell wall, termed as
frustule. Diatoms carry out a significant fraction of the net
primary production taking place in the ocean [Nelson et al.,
1995; Ragueneau et al., 2000; Treguer et al., 1995] and play
a key role in the export of particulate organic matter (POM)
to the deep sea. Because this export removes Si from the
surface ocean, DSi limits diatom production in large areas of
the world ocean [Dugdale and Wilkerson, 1998].
[14] This stimulation of diatom growth in turn means that
the supply of DSi has an important influence on the marine
“biological carbon pump” [Ragueneau et al., 2000, 2006;
Sarmiento et al., 2007], a set of processes in which carbon
incorporated into particulate organic carbon (POC)
through photosynthesis may be exported from the surface
ocean to the deep ocean before its oxidation back to CO2
[Boyd and Trull, 2007; Buesseler and Boyd, 2009; De La
Rocha and Passow, 2013; Honjo et al., 2008; Turner, 2002;
Volk and Hoffert, 1985]. The carbon thus concentrated into
the deep ocean is isolated from the atmosphere for the time it
takes for the surface and deep ocean to mix (~1000 years,
on average). Some of this POC may even be buried in
marine sediments, where it can be sequestered for longer
periods of time. This means that silicate weathering impacts
Figure 2. Simplified schematic of the global C and Si cycle. Carbon land-atmosphere emissions (fossil
fuels and deforestation: 10 " 0.9 Gt C a!1), net ocean carbon uptake (2.4 " 0.5 Gt C a!1), and land uptake
by terrestrial ecosystems (2.6 " 1.0 Gt C a!1) [Peters et al., 2012]. The lateral land-ocean carbon fluxes
are adapted from Ludwig et al. [1996; 1998] and IPCC [2007], estimates for emissions from inland waters
(ranging from 1.2 to 3.2 Gt C a!1) are from Aufdenkampe et al. [2011], and emissions occurring in the
land-ocean transition zone from Laruelle et al. [2010]. Note that the emissions of CO2 from limnic aquatic
systems and the land-ocean transition zone are still poorly constrained and are not recognized in current
Earth System models (cf. the budget apporaches in Aufdenkampe et al. [2011], IPCC [2007], Peters
et al. [2012]). Values for the silicon cycle are compiled after Dürr et al. [2011].
116
HARTMANN ET AL.: ENHANCED WEATHERING
1.3. Proposals for Enhanced Weathering
[16] Enhancing rates of weathering could remove
atmospheric carbon and store it for a significant time in
terrestrial and oceanic systems, effectively accelerating
the natural rate of transfer of carbon out of the atmosphere
(cf. Figure 2). However, the slow natural rates of mineral
weathering are a significant obstacle to overcome. The
kinetics of silicate weathering per mass unit of chosen
rocks can be increased by (1) increasing mineral surface
area (e.g., by grinding), (2) changing the pH of reacting
solutions, (3) increasing temperature, (4) increasing pressure,
(5) choosing appropriate rocks with highly reactive
minerals, (6) changing the flow regime, and (7) making
use of biological metabolism (e.g., certain plant species
remove selectively released elements and change thus the
saturation state of aqueous solutions close to their root
system). A strategy for Enhanced Weathering needs to make
use of some combination of these means for accelerating
weathering rates.
[17] A range of strategies for Enhanced Weathering have
been discussed, including the following:
[18] 1. Spreading finely ground silicate powder, rich in
easily released cations, over the terrestrial surface [Schuiling
and Krijgsman, 2006; Manning, 2008]. This could enhance
natural rates of chemical weathering because the large
surface area of the powdered material would result in rapid
dissolution of the mineral.
[19] 2. Spreading artificial products like iron and steel
slag and cement waste from industrial processes instead
of natural silicate minerals [Renforth et al., 2009]. These
materials dissolve rapidly and also have the potential to
release CO2-consuming cations. Similarly, silicate and
carbonate materials could be treated to produce minerals
(CaO, MgO) that dissolve more rapidly under ambient conditions [Kheshgi, 1995; O’Connor et al., 2005; Renforth
and Kruger, 2013].
[20] 3. Adding reactive minerals (e.g., olivine) to open
ocean surface waters [Köhler et al., 2013].
[21] 4. Spreading suitable material into tidal areas of
coastal zones [Hangx and Spiers, 2009], where wave action physically maintains fresh reactive surfaces, accelerating mineral dissolution and alkalinity production. In
this case, the mechanical decomposition of the grains
has not received much attention [Hangx and Spiers,
2009] but may be important to consider [Schuiling and
de Boer, 2010; 2011].
[22] 5. Pumping CO2 into mafic and ultramafic rock formations to increase chemical weathering rates and the
subsequent carbonation of minerals. This in situ approach
is not discussed here, but has been reviewed elsewhere
[Kelemen et al., 2011; Oelkers et al., 2008].
[23] Attention here focuses on the other low-energy,
large-scale strategies for Enhanced Weathering.
[24] The most suitable silicate mineral for Enhanced
Weathering, given its reactivity and wide natural abundance,
is forsterite (Mg-olivine, Figure 1). It is characterized by
a high abiotic dissolution rate per surface area when
compared to other silicate minerals (Figure 3). Table 1
shows this clearly by comparing the amount of time a 1
mm grain needs to dissolve in aqueous solution at pH 5
[Lasaga, 1995]: a 1 mm grain of forsterite dissolves within
2300 years, while an equivalent grain of quartz requires 34
million years. A 1 mm grain of calcite dissolves in less than
1 year, so in this respect it would be an ideal mineral.
However, carbonate (e.g., calcite) dissolution, as discussed
above, does not necessarily lead to CO2 sequestration (i.e., if
driven by strong acids or if it results in carbonate
reprecipitation). Mafic and ultramafic rocks, which are
<10
Artificial silicate
glasses and gels
10 -9
10
-5
-9
Ca-plagioclase
Olivine
-8
10 10
Pyroxene
-9
10 10
Increasing reactivity
the carbon cycle not only due to direct consumption and
transfer of atmospheric carbon to the ocean associated with
increased alkalinity (the purely chemical effects) but also
potentially via silicon fertilization of the oceanic biological
carbon pump [Köhler et al., 2013].
[15] In addition, the dissolution of minerals associated
with Enhanced Weathering would be expected to release a
range of other elements, some of which are key biological
nutrients (e.g., P, Fe) and some of which are toxins at high
concentrations (but sometimes nutrients at trace concentrations, e.g., Ni, Cr, or Cd). The exact suite and concentration
of elements released will depend on the rocks selected for
dissolution and clearly some caution must be exercised in
this regard. The potential impacts of altering elemental
fluxes to terrestrial and marine systems need to be carefully
considered and further work on this front is needed for
the full range of possible impacts (positive and negative)
to be understood.
-11
-13
10
Amphibole
-12
Na-plagioclase
Orthoclase
Quartz
10
-12
10
-14
Figure 3. The Goldich dissolution series, adapted from
Goldich [1938], showing the variation in dissolution rates for
different minerals. Chemical weathering rates in italics are
from Palandri and Kharaka [2004] at 25# C in mol m!2 s!1
(pH 6); artificial silicate weathering rates are reported in
Renforth and Manning [2009].
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HARTMANN ET AL.: ENHANCED WEATHERING
TABLE 1. Lifetime of a Hypothetical 1 mm Sphere in a Solution at pH 5 in Years for Different Minerals
Mineral
Quartz
Kaolinite
Muscovite
Epidote
Microline
Biotite
Albite
Andesine
Bytownite
Enstatite
Diopside
Forsterite
Dolomite
Calcite
Dissolution Time (a)
34,000,000
6,000,000
2,600,000
923,000
921,000
900,000
575,000
80,000
40,000
10,100
6,800
2,300
1.6
0.1
abundant across the planet, contain a high proportion of
olivine, as well as other minerals, such as pyroxene
(enstatite and diopside in Table 1), with relatively high
dissolution rates. This makes these relatively abundant
rock types (Figure 4) ideal potential targets for Enhanced
Weathering.
2. THEORETICAL ASSESSMENT OF ENHANCED
WEATHERING OF OLIVINE
Forsterite (Mg-Olivine) is one of the most abundant minerals on Earth, and
compared to other silicate minerals, is relatively fast to dissolve at pH 5
[Lasaga, 1995; Renforth et al., 2009].
[25] Quantitative assessment of the scope for using
Enhanced Weathering of olivine to remove CO2 from the
atmosphere is a complex endeavor. Among other things,
the potential scope for CO2 sequestration depends both on
the effective sequestration capacity of the mineral, i.e., how
much CO2 is consumed per gram of olivine weathering, and
on realistic rates of mineral dissolution. It is possible to
place some theoretical constraints on these questions. These
Figure 4. Map showing identified sources of rocks with mafic minerals, like basalt or gabbro, for the
American continents. Data from Hartmann and Moosdorf [2012].
118
HARTMANN ET AL.: ENHANCED WEATHERING
100
3
3
4
þ
!½H ) þ minor compounds;
!
" !
!
"
DIC ¼ HCO-3 þ CO23- ) þ H2 CO*3
(2)
pCO2 = 385 atm
pCO2 = 700 atm
theoretical limit
80
60
40
20
A
0
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.35
B
0.3
0.25
0
1
2
3
4
5
6
Sequestered CO2
7
8
9
10
Ratio C : Olivine [Pg:Pg]
2.1. How Much CO2 Is Consumed Per Gram of Olivine
Weathering?: Chemical Basics of the Marine Carbonate
System
[26] The equations shown in Figure 1 provide a succinct
summary of the overall net effect of weathering over the
long periods of time, when carbonate precipitates locking
carbon into a mineral form. However, these equations do
not capture the complete effect over shorter time scales, in
which dissolved cations from weathering contribute to the
total alkalinity (TA) [Dickson, 1981; Wolf-Gladrow et al.,
2007] of the oceans and not all cation charge supplied
by weathering is balanced by increased oceanic HCO-3
(as illustrated in the simplified equations in Figure 1).
The following equations describe total alkalinity (TA) and
dissolved inorganic carbon (DIC) of the oceans [Zeebe
and Wolf-Gladrow, 2001].
!
"
!
"
(1)
TA ¼ HCO- þ 2 CO2- þ ½BðOHÞ- ) þ ½OH- )
Ratio CO2 : Olivine [Pg:Pg] Olivine [Pg]
constraints are reviewed in this section, using forsterite
(Mg-olivine) as a model mineral. The approach developed
here could easily be applied to other natural minerals or to
artificial silicates with broadly similar conclusions.
Figure 5. The “carbon consumption efficiency” of olivine
weathering, updated from Köhler et al. [2010]. (a) Amount
of olivine necessary for given CO2 sequestration; less
olivine would be needed at higher CO2 partial pressures
in the atmosphere, due to the chemical speciation of the
carbonate system (according to equations (1)–(7)). (b)
Sequestration ratio CO2 to dissolved olivine (Pg:Pg; note
one Pg is one Gt), which decreases with the amount of
CO2 sequestered from the atmosphere. Calculations are
based on a well mixed 100 m deep surface ocean in equilibrium with the atmosphere. Red line: Theoretical limit follows the net equation (equation (8)) of olivine dissolution
without consideration of carbon cycle feedbacks.
The equilibrium constants
!
"
H2 CO*3
;
pCO2
½HCO- )½Hþ )
KH* ¼
3
;
½H2 CO*3 )
½CO23- )½Hþ )
K2* ¼
;
½HCO- )
K1* ¼
(3)
(4)
(5)
3
½BðOHÞ-4 )½Hþ )
;
½BðOHÞ3 )
Kw ¼ ½OH- )½Hþ )
KB* ¼
(6)
(7)
are functions of temperature, salinity, and pressure and thus
differ between seawater and freshwater. The whole carbonate
system shown above works in concert to determine the relative
proportions of the different species of DIC. For present-day
sea surface conditions, the relative molar distribution of DIC
into its three species H2CO*3, HCO-3, and CO23 is about
1:90:9. Note [H2CO*3] = [CO2] + [H2CO3]. Variations in these
proportions can significantly alter the effect of weatheringderived alkalinity on the amount of CO2 uptake from the
atmosphere.
[27] Let us consider this in the case of Mg-olivine,
forsterite (referred to as olivine in the following). This mineral
dissolves in water according to the following reaction:
Mg2 SiO4 þ 4CO2 þ 4H2 O ! 2 Mg2þ
þ4HCO!
3
þ H4 SiO4 :
(8)
[28] This equation seems to indicate that 4 mol of CO2 are
sequestered during the dissolution of 1 mol of olivine,
equivalent to 1.25 g CO2 (or 0.34 g C) per g olivine (the molar
weight of pure Mg-olivine is 140 g mol!1). However,
carbonate system chemistry makes the impact of Mg-olivine
dissolution on the carbon cycle more complicated
than suggested by equation (8), because both DIC and TA
are changed, leading to a new, lower, steady state CO2
concentration. Thus, the ratio of CO2 sequestration to
olivine dissolution will vary with the initial state of the
ocean water and with the amount of olivine dissolved. The
value of 1.25 g CO2 per g Mg-olivine represents an upper
theoretical limit based on the stoichiometry of equation
(8). Seawater, assumed to be initially in equilibrium with
the atmosphere, will become undersaturated with respect
to CO2 by addition of TA from weathering and will slowly
(over weeks to months) reequilibrate by taking up atmospheric
CO2. The amount of CO2 taken up by the ocean is a nonlinear
function of initial TA, pCO2 (atm), temperature, and salinity
[Zeebe and Wolf-Gladrow, 2001]. For large amounts of
olivine, it is also a function of the amount of TA added.
This makes the system seem to some extent complicated,
although the calculation is straightforward for a given initial
seawater composition and a given addition of alkalinity
from weathering. Typical ratios of CO2 consumption as a
function of the amount of olivine-derived alkalinity added
to the global oceans and for different starting atmospheric
pCO2 are shown in Figure 5. In general, for the ranges
modeled here, the efficiency of carbon sequestration is
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HARTMANN ET AL.: ENHANCED WEATHERING
Figure 6. (a) Dependence of olivine dissolution on pH [data from Golubev and Pokrovsky, 2006;
Pokrovsky and Schott, 2000; Rosso and Rimstidt, 2000; Wogelius and Walther, 1991]. Scatter in the data
partly reflects variability in experimental designs, including different proportions of Mg and Fe in the
olivine used in each experiment (note that rates in this plot are normalized to surface area of the minerals).
The abiotic kinetics illustrated here suggest that dissolution rate and thus the total amount of olivine
dissolution that can be expected from an Enhanced Weathering scheme may be pH limited (compare with
the discussion about kinetic limitations in section 2.3). Biotic processes (discussed in section 5) may
increase the amount of potential dissolution for several reasons. One of these is acidity; low pH values
of 4–6 are most common in soil systems, such that dissolution rates are expected to be faster in this setting
compared to other natural environments (note the log scale). (b) Dissolution rates from a range of
minerals, showing the large variability between minerals (and in some cases the same mineral) from a
number of studies [Palandri and Kharaka, 2004, and references therein]. The numbers in brackets behind
the mineral names indicate the sum of listed experiments at 25# C for the given pH-range in Palandri and
Kharaka [2004].
significantly lower than the theoretical limit of 1.25 g CO2
per gram of Mg-olivine.
[29] The surface ocean is supersaturated with respect to
some carbonate minerals. Given this, the input of additional
alkalinity from Enhanced Weathering might be expected to
promote carbonate precipitation (see the right-hand side of
the carbonate equation in Figure 1), which would reduce
or reverse the effectiveness of Enhanced Weathering since
the carbonate precipitation reaction drives CO2 release to
the atmosphere. However, the abiotic rate of carbonate precipitation is limited in the surface ocean by the presence of
2+
3!
cations
sulfate (SO2!
4 ) and phosphate (PO 4 ) anions (Mg
also inhibit calcite precipitation) [Berner, 1975; Morse
et al., 1997; Morse et al., 2007]. The limit to which the
marine carbonate system can be modified before driving
appreciable rates of carbonate precipitation is not fully
understood but is potentially large when distributed globally.
Nonetheless, it is necessary to quantify the exact saturation
limit for various local surface ocean conditions at which abiotic and biotic precipitation of carbonates would occur.
2.2. How Much Can Olivine Weathering Rates Be
Increased?: Abiotic Kinetics of Dissolution and Potential
Limits
[30] Natural rates of mineral weathering and alkalinity
production under ambient conditions are relatively slow and,
as discussed in section 1, the associated CO2 drawdown is
small compared to other fluxes in the global carbon cycle.
However, mineral dissolution rates can vary by several
orders of magnitude, and facilitating rapid dissolution is a
key to any Enhanced Weathering strategy. One of the most
important factors controlling dissolution rates is the surface
area available for reaction; higher surface area per unit mass
means higher dissolution rates and greater alkalinity flux for
a given mass of mineral. However, this is not the only
important factor. The range of dissolution rates for olivine
as a function of pH is shown in Figure 6a. There is clearly
a strong dependence on pH; at low pH, olivine dissolution
can proceed more rapid than at high pH. The scatter around
this pH trend in Figure 6a may partly be attributed to mineral
composition, with the upper range of the scatter representing
forsterite100, effectively pure Mg-olivine. While pH and
mineralogy are important controls on dissolution rate, there
is still a substantial range of rates reported in the literature,
even for individual minerals normalized to standard pH
(Figure 6b). Variability may be due to a range of additional
factors that influence dissolution rate, including temperature,
solution composition, and potentially even the age of
mineral surfaces.
[31] The strong effects of pH and surface area on dissolution
rate mean that finely ground olivine spread on soils will weather
more rapidly than massive rock deposits, both because of the
surface area production and the low pH of soil environments.
This makes this a particularly attractive strategy for designing
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HARTMANN ET AL.: ENHANCED WEATHERING
an Enhanced Weathering scheme. There is little direct
experimental evidence for whether spreading olivine on
soils would lead to enough of an increase in dissolution rate,
as discussed below, but the initial indications are that this
approach could work, especially if focused on humid and
specifically tropical regions.
2.3. Estimating the Potential Gross Global Impacts of
Enhanced Olivine Weathering
[32] There are a few key theoretical considerations when
assessing the broad scope for enhanced mineral weathering
on the land surface.
2.3.1. Dissolution Kinetics and the Effect of Saturation
State
[33] Based on a consideration of solubility of silica, the
runoff water volume, and the constraints placed by potential
changes in pH, Köhler et al. [2010] suggested roughly that
the olivine dissolution technique in the moist, terrestrial tropics will not exceed 1 Gt C a!1 (0.08 Pmol C a!1). Over the
next 50 years, this could sequester approximately 20 matm of
the projected 200 matm rise in atmospheric CO2 under the
A2 emission scenario considering abiotically controlled dissolution. The key theoretical limit imposed is the decrease
in mineral dissolution rate as solutions become progressively more concentrated. When solutions approach saturation, the kinetics of silicate mineral dissolution decrease and
there are certain hints in the literature that dissolution approaches zero or very low rates when certain silica or ion
concentrations, or activities are reached [cf. Lasaga et al.,
1994; Lasaga, 1995; Pokrovsky and Schott, 2000; Van
Cappellen and Qiu, 1997a; b, and references therein]. In
fact, the specific effect of the saturation state and the precipitation of secondary minerals on forsterite dissolution rates
(or other major silicate minerals) under field conditions are
poorly constrained. Basic data for the parameterization of
models and budget approaches for field conditions are
needed to assess the full potential of Enhanced Weathering,
specifically if the complex hydrological conditions in the
soil system (e.g., variability of soil pore water content) are
taken into account. While olivine is considered to be unstable under Earth surface conditions, the solubility of forsterite
is predicted to be 45–60 mmol m!3 using published constants in the LLNL, Minteq, and wateq4f databases. This
is considerably lower than the 2000 mmol m!3 limit imposed
by Köhler et al. [2010]. Note that this discussion considers
kinetics in the absence of biotic processes and organic acids
(cf. discussion below).
[34] Sequestering significant amounts of C through
weathering in humid tropical regions would require
extremely high weathering fluxes from the land surface.
For example, over the catchment area of the Amazon,
achieving sequestration of 1 Gt C a!1 would mean an
area-normalized alkalinity production from weathering
equivalent to 8.7 + 106 mol CO2 km!2 a!1 [Köhler et al.,
2010]. This is slightly higher than the highest (to our
knowledge) reported CO2 flux measured in natural systems
of 6.4 + 106 mol CO2 km!2 a!1, associated with basalt
weathering on the island of Java [Dessert et al., 2003]. This
suggests that the limits inferred by Köhler et al. [2010] are
reasonable upper constraints on the total maximum
potential of this method.
[35] Other studies [Schuiling and Krijgsman, 2006;
Schuiling et al., 2011] suggest that weathering might be able
to exceed the proposed limits. These saturation-based limits
are estimated based on laboratory studies (cf. methods used:
Berger et al. [1994], Daval et al. [2011], Lasaga et al.
[1994], Lasaga [1995], Pokrovsky and Schott [2000], Van
Cappellen and Qiu [1997a,1997b]), and in principle, there
may be ways to overcome such limits in natural settings, such
as through the formation of secondary phases. It remains difficult to assess these effects quantitatively since the range of
biotic and abiotic controls on dissolution rate is not clearly
understood in the context of Enhanced Weathering (see
further discussion below, summarized in Table 2).
[36] Nonetheless, it is clear that sustaining sufficiently
high total weathering fluxes would require maintaining a
sufficient minimum total mineral surface area for reaction.
Given that dissolution is not instantaneous, it might be
necessary to provide a significantly larger amount of silicate
powder than the target annual dissolution rate. This is
illustrated by further considering the case proposed by
Köhler et al. [2010]. They calculate that at least 3 Gt of
olivine per year must be distributed over tropical soils
annually for Enhanced Weathering to consume 1 Gt C a!1.
[37] To put this into the context at a local scale, global
dissolution of 3 Gt of Mg-olivine per year would mean the
distribution and dissolution of up to 600 g m!2 a!1 of olivine
throughout the whole catchment area of the Amazon.
This value assumes that 100% of the olivine that is distributed actually dissolves in the year it is applied, but
this is not likely to be the case. Even far from saturation,
dissolution rates are finite. Based on results of laboratory
experiments (using data provided in Pokrovsky and
Schott [2000]) and assuming a grain size of 75 mm on average, at least 3000 g m!2 a!1 of olivine would need to
TABLE 2. Summary of Major Unknowns About Silicate Mineral Dissolution Rates in the Context of Enhanced Weathering
•
•
•
•
•
•
•
•
Quantitative effect of the approach to saturation state on mineral dissolution rate (for olivine and other target minerals)
Effect of plant uptake of Si (decreasing saturation state of fluids)
Effect of soil processes such as secondary mineral formation, and downward transport of solutes (decreasing saturation state of fluids)
Extent of displacement of applied minerals to depth in agricultural soils
Distribution of acids in soil solutions with depth
Effect of percolation of water through the soil column and associated water residence time
Potential ecosystem-scale feedbacks (e.g., fertilization of terrestrial biological productivity leading to intensification of hydrologic cycle?)
Effect of soil moisture variability (e.g., drying and wetting)
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HARTMANN ET AL.: ENHANCED WEATHERING
be initially applied to offer sufficient surface area of reaction to provide the required fluxes associated with the 1
Gt C a!1 scenario proposed by Köhler et al. [2010]. This
reflects a minimum estimate because it is based on
weathering solutions remaining at pH 5, which is unlikely
at the scale of the soil pore waters, given the likely percolation speeds [Maher, 2010]. Such large quantities may
not need to be applied every year, as long as what is lost
on an annual basis is replaced, but it is likely to be difficult
to achieve sufficient alkalinity fluxes without a substantial
initial application. This is clearly a very simplistic calculation, but it is valuable in demonstrating that there may be kinetic controls that make it difficult to achieve the theoretical
limit with realistic annual application rates. The practicalities of the kinetic limits remain to be worked out. For example, the amount of olivine that would be required would
decrease for smaller grain sizes, as the reactive surface area
per mass of mineral is increased (this is discussed in section
4.2 in detail). Biotic effects may also increase reaction rates
in natural environments. Moreover, these estimates are
based on washed olivine grains, and literature suggests that
the initial dissolution should be higher associated with
freshly crushed minerals, due to ultrafine fragments
produced from the grinding process [Drever, 1997]. In
addition, the material loss due to erosion by flooding or
strong precipitation events needs to be considered.
2.3.2. Effect of Plants
[38] Ecosystem uptake of Si into the plant biomass as
biogenic silica (BSi) [cf. Bartoli and Souchier, 1978;
Bartoli, 1983; Conley, 2002; Fulweiler and Nixon,
2005; Meunier et al., 1999; Street-Perrott and Barker,
2008] might temporarily increase the total potential of
mineral dissolution while decreasing the DSi soil solution concentration by shifting soil solutions farther away from saturation. But the amount of Si that can conceivably be
sequestered in terrestrial biomass is limited. Globally,
the uptake of DSi by ecosystems is currently estimated at
60–200 Tmol Si a!1 globally [Conley, 2002]. This would
equate to the uptake of the Si released from dissolution of
8.4–28.1 Gt a!1 of forsterite. While this seems large, it must
be taken into account that total land area suitable for olivine
distribution is limited (see below). In addition, the uptake of
DSi by terrestrial ecosystems is counterbalanced by an equivalent loss to the soil systems, unless there is progressive
accumulation of BSi in the biomass reservoir. The amount
of DSi that could be stored in the form of additional BSi is
not known and depends largely on the plant communities
where Enhanced Weathering would be implemented and
their capacity for silica accumulation.
2.3.3. Effects of Soil Processes
[39] Further processes, like downward transport of
dissolved elements in the soil column (besides efflux from
the soil system through runoff) and precipitation of silica [cf.
Sommer et al., 2006] at depth in soils (i.e., below the zone
where the olivine or other silicates are applied), affect the dissolution rate of applied minerals. The transport of dissolved
products of applied minerals from upper soil layers driven
by the downward percolation of water through the soils
would diminish the possibility of reaching saturated conditions under which mineral dissolution will no longer occur
at significant rates. Moreover, depending on the soil type
and conditions, clay formation may also depend on the
DSi concentration and could then affect the rate of applied
mineral dissolution. Field experiments conducted in various
environments and taking biological activity into consideration are needed to address such questions about this potential Si saturation limit and its effect on the weathering of
olivine or other applied silicate minerals in soil environments. The review of Sommer et al. [2006] offers further
detail on this topic.
2.3.4. Effects on pH of Natural Waters
[40] Another potential limitation on olivine weathering as
an Enhanced Weathering technique is the need to avoid
changes in soil and river pH affecting ecosystems negatively.
Dissolution of olivine or other cation-bearing silicates
increases the pH of the surrounding fluids, and both terrestrial
and aquatic (freshwater and marine) ecosystems are sensitive
to pH variations [Doney et al., 2009; Driscoll et al., 2001;
Fabry et al., 2008; Mayes et al., 2005; Rost et al., 2008].
Avoiding extreme shifts in the pH of natural waters places
limits on the amount of olivine weathering that can be
proposed for a given river basin (cf. example calculations
in Köhler et al. [2010]).
2.4. Enhanced Weathering by Distribution of Olivine
in the Open Oceans
[41] The limits imposed by needing to avoid large shifts
in pH in freshwater systems might be avoided by dissolving
olivine in the surface ocean [Köhler et al., 2013] where the concentration of DSi is well below the saturation level and
much larger volumes of water are involved. DSi concentrations of the modern oceans are on average ~5 mmol m!3
(5 mM) in the surface ocean [Laruelle et al., 2009]. Even
in the Southern Ocean, an exceptional region where surface
water concentrations can be as high as 75 mmol m!3
(75 mM), concentrations remain well below amorphous
silica saturation [Koltermann et al., 2011] of roughly
1000 mM.
[42] Direct dissolution of olivine in the open ocean might
significantly increase the realistic scope of Enhanced
Weathering with olivine (or other minerals). The CO2
sequestration per amount of olivine being dissolved is
slightly smaller if olivine is dissolved in the ocean compared
to on land, but this effect is relatively small, and the benefit
would be a faster rise in surface ocean pH (Figures 7d and 7e),
a very welcome outcome for counteracting ocean acidification
[Doney et al., 2009]. However, surface ocean pH is approximately
7.8–8.3, and dissolution would proceed at a much slower rate
than in tropical soils (Figure 6), thus requiring smaller
mineral grain sizes for comparable dissolution rates relative
to the application of the minerals onto soils. Moreover,
potential complications such as the settling of grains into
the deep ocean prior to their complete dissolution would
have to be carefully assessed. All of the simple modeling
scenarios presented here ignore the potential effects of
Enhanced Weathering on the marine and terrestrial
122
pH surface ocean [-]
pCO2 [ atm]
Emissions [PgC a-1]
HARTMANN ET AL.: ENHANCED WEATHERING
30
25
data
A2 emission scenario
20
15
10
(a)
5
0
1100
1000
900
800
700
600
500
400
300
200
8.2
0
ice cores
measurements
climate models
(d)
-10
-20
(b)
theoretical limit 3Pg olivine a-1
-30
3Pg olivine a-1 @ land
3Pg olivine a-1 @ ocean
8.1
0.02
8.0
7.9
7.8
7.7
7.6
0.01
A2 baseline
3Pg olivine a-1 @ land
3Pg olivine a-1 @ ocean
1800
1900
(c)
2100 2010
2000
Time [years AD]
(e)
2020
2030
2040
2050
0.0
2060
Time [years AD]
Figure 7. Modeling results that simulate the consequences of olivine dissolution with the BICYCLE-model, a box model
for the global carbon cycle [Köhler et al., 2010]. (a) The sum of anthropogenic emissions from fossil fuel combustion
(1750–2000 AD) [Marland et al., 2005] and land use change (1850–2000 AD; before 1850 AD: linear extrapolation of land
use change to zero in 1750 AD) [Houghton, 2003]. For 2000–2100 AD, the A2 emission scenario is used [Nakicenovic and
Swart, 2000]. (b) Global atmospheric CO2, including past data from the Law Dome ice core [Etheridge et al., 1996] and
instrumental measurements on Mauna Loa for 1958–2008 AD [Keeling et al., 2009]. Forward simulation results of the
A2 emission scenarios are shown with passive (constant) terrestrial carbon storage. The gray area covers the range of results
from coupled carbon cycle–climate simulations for the A2 emission scenario C4MIP [Friedlingstein et al., 2006]. (d) Impact
of enhanced olivine dissolution for 2010–2060 AD on pCO2 showing differences of simulated atmospheric pCO2 versus the
A2 emission baseline (∆pCO2 = 0) for two different weathering scenarios (3 Gt olivine per year dissolved on land or in the
open ocean, as shown in legend). Olivine dissolution on land implies the extraction of CO2 out of the atmosphere and the
riverine input of bicarbonate into the surface ocean following equation (8), while in the case of open ocean dissolution only
an input of alkalinity into the surface ocean is generated, which changes the marine carbonate system such that CO2 is taken
up by the ocean and pH is increased. The theoretical upper CO2 sequestration limit is indicated by a red line in Figure 7d.
This limit ignores the effects of the carbonate chemistry on the olivine dissolution and uses only the net dissolution
equation (equation (8)), which implies that 1 mol of olivine introduces 4 mol of TA and DIC into the ocean. (c, e). Mean
pH of the global surface ocean for the same scenarios as in Figures 7b and 7d, respectively. The light gray box in
Figures 7b and 7c covers years 2010–2060 AD, which are in focus in Figures 7d and 7e. Further information can be found
in Köhler et al. [2010].
ecosystems, including the effects on the biological carbon
pump and its capacity to draw down CO2 through removal
of organic biomass into the deep ocean. To the extent that
it is currently possible, these aspects are discussed in detail
in sections 4 and 5.
3. EMPIRICAL INSIGHTS INTO ENHANCED
WEATHERING
[43] To date, no results from field experiments exploring
the weathering of fine-grained olivine added to soils and
the consumption of CO2 associated with it have been
published, and the authors are aware of only one study using
pot-experiments (ten Berge et al., 2012). However, experimental evidence from studies focusing on related topics
sheds light on the potential of olivine for Enhanced
Weathering as a geoengineering technique and the possible
consequences for global biogeochemical cycles.
3.1. Lessons From Artificial Silicates
[44] One important line of evidence providing information
relevant to understanding Enhanced Weathering comes from
studies on the dissolution of anthropogenic material (including
artificial silicates) and the formation of carbonate minerals
within these materials in the natural environment. Silicate
compounds are a product of numerous human activities,
including mining (quarry fines and tailings), cement
production and use (cement kiln dust, construction, and
demolition waste), iron and steel production (slag), and coal
combustion (fuel ash and bottom ash) [Renforth et al., 2011]
and considerable work has been done to understand the fate
and ecological impact of these by-products on the natural
system. These materials are usually associated with
(or wholly consisting of) amorphous gels or glasses and
meta-stable crystalline phases (e.g., “larnite,” Ca2SiO4 and
“alite,” Ca3SiO5). Given the complex mineralogy of the
materials used in these experiments, computation of the
weathering rate of artificial silicates is difficult. In addition
to the work that has been done with artificial silicates, a
substantial number of laboratory, field, and modeling
studies have investigated the mineralogy, environmental
chemistry, and/or carbonation of cement. These include
studies of raw clinker calcium silicates and hydrated
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HARTMANN ET AL.: ENHANCED WEATHERING
calcium silicate gels [Bertron et al., 2005; Chen et al., 2004;
Galle et al., 2004; Hodgkinson and Hughes, 1999;
Huntzinger et al., 2009; Renforth and Manning, 2011; Shaw
et al., 2000a; Shaw et al., 2000b], slags, and other silicate
glasses [Bayless and Schulz, 2003; Fredericci et al., 2000;
Gee et al., 1997; Hamilton et al., 2001; Harber and Forth,
2001; Huijgen et al., 2005; Mayes et al., 2008; Mayes
et al., 2006; Oelkers, 2001; Oelkers and Gislason, 2001;
Parsons et al., 2001; Rawlins et al., 2008; Roadcap et al.,
2005; Sobanska et al., 2000] and ashes [Dijkstra et al.,
2006; Goodarzi, 2006; Grisafe et al., 1988; Gunning
et al., 2010; Koukouzas et al., 2006; Lee and Spears, 1997].
All studies suggest elevated reactivity in comparison to fully
crystalline natural silicates.
[45] While silicate glasses and gels are the largest component
of some anthropogenic material streams (the total quantity
of which may be 10–20 Gt a!1; Renforth et al., 2011), they
are often also associated with other minerals. Free lime
(CaO) and portlandite (Ca(OH)2) are typical constituents
of cements, slags and ashes (usually <15% w/w) [Das
et al., 2007; Koukouzas et al., 2006; Scrivener et al.,
2004], and readily carbonate in the presence of dissolved
CO2. Nonetheless, the majority of carbonate mineral formation in these waste materials is derived from the dissolution
of the poorly crystalline silicate minerals. Waste materials
such as these may be able to capture 190–332 Mt C a!1
[Renforth et al., 2011]. This total carbon capture only
mitigates a fraction of the carbon emissions produced
during manufacturing.
[46] Rapid carbonate mineral formation has been
observed during field investigations of the weathering of
artificial silicates [Dietzel et al., 1992; Kosednar-Legenstein
et al., 2008; Macleod et al., 1991; Mayes et al., 2006;
Renforth et al., 2009; Wilson et al., 2009] (Figure 8).
Renforth et al [2009] investigated the formation of carbonate
in soils formed on demolition waste and slag. Figure 8 quite
visibly shows carbonate formation at these sites, which is a
product of rapid material weathering (equivalent to 2500 t C
km!2 a!1). In natural soils, such carbonate formation would
take 100s to 1000s of years, but the rapid weathering rates of
waste materials results into the observation that such a mass
of carbonate is forming in only tens of years. Similarly, Wilson
et al. [2009] report the sequestration of 11 Mt of atmospheric
CO2 in serpentine-rich tailings at the Clinton Creek asbestos
mine in Canada in 30 years. Wilson et al. [2010] interpret the
stable carbon and oxygen isotope signatures in carbonates to
suggest that it was the supply of carbonate ions (from the
speciation of CO2 dissolved into the aqueous phase) limiting
mineral carbonate precipitation, rather than the supply of
Mg2+ from the weathering of serpentine.
[47] These laboratory and field investigations of artificial
silicates suggest rapid weathering rates result at least in part in
carbonate precipitation and thus carbon dioxide sequestration.
The potentially high weathering rates identified for artificial
silicates are more than an order of magnitude higher than the
rates associated with natural silicate minerals (see Figure 3)
and the associated CO2 sequestration is similarly much
higher than the largest identified CO2 sequestration rates of
around 75 t C km!2 a!1 associated with natural weathering,
in Java and the Philippines [Dessert et al., 2003; Schopka
et al., 2011].
[48] To some extent, the chemical weathering of artificial
silicates deposited on the Earth surface can be considered as
a practiced (albeit unintentional) application of Enhanced
Weathering. Since the early 1800s, approximately 100 Gt
of anthropogenic silicate material has been produced
Figure 8. (left) A “hardpan” of carbonate formed on waste slag mounds at former steelworks in Consett, United Kingdom. (right)
Carbonate precipitation in waters egressing from a waste landfill in Scunthorpe steelworks (photograph courtesy of Carla-Leanne
Washbourne). In both cases, rainwater has percolated through the material (dissolving Ca2+ and Mg2+) and contact with DIC
promotes the precipitation of carbonate.
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HARTMANN ET AL.: ENHANCED WEATHERING
[Renforth et al., 2011] and is either currently still in use or
has been deposited on land (in landfills) or in the ocean.
Optimizing the carbonation of these materials could on its
own enhance the removal of CO2 from the atmosphere,
but optimization requires a better understanding of how
and why carbonation rates vary among materials and
environmental conditions. Fortunately, the rapid rates
associated with artificial materials make this variability
relatively easy to study, and lessons learned from such
research (i.e., in terms of what most effectively increases
mineral dissolution and subsequent carbonation) promise to
have much wider applicability to Enhanced Weathering in
general. This is thus an obvious priority area for further work.
3.2. Lessons From Agriculture: Agricultural
Enhancement of Weathering Rates and the Role of Liming
[49] Additional information about Enhanced Weathering
comes from our knowledge of weathering and CO2
consumption associated with agriculture. There are indications
that agricultural activities enhance weathering rates, even
without the addition of reactive minerals as proposed in
Enhanced Weathering strategies, though there are only a
limited number of studies that have considered the impact of
agricultural activities on weathering and major gaps in
knowledge remain. The studies that have been done
converge in suggesting that agricultural use of land increases
weathering fluxes. Paces [1983] assessed the mass balance
of solutes in two adjacent catchments in central Europe, one
agricultural and one forested, and found that the Na flux from
the agricultural catchment was 2.6 " 1.9 times higher than
the flux from the forested catchment. When accounting
for differences in the exposure of the Na-bearing oligoclase
minerals, the dissolution rate constant in the agricultural
catchment was found to be approximately 4.7 times higher
than in the forested catchment. Similarly, Pierson-Wickmann
et al. [2009] found that weathering rates under agricultural
land in Brittany, France, were significantly elevated relative
to trends for given runoff values for other catchments from
a global compilation. Other evidence for the impact of
agricultural activities on weathering rates can be observed
in the long-term (~100 years) trend of increasing DIC
concentration in the Mississippi River or from comparisons
between forested and agricultural areas [Barnes and
Raymond, 2009; Raymond et al., 2008]. Besides agricultural land use and practices, urban areas add to the observed
increased DIC fluxes [Barnes and Raymond, 2009; Moosdorf
et al., 2011], although the contribution of suggested sources
(enhanced weathering in urban green spaces, leaking sewer
systems, contribution from artificial materials, groundwater
resources for water supply, etc) to the global C-budget
remains to be quantified.
[50] Identifying the mechanism of agriculturally Enhanced Weathering is not straightforward. One significant
effect of agricultural activity is to increase the effective
discharge from streams and rivers, through irrigation and a
reduction in evapotranspiration [e.g., Raymond et al., 2008].
Watershed-scale weathering fluxes are closely related to
discharge, which (especially for peak discharge) is modified
through irrigation and vegetation removal. Attention has
also focused on agricultural acidification facilitating mineral
dissolution, for example associated with the nitrification of
nitrogen-rich fertilizers [Perrin et al., 2008; PiersonWickmann et al., 2009; Semhi et al., 2000]. In this case,
Enhanced Weathering may not always lead to the sequestration of carbon, if DIC is associated with dissolution of
carbonates by nitric acids (see Figure 1). There may also be
significant effects on weathering rates from agricultural tillage,
which exposes less weathered minerals from deeper soils and
may enhance dissolution rates, but this latter effect is poorly
quantified for larger areas.
[51] Together, these effects reflect incidental anthropogenic
Enhanced Weathering as a side effect of agricultural land use
(cf. comments of Mayorga [2008] on the alteration of DIC
fluxes). Better understanding of tillage and acidification as a
result of N-fertilization and how they contribute to carbon
fluxes associated with agriculture is clearly critical to
accurately assessing the potential for CO2 sequestration of
adding new minerals to soils.
[52] One other agricultural practice relevant to
understanding Enhanced Weathering is agricultural liming.
Agricultural lime (which is mostly carbonate minerals from
crushed limestone, but sometimes also contains calcium or
magnesium oxides) is often applied to buffer soil pH within
a range favorable for crop growth [Hamilton et al., 2007] or
to counteract soil/stream water acidification [Hindar et al.,
2003; Huber et al., 2006; Kreutzer, 1995; Rundle et al.,
1995]. Several studies have explored the fate of agricultural
lime applied to soils and tried to quantify its effect on CO2
drawdown. Dissolution of carbonate minerals in agricultural
lime is effectively a kind of Enhanced Weathering (cf.
equations in Figure 1), where the net effect on CO2 depends
on whether dissolution is driven by carbonic acid, in which
case dissolution sequesters CO2 from the atmosphere, or by
other acids, such as HNO3 or H2SO4. Dissolution by the
other acids leads to a loss of alkalinity in comparison to
dissolution by carbonic acid (cf. Figure 1) and may result
in the addition of CO2 to the atmosphere [Hamilton et al.,
2007; Perrin et al., 2008; Semhi et al., 2000]. This makes
it difficult to accurately account for the net effect of liming
practices, even when they can be directly attributed to
measurable increases in riverine element fluxes (e.g., Ca2+
and bicarbonate) [Hartmann and Kempe, 2008; Oh and
Raymond, 2006]. A key distinction to liming (including
the application of carbonate rocks) is that Enhanced
Weathering would favor the use of silicate minerals, because
these would not act as a direct CO2 source even if they were
dissolved by a strong acid (Figure 1). Studying the effect
of liming further requires recognizing that lime addition
changes the capacity of soils to act as a CO2 sink by storing
organic carbon in the long term. While short-term studies
provide partly contradicting results, a long-term study on the
application of liming (~100 years) provides evidence for a
positive effect on soil organic carbon storage for grassland
areas [Fornara et al., 2011]. Despite the many unresolved
uncertainties, the historical practice of liming provides an
appealing analogue for studying Enhanced Weathering
125
HARTMANN ET AL.: ENHANCED WEATHERING
and its potential effects. In order to identify the conditions
under which liming acts as a net carbon sink, it may be fruitful to combine data from the many relevant studies undertaken in agricultural science.
3.3. Need for Specific Experiments
[53] It should be emphasized that, although field
“experiments” such as agriculturally modified weathering
and the recarbonization rates of silicate materials associated
with mining (as discussed in section 3.1) provide some
guidance, it remains difficult to make reliable quantitative
estimates of dissolution rates associated with potential
Enhanced Weathering schemes. This is because the interaction
of aqueous solutions, minerals, physical soil properties,
plant effects, and climate variability is difficult to estimate
or to model, specifically if relevant information about the
physical properties of a site are weakly defined [cf. discussion
in Godderis et al., 2006; Godderis et al., 2009]. Such
estimates will only be possible with controlled studies
considering the range of processes affecting the Enhanced
Weathering rate. There are several factors which complicate
estimations of how quickly minerals will dissolve once
applied to the land surface. In agricultural areas, the
material, which has been applied directly to the soil surface,
will soon be displaced into the upper soil horizons by tilling
and other agricultural practices or be removed due to physical erosion. At the soil surface, the dissolution rate of this
material will likely be controlled of the amount and chemistry of rainwater. However, mineral powder tilled below the
soil surface will additionally be affected by organic acids
present in soils as well as by processes of ion exchange
related to soil properties and the metabolic activity of soil
organisms. In the lower horizons of the soil, CO2 partial
pressure is significantly elevated with respect to the atmosphere, due to the release of CO2 through plant roots and
to aerobic respiration that occurs within the soil and can
temporarily reach levels of about 50,000 ppmv, depending
on land cover, soil properties, climate, and season [Flechard
et al., 2007; Hashimoto et al., 2007; Manning and Renforth,
2013]. Understanding the distribution and quality of acids in
the soil solution and the coincident movement of water
through the soil needs some focus in future research. All
of these effects will need to be carefully considered in
experimental tests of Enhanced Weathering in order to
derive the parameters that are needed for accurate models
which predict the consequences of Enhanced Weathering
applied at the large scale (Table 2).
4. PRACTICALITIES OF ENHANCED WEATHERING
ON LAND AS A CLIMATE ENGINEERING METHOD
[54] Few studies holistically evaluate the engineering
requirements of Enhanced Weathering (including mining,
crushing and milling of rocks, transportation, and application),
though some have begun to consider such practicalities; in
particular, Hangx and Spiers [2009] investigated the
potential of spreading olivine on coastal areas and Renforth
[2012] investigated the engineering requirements of
deploying Enhanced Weathering at a UK national scale.
Assuming optimistic weathering rates and intranational
transport distances, Renforth [2012] concluded that the
energy (expressed as total thermal) requirements for
Enhanced Weathering would be 2.9–91.7 GJ t!1 of C and
potentially cost between 88–2120 US$ t!1 of C. The range
of the estimate is largely due to uncertainty in the grinding
requirements (see below). Using these figures, a global
industry that sequesters 1 Gt CO2-C per year may have an
energy demand equivalent to 0.7%–19.4 % of global energy
consumption. Here the main outcomes of these studies are
reviewed together with other appropriate literature to
outline the potential engineering implications of a globally
operated Enhanced Weathering scheme.
4.1. Resources
[55] Optimization of an Enhanced Weathering scheme
requires identifying a highly suitable location for mineral
application and connecting it to appropriate mineral resources.
4.1.1. Application Sites
[56] Silicate mineral dissolution rates in the environment,
which are key to the feasibility of Enhanced Weathering, are
known to depend on climate and mineral supply [Dessert
et al., 2003; Hartmann, 2009; Hartmann et al., 2010;
Hartmann and Moosdorf, 2011; West et al., 2005; White
and Blum, 1995]. This means that optimal application sites
would be (i) warm and (ii) wet (such as in the humid tropics)
and (iii) have a presently limited supply of readily
weatherable cation-releasing minerals. Such areas with
deeply weathered soils, where the upper soil is hydrologically
disconnected from the weatherable rock [c. f. Edmond et al.,
1995; Stallard and Edmond, 1983; 1987; West, 2012; West
et al., 2005], are shown in Figure 9a. In addition, practicalities, including the ease of mineral application and the
availability of useful infrastructure (roads, rail, inland
waterways, and spreading technology for mineral addition
to the land), make agricultural land (Figure 9b) most feasible
for use. Thus, the optimal locations for mineral application
to soil are likely to be on agricultural land in the humid
tropics (see Figures 9a, 9b).
4.1.2. Mineral Resources
[57] An ideal Enhanced Weathering scheme would utilize
nearby sources of rapidly dissolving silicate minerals. In
general, mafic or ultramafic rocks, like peridotite, basalt,
gabbro, or dunite (see Figure 4), have the highest content
of silicate minerals, such as olivine, that weather rapidly.
Moreover, mafic and ultramafic rocks have higher cation
contents than other silicate rocks, like granite or rhyolite,
so-called felsic rock types (Table 3), and are thus the best
suited for use in Enhanced Weathering.
[58] Mafic and ultramafic rocks are abundant at the
Earth’s surface (Figure 4) and are composed of numerous
minerals. Peridotite, a rock that is dominated by the mineral
olivine, would be the most obvious candidate for use in
Enhanced Weathering, except that its distribution is relatively
limited compared to other mafic rock types. Basalts,
which contain mafic minerals, are primarily composed of
Ca-plagioclase feldspars, pyroxene, and olivine, with minor
126
HARTMANN ET AL.: ENHANCED WEATHERING
a
Soil types
Gleysols
Histosols
Ferrasols
Other deeply weathered soils
b
Figure 9. (a) Areas with soils of low cation content or high groundwater table and/or where the hydrologically active
surface is disconnected from rock material with significant amounts of elements (Ca, Mg, K, Na) available for cation
weathering and thus CO2 consumption. These soil types cause a shielding effect for natural chemical weathering if compared
to rock surfaces in direct contact with surface hydrological processes, as for example in steep, tectonically active areas with
high physical erosion rates [Hartmann et al., 2010a]. These locations are well suited for applying minerals to soils for
Enhanced Weathering. Soil types distinguished here are gleysols, histosols, ferrasols, and other deeply weathered soil types
identified in the Harmonized World Soil Data Base [FAO et al., 2008]. (b) Land cover distribution [based on data from
Bartholome and Belward, 2005; from Hartmann and Kempe, 2008].
proportions of iron (oxy)hydroxides and offer a more widely
available alternative. The largest areas of basalt [Oelkers
et al., 2008] are the flood basalts (e.g., Deccan Traps, India,
and Siberian Traps, Russia), which cover several hundred
thousand to a million square kilometers at a depth of
1–2 km.
[59] The distribution and abundance of ultramafic rocks
are often better known on national rather than global scales.
For example, Goff et al. [2000] and Krevor et al. [2009]
have presented the location, within the United States, of
ultramafic rocks. In the United States, these rocks, which total
approximately 200 km3 of material (see Figure 4), are
predominantly confined to some mountainous regions (the
Appalachians in the eastern United States and, in the
western United States, the Josephine ophiolite in Southern
Oregon, Trinity ultramafic sheet and parts of the Sierra
Nevada in California, and Twin Sisters dunite and Ingalls
complex in Washington). To give another example on the
national scale, Koukouzas et al. [2009] have estimated the
carbonation potential of ultramafic rocks in the Vourinos
ophiolite complex in Greece as totalling approximately 6
Gt C. Furthermore, Renforth [2012] estimates that 33 Gt
of ultramafic rocks, which may be able to capture approximately 7 Gt C, are potentially extractable in the United
127
HARTMANN ET AL.: ENHANCED WEATHERING
TABLE 3. The Geochemical Composition of Some Igneous Rocks
Weight %
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
CO2
S
SO3
H2O+
C
Cl
Sum
Peridotite (Close to Lherzolite)
Gabbro
Granodiorite
Granite
Basalt
Andesite
Rhyolite
42.43
0.63
4.25
3.62
6.61
0.41
31.37
5.07
0.49
0.34
0.10
0.30
0.02
0.02
4.24
0.02
0.07
100.00
50.34
1.12
15.54
3.02
7.65
0.12
7.62
9.62
2.40
0.39
0.93
0.24
0.02
0.02
0.86
0.02
0.07
100.00
66.07
0.54
15.73
1.38
2.73
0.08
1.74
3.83
3.75
2.73
0.18
0.08
0.02
0.02
1.04
0.02
0.07
100.00
71.20
0.31
14.30
1.21
1.64
0.05
0.71
1.84
3.67
4.06
0.12
0.05
0.02
0.01
0.77
0.03
0.02
100.00
49.11
1.84
15.71
3.78
7.12
0.20
6.72
9.45
2.90
1.10
0.35
0.11
0.07
0.09
1.38
0.01
0.07
100.00
57.71
0.87
16.95
3.26
4.02
0.14
3.32
6.76
3.47
1.61
0.21
0.05
0.36
0.04
1.17
0.02
0.05
100.00
72.70
0.28
13.25
1.48
1.11
0.06
0.39
1.14
3.54
4.29
0.07
0.08
0.06
0.01
1.41
0.02
0.11
100.00
Compiled in Hartmann et al. [2012] [data based on Le Maitre, 1976; Max Planck Institute for Chemistry, 2006; Ricke, 1960; Taylor, 1964]. From the left to
the right: mafic to felsic plutonic rocks (peridotite to granite) and volcanic rocks (mafic basalt in comparison to more felsic types of volcanics like rhyolite).
For igneous as well as volcanic rocks the content of Ca and Mg decreases from left to right.
Kingdom. However, the amenity value of these formations
suggests that only a fraction of this potential is exploitable.
[60] While the quantity of material available for Enhanced
Weathering may not be a limiting factor, getting it to a
suitable location for weathering certainly could be. Efficient
deployment of Enhanced Weathering on a global scale
requires a global and relatively high-resolution compilation
of geochemical rock properties that shows local and
regional heterogeneities. As a starting point, global-scale
assessments could be facilitated by using geochemical
information from the mapping of exposed rocks (applying
standardized sampling grids and interpolating between
sampled sites, as has been done for Europe by the FOREGS
program [Imrie et al., 2008]) or by compiling regional and local
geological mapping studies and assembling the geochemical
properties of the rocks based on geological/lithological maps
[Hartmann et al., 2012; van Straaten, 2002].
[61] Regardless of the distribution of minerals suitable for
use in Enhanced Weathering, large scale application of this
geoengineering technique will require new mines; the
current total global production of olivine is only ~8 Mt a!1
(personal communication with the Åheim mine in Norway), 3
orders of magnitude less than necessary for geoengineeringscale undertaking of Enhanced Weathering (see section 2).
The environmental costs of such substantial additional
mining, through such destructive procedures such as mountain
top removal, need to be considered, as does the mobilization
of potentially hazardous metals during the dissolution of
mafic and ultramafic rocks. For example, certain dunite
rocks contain relatively high amounts of nickel (Ni) and
chromium (Cr) [Max Planck Institute for Chemistry, 2006]
and, as has been seen many times before in mine drainage,
if mineral weathering releases metals at high rates, they will
prove harmful to nearby ecosystems [Alloway, 2012]. At
lower rates, however, many of the metals that would be
released (e.g., Fe, Ni, Co, Zn) would serve as vital nutrients
for the growth of autotrophs in terrestrial and some marine
ecosystems, an effect discussed at greater length in sections
5 and 6. An overview of the significantly variable elemental
composition of rocks can be found, e.g., in the GEOROC
database [Max Planck Institute for Chemistry, 2006].
4.2. Material Processing
[62] One of the main challenges to assessing any Enhanced
Weathering strategy is determining the cost of processing
the material for reaction. In particular, crushing rock
(“comminution”) to the particle size necessary for rapid
dissolution is an energy-consuming endeavor. Some of the
energy consumed in comminution is stored as potential
energy of the new surface area. However, rock comminution
equipment consumes substantially more energy than the
theoretical free energy production of the new surface,
resulting in poor efficiencies (<5%) [Fuerstenau and Abouzeid,
2002]. Most of this energy is lost in heat, vibration, and
noise. Improvement of energy efficiencies in comminution
would improve the feasibility and lower the cost of
Enhanced Weathering.
4.2.1. Practical Lessons in Grinding
[63] Rocks containing silicate minerals have long been
extracted for use as aggregate that is mainly used in
infrastructure development (as road base, filler in concrete,
earthworks, etc.). There is a lot of information available
from the aggregate industry about the physical properties
(particularly the particle size distribution and shape) of the
material following comminution. The classic formula
developed by Bond [1952] and verified by laboratory scale
crushing experiments uses particle size reduction to predict
energy use in comminution:
10Wi 10Wi
W ¼ pffiffiffiffiffiffiffi ! pffiffiffiffiffiffiffi ;
P80
F80
(9)
where Wi is the material specific work index (kWh t!1), P80
represents the particle diameter to which 80% of the product
passes and F80 denotes the same limit in the feed. P80
128
HARTMANN ET AL.: ENHANCED WEATHERING
represents a definable upper limit to the particle diameter of
the resultant material. Part of this size fraction is often referred to as “fines” (material that is too small for use as aggregate, usually <4 mm), which is a substantial problem in
quarries. The size and subsequent “quality” of the aggregate
is carefully controlled by crusher operational parameters and
screens that separate out the required size fraction. Efforts
are made to reduce the production of the waste fines, which
are stockpiled and sometimes used in redevelopment of the
site [Woods et al., 2004]. Extraction and processing of metalliferous ore is not constrained in the same way, and in this case
the production of fine particles (<50 mm) is desirable.
[64] Processing rock for Enhanced Weathering would
be done in several stages. First, low-energy (mainly electrical ~5 kWh t!1) blasting and primary crushing would
be used. The majority of energy (electrical) in comminution (10–316 kWh t!1) [Renforth, 2012] would be used
in the second step for secondary or tertiary crushing/grinding
to produce the small grain sizes necessary for rapid
dissolution.
4.2.2. Theoretical Constraints on Mechanical Grinding
[65] Expressing energy consumption as a function of
surface area is particularly useful for Enhanced Weathering,
because weathering rates can also be expressed in terms of
material surface area, so energy costs and weathering rates
can be directly related. There have been a limited number of
studies investigating surface area changes during comminution
[see, e.g., Axelson and Piret, 1950; Balá!z et al., 2008;
Fuerstenau and Abouzeid, 2002; Haug et al., 2010;
Stamboliadis et al., 2009] (Figure 10). The relationship
between energy consumption (y) and the resulting surface
area (x) is exponential, following (Figure 10):
y
$
%
& 2'
m
kW h
¼ 76:854 " 7:063 e0:9x"0:002 g
& 2
'
t
r ¼ 0:995 :
(10)
[66] As discussed previously (section 2), the maximum
carbon capture potential of olivine is 340 kg C t!1 of
mineral. If material other than pure olivine (e.g., ultramafic
to mafic rocks such as dunite, which contain a small proportion
of other minerals) is used, the net efficiency will decrease
[~200 kg C t!1 (material) may be typical for ultrabasic rocks;
Renforth, 2012, and references therein]. Therefore, a conservative feasibility analysis of Enhanced Weathering must
include the lower carbon capture potential. Furthermore,
this carbon capture potential may be reduced if there is
appreciable resulting carbonate precipitation in soils or the
surface ocean.
[67] Fossil fuels emit approximately 0.06–0.27 kg C per
kWh produced (0.06 kg C per kWh for road fuel, 0.11 kg
C per kWh for grid electricity in the United Kingdom,
~0.27 kg C per kWh for electricity from coal combustion)
[see Renforth, 2012, and references therein]. Therefore, the
maximum energy that can be “spent” on a material (including that which is required for extraction, processing, and transport) is between 740 and 3330 kWh t!1 of material (assuming
Figure 10. Crushing energy (kWh t!1) against generated
surface area [adapted from Axelson and Piret, 1950; Balá!z
et al., 2008; Haug et al., 2010; Stamboliadis et al., 2009].
There is relatively little data for the intermediate-energy
grinding (0.1–5 m2 g!1 surface area) and additional studies
in this range would be a particular benefit to assessing
Enhanced Weathering efficiencies. It is also possible to
further increase dissolution rates with “mechanochemical”
activation, in which the silicate framework at the surface of
the mineral is altered chemically [Balá!z et al., 2008; Haug
et al., 2010]. However, the energy requirements associated
with mechanochemical activation are likely to limit the
feasibility for Enhanced Weathering.
the material can capture ~200 kg C t!1). Exceeding this energy
budget would result in material processing/transportation
emissions surpassing the carbon drawdown from Enhanced
Weathering. The exact energy budget for Enhanced Weathering
depends on site specific information including extraction
and application site infrastructure and technology, fuel
mix, distance, and transport mode between extraction and
application site and comminution requirements (which
requires a greater understanding of weathering rates in soils).
Most crushing and grinding practices use ~102 kWh t!1 of
material (Table 4) and are generally within the range presented
above.
[68] Accounting for the energy consumed in processing
is a key part of robustly determining the carbon capture
efficiency of Enhanced Weathering strategies. This will
need to be actively monitored since grinding efficiency
may vary for different specific materials (e.g., olivine from
different dunite deposits). This is important due to the
trade-offs between the energy consumed in grinding and
the rate of dissolution. For example, using the energy
consumption described by equation (9), the net CO2 sequestration efficiency of olivine application to soils would be reduced by 5%–10% because of CO2 emissions related to the
mining and grinding of olivine [Hangx and Spiers, 2009],
assuming the use of 10 mm grain size particles. Using a
37 mm grain size would require less energy for crushing,
so the loss in efficiency would only be 0.7%–1.5%, but
the dissolution rate of such larger sized particles is expected
to be significantly slower, delaying the sequestration effect.
129
HARTMANN ET AL.: ENHANCED WEATHERING
TABLE 4. Typical Energy Requirements for Various Crushing Technologies
Feed Particle Size (mm)
Product Particle Size (mm)
Capacity (t h!1)
Energy Use (kWh t!1)
18–65
7–12
1–2
<37
12–225
15–65
Centrifugal mills
Ball/stirred media mills
104–105
20–30
11
150
0.1–35a
Impact crushing
Cone crushing
Jaw crushing
104–105
105
105–106
102–103
102–103
103
130–1780b
80–1050b
60–1600b
6.6–11.0
7.6–36.0
150.0
13.0–233.0a
20–100b
0.6–1.5b
0.3–1.5b
1.0–1.4b
Crushing and Grinding Technology
Roller mills
a
Indicative values derived from Kefid technical data sheets.
Indicative values derived from Metso technical data sheets.
Sources: Wang and Forssberg [2003], Lowndes and Jeffrey [2009], Fuerstenau and Abouzeid [2002], O’Connor et al. [2005].
b
4.3. Transportation and Infrastructure
[69] Transportation of large quantities of milled rock
requires extensive distribution networks and infrastructure
[Hangx and Spiers, 2009]. Agricultural areas in industrialized
and some emerging countries have existing supply channels
and basic infrastructure that could be easily modified for
use by Enhanced Weathering programs. Some areas of
Africa and Southeast Asia, though environmentally well suited
for Enhanced Weathering, would need to build new supply
channels, since there is currently little use of agricultural
fertilizers and little associated infrastructure that could be
co-opted for dispensing powdered minerals [Hernandez
and Torero, 2011; van Straaten, 2002].
[70] The amount of silicates that need to be transported
and dissolved to implement Enhanced Weathering on a
geoengineering scale is huge and may require expansion
of current infrastructure even in areas where infrastructure
is well developed. The olivine weathering scheme discussed
here involves the production, transportation, and distribution
of 3 Gt olivine per year over soils. By comparison, in 2010,
a total of 8.3 Gt of goods for international trade were loaded
at the world’s ports (i.e., a value which does not include goods
for intranational trade [UNCTAD, 2011]). Thus Enhanced
Weathering requires a transport industry of a scale similar to
that in use for international commerce.
4.4. Application and Monitoring
[71] The application of weatherable minerals on land
would preferentially be conducted using established
agricultural infrastructure, including the supply chains for
fertilizers. Application of ground minerals to forested
regions would be a challenge since application could probably
only be done from the air, at considerable expense, both
financially and in terms of the carbon efficiency of CO2
sequestration (Table 5). Application from the air would
also require new infrastructure or adaptation of existing
infrastructure such as fire fighting planes, as well as a
means for monitoring the amount of mineral that reaches
the target soil system (a certain but unknown amount will
be laterally transported by wind and leave the optimal
target area).
[72] Indeed, independent of what minerals are used and
whether they are applied on agricultural or forested land,
TABLE 5. Emissions From a Range of Freight Transport
Freight/Haulage Transport Method
Airfreight
Road heavy goods vehicle
Diesel rail
Electric rail
Inland waterways
Large tanker
gCO2 Emissions km!1 t!1
602
62
15
14
31
7
Sources: McKinnon and Piecyk [2009], McKinnon and Piecyk [2010],
Institution of Mechanical Engineers [2009].
careful monitoring will be of paramount importance. This
would have to include monitoring of the following: (1)
the chemical impact of the released solutes (including
potential contaminants like heavy metals) and shifts in
pH in soils and freshwater systems, (2) the physical
impacts of dust particles on organisms both in aquatic
systems and in the air, and (3) the dissolution of the
mineral powders.
[73] Although it will not be easy to monitor all of these
aspects, tracking the dissolution of the minerals may
be the hardest. It is currently difficult to make accurate
predictions for how quickly the applied minerals will
dissolve, much less to monitor this effect over large areas
of the land surface. Any implementation of Enhanced
Weathering will require the development of carefully
considered monitoring techniques for efficiency and risk
assessment, as well as application strategies, which do not
exist to date (cf. Appendix).
5. POTENTIAL FEEDBACKS WITH THE TERRESTRIAL
BIOSPHERE
5.1. Plants and Weathering: Beyond Abiotic Kinetics
[74] In general, life across a range of scales, from
microbes to forests, has been found to naturally increase rates
of silicate mineral dissolution and associated drawdown of
atmospheric CO2. A key question for Enhanced Weathering
is what impact biological activities would have on readily
weatherable minerals applied to soils: Biological activity
might increase the dissolution rate of applied minerals on the
one hand, or mineral addition might reduce the biological role
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HARTMANN ET AL.: ENHANCED WEATHERING
in weathering by altering the nutrient status of ecosystems on
the other hand.
[75] Field studies have quantified to first order how the
presence of plants and associated ecosystems affects mineral
weathering rates. Moulton et al. [2002] compared
weathering-derived element fluxes from streams draining
small Icelandic catchments (basalt) that were either
vegetated with birch or conifer or covered only by lichens.
They found elemental fluxes 2–5 times higher with forest
vegetation compared to lichens. Bormann et al. [1998] used
a mesocosm experiment at the Hubbard Brook Experimental
Forest in New Hampshire, USA, where “sandboxes” with
uniform granitic substrate were planted with trees or left
bare. They found weathering release rates were significantly
higher under the trees (18+ higher for Mg and 10+ for Ca).
In experiments growing different plants on basaltic substrate under laboratory conditions, Hinsinger et al. [2001]
found that plants enhanced the release by chemical
weathering of many elements by a factor of 1–5 relative to
a salt solution, providing experimental evidence to
support the field observations from Iceland and New
Hampshire. These effects may differ depending on the tree
species, for a variety of reasons such as differences in plant
uptake rates of elements, soil pH, and mycorrhyzal assemblages, though the relationships remain to be well understood. Some studies have hinted at higher weathering fluxes
associated with angiosperms compared to gymnosperms,
but this picture is not entirely conclusive (see compilation
of data by Taylor et al. [2009]). There is also evidence that
lichens enhance weathering rates over that of bare rock,
although there is a lack of well-controlled field studies to
quantify this effect [cf. Brady et al., 1999; McCarroll and
Viles, 1995].
[76] Plants drive higher weathering rates for a number
of reasons [Manning and Renforth, 2013]. In the process
of taking up nutrient elements, they alter soil solution
chemistry and change the saturation state of minerals, favoring dissolution. Through root respiration, they directly release CO2 into soils, increasing acidity and enhancing mineral dissolution rates (see Figure 11). Moreover, through
mycorrhizal assemblages, they release organic compounds
that accelerate mineral dissolution [Leake et al., 2008].
[77] Some of the key mechanisms of biotic weathering
enhancement involve symbiotic interactions, with plant, fungus,
and bacteria communities working closely in tandem with each
other. Many interactions specifically target the release of nutrient elements from minerals. In several studies, fungal hyphae
have been shown to penetrate into silicate minerals to elicit
the release of Ca and P from apatite [Jongmans et al., 1997;
Van Breemen et al., 2000], which is only found in trace amounts
within most rocks but contains high concentrations of these
critical nutrients. Mg and Fe are also important plant nutrients,
but biological weathering of Mg- and Fe-bearing minerals
(such as olivine) has not been carefully examined. The
extent to which ecosystems facilitate the weathering of
minerals such as olivine may depend on ecosystem nutrient
status, with, for example, nutrient stress potentially driving
greater biological enhancement of weathering. This should
be an active area of research in terms of understanding how
schemes for Enhanced Weathering will function in practice.
5.2. Release of Silicon and Its Effects on Terrestrial
Ecosystems
[78] The use of silicate minerals for Enhanced Weathering
will result in the production of significant quantities of
dissolved silicon. This excess silicon will not be confined
to soil solutions, rivers, and other aqueous systems but will
work its way into many other biogeochemical reservoirs
and may affect a range of processes in the terrestrial silica
cycle (Figure 12).
[79] Silicon is considered to be a beneficial nutrient for
plants in general, and in some cases, it is essential for
Photosynthesis
CO2 in soil gas
Diffusion
Mineralisation
Atmospheric
CO2
Microbial
Biomass
Plant
Roots
Plant
exudation
Dissolution
Uptake
Soil
Solution
Excretion
Degassing
Organics+ CO2
Organics
Soil
Particles
Desorption &
Dissolution
Leaching
River
Water
Sorption
Ground
Water
CO2
Figure 11. A broad conceptualization of organic carbon and inorganic CO2 dynamics in the environment.
Organic carbon in the soil solution (as low molecular weight organic acids) which is exuded by soil flora and
fauna contribute substantially to weathering of soil particles. Adapted from Jones et al. [2003].
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HARTMANN ET AL.: ENHANCED WEATHERING
growth [Epstein, 1994, 1999, 2009]. An ample supply of
usable silicon improves the water use efficiency and
drought stress resilience of certain plants, increases their
rate of photosynthesis under drought stress, and enhances
resistance to certain diseases and infestations [Agarie
et al., 1992; Chen et al., 2011; Crusciol et al., 2009; Datnoff
et al., 1991, 1992, 1997; Deren et al., 1994; Gao et al.,
2004; Korndorfer et al., 1999; Savant et al., 1997a,b]. This
means that, in soils containing relatively low amounts of
“plant-available silicon”, Enhanced Weathering could
improve plant health and growth. Low concentrations of
plant-available silicon are found in highly weathered soils
with low base cation contents, predominantly soils in
regions of the humid tropics as well as histosols with high
organic matter content [Datnoff et al., 1997; Nanayakkara
et al., 2008; Savant et al., 1997a]. Many agricultural fields
are also depleted in plant-available silicon because of the
repeated harvesting of crops that results in the export of
the silica found within plants [Datnoff et al., 1997;
Nanayakkara et al., 2008; Savant et al., 1997a]. Agricultural
fields have often been fertilized with nitrate and phosphate
fertilizers for years, but not with silica releasing minerals,
driving these fields into silicon limitation.
[80] Silicon present in dissolved form in water is absorbed
by plants as monosilicic acid, Si(OH)4 [Jones and
Handreck, 1967], where, at various deposition sites within
the plants, it polymerizes into a silica gel that further
condenses to form amorphous, hydrated silica solids known
as phytoliths in land plants [Yoshida et al., 1962]. This
biogenic silica (BSi) fulfils several functions. It contributes
to increases in cell wall structure, helping to defend plants
against biotic stress like insect pests [Alvarez and Datnoff,
2001; Deren et al., 1994; Epstein, 1999; 2009; Ma, 2004].
Abiotic stress reduction seems to be provided by enhancing
the uptake of phosphorus (in the case of rice plants), reducing toxicities associated with Mn, Fe, and Al, increasing the
mechanical strength of stems, improving the plant growth
habit (overall shape, structure, and appearance of the plant),
and reducing the shattering of grains [cf. references in
Nanayakkara et al., 2008]
[81] Some of the observed positive effects of abundant BSi
within a plant might be due to the physical properties of the
phytoliths. Such silica nanoparticles possess a significant adsorption surface that could affect the wetting properties
of xylem vessels (the conduit through which water is
transported through vascular land plants) and thus could
improve the water use efficiency of these plants [Gao et al.,
2006; Gao et al., 2004; Wang and Naser, 1994; Zwieniecki
et al., 2001]. Increased resistance to drought has been reported
for several plant species, like sorghum bicolor, maize, rye, and
rice, when plant-available silicon has been supplied [Chen
et al., 2011; Gao et al., 2006; Hattori et al., 2005, 2009].
The addition of silicon to soils by enhanced chemical
weathering may increase water use efficiency by as much as
35%, depending on plant species [Gao et al., 2004]. This
effect could mean that Enhanced Weathering, by increasing
Si supply, might alter local hydrologic cycles, but this has
not been carefully considered. Better understanding of this
effect will be important for assessing the complete consequences of mineral application to soils.
[82] Through the release of silicon, Enhanced Weathering
could result in additional drawdown of CO2 by stimulating
plant growth in nonhumid areas, because of its positive
effect on the water use efficiency and in areas where silicon
Agricultural Si exportation
0.22
ATMOSPHERE
BIOMASS
Biogenic silica
(phytoliths and micro-organisms)
1.7-5.6
RIVERS
SOIL
SOLUTION
Eolian Si particles
SOIL
0.01
Biogenic silica
Suspended
Si particles
Primary minerals
Dissolved Si
(H4SiO4)
Secondary minerals
Dissolved Si
(H4SiO4)
OCEANS
0.28
(phyllosilicates, poorly crystalline silicates,
silica polymorphs)
Fe, Aloxy-hydroxides
0.14
Dissolved Si
(H4SiO4)
6.7
Biogenic silica
(diatoms)
Diatoms
GROUNDWATER
Silica polymorphs
(silicification)
Dissolved Si
(H4SiO4)
0.16
Feoxides
Figure 12. Silicon is transferred within the terrestrial system as indicated by this figure, adapted from
Cornelis et al. [2011] (figure courtesy of Jean-Thomas Cornelis). The compartments of the biogeochemical Si cycle on continents were modified from Basile-Doelsch et al. [2005]. Solid line: transport; dashed
line: dissolution; small dotted line: neoformation/precipitation; pointed-dotted line: adsorption/desorption.
Numbers on arrows show interpool Si transfers in 1012 kg Si a!1 [Matichenkov and Bocharnikova, 2001;
Treguer et al., 1995].
132
HARTMANN ET AL.: ENHANCED WEATHERING
is limiting to plant growth. The extent to which Enhanced
Weathering could release the terrestrial biosphere from
silicon limitation is not yet known, as it has only been recently
recognized that land plants could be silicon limited, leaving
many silicon limited regions to be identified and mapped.
[83] It is important to recognize that most of the studies
referred to in this section focused on the effect of silicon
availability on agricultural ecosystems. Studies have
investigated the impact of plant available silicon on the
development of trees and their physiological properties for
only a few species. However, recent evidence suggests that
trees impact the local silica cycle differently than shrubs and
grasses (cf. the work of Cornelis: Cornelis et al. [2010a],
Cornelis et al. [2010b], Cornelis et al. [2010c], Cornelis
et al. [2011a], Cornelis et al. [2011b]), perhaps in part
because BSi from forests is 10–15 times more soluble than
BSi from grasses, owing to its greater specific surface area
[Wilding and Drees, 1974; Cornelis et al., 2010c].
5.3. Release of Other Nutrients and Effects on
Ecosystem Productivity
[84] While dissolution of Mg-silicates like olivine will
primarily supply Mg and Si to soils (Figure 1), it will also
release many other elements that will have effects on
ecosystems. By mass, an ultramafic rock may contain up
to 5% Fe, 0.06% Mn, 0.02% P, and 0.02% K [Green,
1964], and so weathering of 3 Gt per year of olivine could
release 150 Mt Fe and up to 1 Mt of Mn, P, and K. This
could spur plant growth by providing essential nutrients,
thereby driving further sequestration of carbon in the terrestrial
reservoir by building up standing stocks of organic carbon in
biomass and soils.
[85] The net impact of such mineral fertilization on the
terrestrial carbon pool in agricultural ecosystems is already
relatively well understood [e.g., Alvarez and Datnoff,
2001; Ma and Takahashi, 1990; van Straaten, 2002],
because the optimization of crop yield and thus growth rates
is a major objective in agricultural science. For example, the
net rice yield can be increased by 10%–50% by application
of silicon fertilizers, depending on the local conditions [cf.
Alvarez and Datnoff, 2001]. The application of suitable
rocks for mineral fertilization has been discussed for
decades [van Straaten, 2002; Walthall and Bridger, 1943],
and the large number of studies on this topic is more than
that comprehensively surveyed here.
[86] Much less is known about the potential impact of
Enhanced Weathering on the carbon content of forested
regions. Tropical forested regions contain about 25% of
the total terrestrial biomass [Jobbagy and Jackson, 2000]
and account for at least 33% of the global terrestrial net
primary production (NPP) [Beer et al., 2010; Grace et al.,
1995; Phillips et al., 1998]. These forests are located in
the most suitable areas for carrying out Enhanced
Weathering, and changes in their productivity associated
with increasing nutrient supply could be significant in terms
of the global carbon cycle. The main plant nutrients are N,
P, and K, and a poor supply of any of these nutrients may
limit productivity [Hyvonen et al., 2007; Tripler et al., 2006].
The mafic and ultramafic rock powders being considered
for Enhanced Weathering contain minor proportions of
P-rich and K-rich minerals, but little if any N, though
trace metals that are present in silicate rocks are required
in N-fixing enzymes [Ragsdale, 2009]. If tropical forest
ecosystems are P- or K-limited, then the P and K supply
from Enhanced Weathering should affect the carbon pool of
forested ecosystems. Cleveland et al. [2011] conducted a
meta-analysis for tropical forests because some studies
have suggested that NPP in tropical forests is limited by P
[cf. references in Cleveland et al., 2011], while others have
argued that tropical forests often have a labile P pool in the
surface soil sufficient to avoid P limitation of NPP in these
systems. The overall result was that the lack of spatially
explicit knowledge of how tropical forest systems will react
to enhanced P availability and possibly also K availability
[Tripler et al., 2006] calls for a series of large-scale nutrient
manipulations experiments to clarify this issue [Cleveland
et al., 2011].
[87] Based on this, we recommend that during Enhanced
Weathering exercises, the effect on NPP and biomass per
unit area should be monitored so that the potential surplus
in C-sequestration due to biomass (and soil carbon) increase
may be evaluated.
5.4. Wider Consequences for Agricultural Systems
[88] When minerals are spread on agricultural land during
Enhanced Weathering, a significant additional benefit may
be the fertilization of crops [cf. van Straaten, 2002,
outlining the concept of “rocks for crops”] because silicate
minerals contain most of the nutrients required by plants
(with the exception of N). Powdered silicate rocks have
even been considered as an alternative to conventional fertilization in areas where fertilizers are not available or are too
expensive for many farmers, and in organic agriculture
[Coroneos et al., 1995; Leonardos et al., 1987; Leonardos
et al., 2000; Von Fragstein et al., 1988; Walthall and Bridger,
1943]. The potential for silicates to supply K [Manning, 2010]
is notable because this critical nutrient is rapidly depleted from
agricultural soils, particularly in the tropics. Manning [2010]
concludes: “the present high cost of conventional potassium
fertilisers justifies further investigation of potassium silicate
minerals and their host rocks (which in some cases include
basic rocks, such as basalt) as alternative sources of K,
especially for systems with highly weathered soils that lack a
significant cation exchange capacity.”
[89] Both the time frame and exact extent of the fertilization effect of adding powdered silicate rocks to agricultural
lands needs to be better assessed, considering this as part of
the geoengineering strategy of Enhanced Weathering. Many
studies have concluded, for example, that slow dissolution
and nutrient release rates from silicate minerals limit their suitability for agricultural applications [e.g., Blum et al., 1989a;
Blum et al., 1989b; Von Fragstein et al., 1988], while others
have concluded that in some environments the relatively high
dissolution rates of the minerals makes them suitable as longterm, slow-release fertilizers [e.g., Leonardos et al., 1987;
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HARTMANN ET AL.: ENHANCED WEATHERING
Nkouathio et al., 2008]. The key lies in identifying which
combinations of plants, soils, minerals, and climatic conditions result in high nutrient release rates that stimulate plant
growth and crop yields. Accomplishing this is made difficult
by mineral dissolution in soils being governed by a series of
interactions that are difficult to simulate in laboratory experiments [Harley and Gilkes, 2000; van Straaten, 2002]. What is
clear is that targeted application of silicate minerals to
agricultural soils may have synergistic effects on primary
productivity, industrial fertilizer use, and crop yields, and
well-designed Enhanced Weathering schemes strategies
would take advantage of this.
6.
THE COASTAL AND THE MARINE SYSTEM
6.1. Total Alkalinity and pH
[90] If Enhanced Weathering is carried out on a
geoengineering scale, total alkalinity (TA; see section 2,
equation (1) above) and pH in the ocean will change due
to the input of the products (Mg2+, Ca2+, H4SiO4) from
silicate rock weathering. The input of Mg2+ and Ca2+ leads
to an immediate increase of TA (equation (1), section 2.1).
The related change of pH can be calculated under the
assumption of equilibration of CO2 partial pressures between
atmosphere (at a certain value given) and the ocean. The
“one-time-input” weathering of 10 Gt olivine (e.g., pure forsterite
(Mg2SiO4): 10 + 1015 g forsterite-olivine + 1/140 mol/g
forsterite + 2 mol magnesium/mol forsterite) would result in
an input of 1.4 + 1014 mol Mg2+.
[91] If this input were evenly distributed over the
whole ocean surface (taken here as the upper 50 m of
the water column), the impact on TA and pH would be
relatively small (∆TA = 8 mmol kg!1, ∆pH = 0.001 from
an initial mean state of DIC = 2010 mmol kg!1, TA = 2280
mmol kg!1, T = 20# C, Salinity = 34). However, changes in
TA and pH would increase over time if the same amount of
olivine was weathered every year over a longer period. If
the “one-time-input” is restricted to a much smaller volume, for example just the coastal regions, the local
changes in TA and pH would be much higher (∆TA =
790 mmol kg!1, ∆pH = 0.11 for 1% of the upper ocean
volume). The extent of the change in TA and pH in the
surface ocean over time will depend in part on circulation
and mixing and thus has to be calculated using an ocean
circulation model. Such detailed analysis remains to be
done, so much remains to be understood about how
Enhanced Weathering would influence the oceanic
alkalinity system and potentially offset the decreasing
pH associated with ocean acidification [Köhler et al.,
2013]. Specifically, in local coastal areas affected by
“acidification” due to CO2 increase in the atmosphere,
the Enhanced Weathering strategy might be considered
to limit the consequences of acidification.
6.2. Alteration of the Si Fluxes to the Coastal Zone and
Influence on the Biological Carbon Pump in the Oceans
[92] In addition to changing alkalinity and pH, global scale
application of Enhanced Weathering would significantly alter
dissolved silicon (DSi) fluxes to the coastal zones. Silicon
released by weathering on land may be transmitted, via
runoff, to rivers. Some but not all of the DSi delivered to
rivers is likely to be taken up as biogenic silica (BSi)
produced by diatoms and marshland plants in the river, as
well as in lakes, reservoirs, and estuaries [Humborg et al.,
1997, 2000; Ittekkot et al., 2000]. Still, a portion of the
DSi is expected to make its way into the ocean [Laruelle
et al., 2009], as recent retention of DSi in the land system
is estimated to be only about 20% [Beusen et al., 2009].
This proportion may vary locally because of varying
degrees of N and P limitation in many large river systems
draining to the coastal zone, depending in part on the
industrialization stage of the catchment and anthropogenic
nutrient inputs [Beusen et al., 2005; Harrison et al., 2010;
Hartmann et al., 2011; Mayorga et al., 2010]. Moreover,
while it is likely that significant amounts of BSi deposited
in the floodplains of rivers are redissolved, a significant
proportion might be stored in floodplain deposits, as results
from the Congo river indicate [Hughes et al., 2011].
However, this amount is globally uncertain and more research
is needed to understand the fate of DSi during transport
from its point of mobilization to the costal zones [Hughes
et al., 2011]. If all Si is released during the dissolution of 3
Gt of olivine in humid tropical areas (based on the scenario
described in section 2) and if all of this Si reaches the coastal
zone, then the annual DSi fluxes to the coastal zone in humid
tropical areas would increase, on average, by a factor of >3.4
over current values [cf. Dürr et al., 2011] (see Figure 13).
Regionally the increase could be higher. Assuming an area
specific and runoff weighted using Fekete et al. [2002] equal
release of silica into rivers for the humid tropical areas
(after Holdridge 1967, digital version by Leemans 1992) as
described by the scenario above, DSi-fluxes would increase
for the Amazon, Orinoco and Congo by a factor of 8.6, 8.2
and 6.1 above current values, respectively. More than 50%
of this additional flux would be delivered by these three rivers (34% by the Amazon alone), plus the Mekong, Ganges/
Brahmaputra, Salween and Irrawaddy in SE Asia. Most of this
additional flux (73.4%) would reach coastal zones directly
connected to open oceans (see below), but the remainder
would be delivered to areas connected to regional and marginal seas [Meybeck et al., 2007] that would probably retain
most of the DSi (see Figure 13). SE Asia regional seas would
be responsible for the major share of the retention (10.1% to
the South China Sea and 6.0% to the Sunda/Sulu/Banda seas;
in total 21.6% of the incoming additional flux).
[93] Several questions then arise. Will this extra DSi be
entirely transmitted to and retained in nearshore sediments
as BSi that has been produced by silicifying organisms (like
diatoms) in the vicinity of the river plume, or will it serve as
a silicon source to more distant areas of the ocean? Will this
extra DSi alter marine food web structures by favoring the
growth of diatoms, which, uniquely among the major
marine phytoplankton, require DSi as a nutrient for growth?
And, lastly, would such additional input of DSi to the ocean
have any stimulating effect on the biological pumping of
carbon out of the surface ocean, thereby lowering atmospheric
concentrations of CO2?
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HARTMANN ET AL.: ENHANCED WEATHERING
Figure 13. Catchments of basins contributing most of the dissolved silica fluxes to the coastal zones are
located in regions favorable for the Enhanced Weathering procedure. Specifically in Southeast Asia a
significant amount of additional dissolved silica would be most likely intercepted by closed or
semienclosed regional seas (figure after Ragueneau et al. [2010] and Dürr et al. [2011]. Boundaries of
humid tropics and additional areas classified as warm and humid after Holdridge [1967], digital version
by Leemans [1992], are indicated by red lines. Retention of dissolved riverine material by regional and
marginal seas after Meybeck et al. [2007]. Relative fluxes are normalized according to the global mean
value. Thus the value “1” indicates the global average. River catchments were aggregated according to
the COSCAT segmentation scheme [Dürr et al., 2011].
[94] There is evidence to suggest that the enhanced delivery
of DSi to the ocean by rivers would result in local, if not
regional increases in the inventory of DSi in surface waters.
For example, the natural DSi load in the Congo River (a flux
of 3.5 + 1011 mol of Si per year) is enough to raise DSi concentrations along a 1000 km stretch of coastline by 5–10 mM
[Bernard et al., 2011]. In addition, the dissolution of BSi
(largely produced from river-sourced DSi) from the sediments of the Congo River fan provides a diffusive supply
of DSi to this area, elevating the DSi concentration of
deeper waters by several mM [Ragueneau et al., 2009]. Similarly, the 1.1 + 1012 mol per year of DSi delivered by the
plume from the Amazon and Orinoco Rivers is enough to
raise DSi concentrations in the Caribbean by 10 mM
[Bernard et al., 2011]. These are extreme cases, as the
fluxes are large, but they illustrate that a doubling of the
silicon flux to the ocean in specific areas could have far
reaching influences of DSi concentrations in surface
waters— exactly where it could be used by the obligately
photosynthetic diatoms to fuel their growth.
[95] In contrast to this, however, is the recent work of
Laruelle et al [2009], who used a box model to study the
impact of increasing temperatures (due to global warming)
and the retention of BSi in terrestrial freshwater systems
due to damming. The scenarios modeled, while focused on
potential changes to the silica cycle in the near future, give
some insight into the extent to which additional quantities
of silicon from weathering can be transmitted from land to
sea. In the model, increased temperatures, which resulted
in higher weathering rates, led to consequently increased
fluxes of reactive silicon toward the ocean in rivers. In the
absence of increased damming, concentrations of DSi
significantly increased in the coastal zone (although this
may have been due not to the additional silicon per se, but
to the higher dissolution rate of BSi at higher temperatures,
decreasing the retention of BSi in estuaries). In the model,
this additional silicon did not result in an increase in DSi
concentrations in the open ocean, although again this was
due to the increase in temperature which, in the model, led
to increased rates of production of BSi. When included,
the projected increase in river damming diminished silicon
fluxes to estuaries and the coastal zone even in the face of
elevated weathering rates. It would be interesting to use
such a model to explore the consequences of increasing
weathering fluxes, independent of changes in temperature
and subjected to various different damming scenarios, to
see to what extent a sustained input of double the
weathering flux of silicon could be transmitted to the coastal
zone and open ocean.
[96] It is highly probable, however, that increasing the
DSi flux in rivers may shift the ecological balance in rivers,
lakes, and coastal systems back toward the “natural” order
that has been disrupted by damming and agricultural runoff.
The 1960s through 1980s saw an explosive growth in dam
building [Rosenberg et al., 2000], and now about 30% of
the global sediment discharge is retained behind dams rather
than being transported downstream [Vörösmarty and
Sahagian, 2000]. The trapping of amorphous (including
biogenic) silica, which is easily soluble, deprives downstream
areas of a significant portion of their DSi supply [Humborg
et al., 1997, 2000; Ittekkot et al., 2000]. As a result, silicon
fluxes to the ocean from rivers have decreased over the last
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HARTMANN ET AL.: ENHANCED WEATHERING
century. At the same time, nitrate and phosphate fluxes to
the coastal ocean have more than doubled due to runoff
from agricultureor wastewater treatment plants [Meybeck,
1998]. By releasing diatoms in the coastal ocean from nitrate and/or phosphate limitation, the total amount of BSi
production in coastal waters has been increased, further
reducing DSi concentrations in the coastal ocean that is
already being starved of silicon inputs from rivers. With
lack of additional silicon input, the net result has been a shift
of large freshwater systems (like the Great Lakes) and some
coastal areas and seas (like the Baltic Sea and the Mississippi River plume) out of nitrogen or phosphorus limitation
and into silicon limitation [Conley et al., 1993; Nelson and
Dortch, 1996; Turner and Rabalais, 1994] and away from
diatoms as the dominant primary producers toward groups like
dinoflagellates, which are more likely to be toxic and/or
prone to fall into the “harmful algal bloom” (HAB) category.
It would be reasonable to expect that significant extra input
of DSi to lakes, rivers, and the coastal ocean would reverse
the decade-long trend away from diatoms in these areas.
Whether Si release associated with Enhanced Weathering
would avoid dams and reach the oceans depends on the
location where minerals are applied.
[97] If significant inputs of DSi into the coastal ocean
and adjacent seas promote the growth of diatoms, will an
increase in the pumping of carbon out of the surface ocean
also occur? Our understanding of the myriad interacting
processes and factors which control the production and
destruction of rapidly sinking particles in the ocean is not
yet at the point where we can make definitive predictions,
especially for the coastal zone which would be the most
direct recipient of the additional DSi. The answer will, both
regionally and globally, depend on several factors, including whether diatom growth is silicon limited (and thus stimulated by additional inputs of DSi). Also likely to play a key
role is whether the extra diatom production occurs fairly
continuously or in pulses (blooms), which stand a greater
chance of forming and exporting large, rapidly sinking
particles. And lastly, the extent to which additional dissolved
silicon will result in enhanced particulate organic carbon
(POC) flux out of the surface ocean will depend on whether
the local food web structure favors export (e.g., in the form
of appendicularian houses and salp fecal pellets) versus
retention and recycling of POC in the upper water column.
[98] There is some evidence from the open ocean that, when
diatoms dominate primary productivity in the ocean, they enhance the flux of POC out of the euphotic zone and into the
deep ocean. This can be seen in a comparison of POC fluxes
at the Hawaiian Ocean Time series (HOT) station ALOHA
in the oligotrophic subtropical Pacific central gyre and at the
K2 site in the northwest Pacific subarctic gyre [Buesseler
et al., 2007]. At the K2 site, which was dominated by diatoms, primary production was more than twice as much than
at station ALOHA and a slightly greater proportion of this
primary production was exported through the base of the
euphotic zone (16% versus 12%). In addition, 51% of this
exported POC was transferred through 500 m depth at K2
versus 20% at station ALOHA. Similarly high export efficiencies (25%–40%) have been observed between 100 and
750 m depth related to a diatom bloom in the North Atlantic
[Martin et al., 2011]. Although differences in seasonality
and food web structure between the higher and lower
exporting sites may contribute to these differences in the
strength and efficiency of the biological pump, at face value
they suggest that diatom-dominated systems result in
enhanced export of POC out of the surface ocean. Another
study, based on a greater number of sites and more deeply
deployed sediment traps, has noted that the silica dominated
portion of the North Pacific (e.g., sites like K2) transports,
on average, 214 mmol C m!2 a!1 as POC to depths of
1 km, while calcium carbonate dominated portions of the
North Pacific (i.e., sites more comparable to station
ALOHA) export on average only 39 mmol C m!2 a!1
[Honjo et al., 2008]. Studies incorporating plankton
functional types with global circulation models suggest that
diatoms are responsible for nearly the majority of POC export
in the ocean [Jin et al., 2006].
[99] Although the above studies have all focused on the
open ocean, diatoms are also often ecologically dominant
and key contributors to particle flux in coastal zones and
river plumes. It is estimated that, despite their relatively
small area compared to the rest of the ocean, coastal zones
comprise about 50% of both the production and sedimentary
burial of BSi in the ocean [DeMaster, 2002; Shipe and
TABLE 6. Summary of Potential Side Effects of Enhanced Weathering (Considering Local Conditions)
Possible beneficial side effects
• Increasing pH of ocean waters, counteracting CO2-induced acidification
• Supply of Si to coastal oceans
○ May counteract Si limitation and decrease harmful algal blooms
○ May act as “ocean fertilization” to enhance sequestration of atmospheric CO2 through the organic carbon biological pump
• Supply of Si and other nutrients (principally Fe, Mn, P, K) to terrestrial ecosystems
○ May increase terrestrial productivity and lead to greater sequestration of CO2 in terrestrial biomass
○ May increase crop production
○ May provide additional income for farmers through CO2 certificate trading
Possible problematic side effects
• Change in pH of soils and surface waters (streams, rivers, lakes), affecting terrestrial and aquatic ecosystems
• Change in Si concentration of surface waters, affecting ecosystems via altered nutrient ratios
• Release of trace metals associated with target minerals (particularly Ni, Cr in case of olivine application)
• Generation of dust
• Socioeconomic and sociopolitical consequences for agricultural communities of a new, large-scale industrial and financial enterprise.
• Environmental costs of an up to three orders of magnitude increase in olivine mining, globally
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TABLE 7. Key Target Research Areas for Progress in the Science Behind Enhanced Weathering
Better understanding of representative dissolution rates and their controls (see Table 2 in section 3)
Assessing effect on alkalinity and cation supply on processes controlling carbonate precipitation in coastal and oceanic water bodies
Identification of key mineral resources and their distribution
• Optimizing techniques for grinding
• Understanding main potential side effects (see Table 6)
• Identifying and quantifying key large-scale feedbacks
○ influence of changing nutrient status on plant-weathering systems
○ changes in productivity of agricultural and terrestrial ecosystems, and effects on the hydrological cycle
○ Si and alkalinity fluxes to the oceans and their effects (e.g., change in nutrient elemental ratios like N/Si or P/Si and altered proton fluxes (pH))
• Quantifying effects of enhanced alkalinity fluxes on the carbonate system in the coastal and open oceans
• Developing overall dose-response relationship quantifying the net “carbon consumption efficiency” of different scenarios of mineral application
• Developing techniques for being able to monitor mineral dissolution over large spatial scales accurately
• Developing effective monitoring strategies for local to regional alterations of biogeochemical fluxes, as part of building institutional structures for sustainable
global management of material fluxes
•
•
•
Brzezinski, 2001; Treguer and De La Rocha, 2013]. As continental shelves and slopes are also host to roughly 50% of
the POC flux to the seabed [Dunne et al., 2007], this implies
a potentially strong link between diatoms and the biological
carbon pump in coastal regions. That riverborne nutrients
may stimulate phytoplankton growth in river plumes, not only
in coastal regions adjacent to river mouths, but further at sea as
well, can be seen in the elevated concentrations of BSi and
significant contribution of diatoms to primary production in
these plumes [e.g., Shipe et al., 2006].
[100] These studies all illustrate cases where more DSi
promotes more diatom growth and greater capacity and
efficiency to the export of POC to the deep sea (i.e., away from
the atmosphere). In contrast, there is the entirety of the Southern
Ocean which clearly demonstrates that a high availability of DSi
in surface waters need not necessarily result in a high flux of POC
to depth. Concentrations of DSi in Southern Ocean surface
waters are remarkably high (up to 75 mM) due, in part, to the
upwelling of subsurface waters with significantly high DSi concentrations. This excess of DSi, in conjunction with other environmental parameters, does result in a phytoplankton community
largely dominated by diatoms. However, due to phytoplankton
growth limitation by a combination of the low availability
of trace metals like iron, the low availability of light related to
the extremely deep surface mixed layers, the low temperatures,
and the high grazing pressure relative to growth rates,
overall primary production is low at 5 mol C m!2 a!1
[Honjo et al., 2008] compared to the global ocean average
of 12 mol C m!2 a!1 [Field et al., 1998]. Roughly 1% of
this net primary production in the Southern Ocean is exported
to a depth of 2 km, for a flux of 69 mmol C m!2 a!1, a value
that is slightly more than half the global mean value of
120 mmol C m!2 a!1 [Honjo et al., 2008], but at the same
time indicative of a relatively efficient biological pump.
[101] There has also been discussion of open ocean distribution and dissolution of Si-bearing minerals as a geoengineering
strategy (see section 2.4). In terms of adding DSi to the ocean,
this approach would potentially overcome the bottleneck represented by river damming [Laruelle et al., 2009]. Modeling
results [Köhler et al., 2013] with a complex ecosystem model
embedded in a state-of-the-art ocean general circulation model
suggest that addition and dissolution of silicate minerals in the
surface ocean might change the phytoplankton species
composition in the ocean toward diatoms. This study suggests
that open ocean dissolution of olivine is de facto an ocean fertilization, which might also potentially have side
effects typically associated with them, e.g., increase in anoxic
conditions in intermediate water depths [Lampitt et al., 2008].
7.
CONCLUSIONS
[102] The rapidly rising concentrations of atmospheric
CO2 are projected to significantly alter Earth’s climate in a
way that could be detrimental to human society and other
sensitive ecosystems. At the same time, rising CO2 is
acidifying the oceans, causing harm to calcifying organisms,
and thereby disrupting marine food webs. Herein we have
critically examined the geoengineering method of
Enhanced Weathering that has been proposed as a means
of removing CO2 from the atmosphere. We have attempted
to address the practical issues and feasibility of the
technique, its potential ecological impacts (positive and
negative), and the infrastructure and management structures
needed to both carry it out and monitor its effects.
[103] It is worth noting that, on a relatively small scale,
techniques akin to Enhanced Weathering have been in use
for perhaps millennia in the form of applying limestone or
siliceous rock powder to condition or fertilize agricultural
fields to improve productivity. These activities, together
with preliminary modeling and feasibility assessments,
suggest that Enhanced Weathering is a promising CDR
(carbon dioxide removal) technique and could be deployed
as one of a portfolio of several CDR techniques to avoid or
diminish impending climate change and ocean acidification.
[104] Silicates rich in cations (particularly Mg2+ and Ca2+
which are most concentrated in mafic and ultramafic rocks,
with exception of carbonate rocks) are most appropriate for
Enhanced Weathering. Olivine in particular represents an ideal
silicate mineral. As a result of weathering, CO2 is converted to
bicarbonate and carbonate ions, which increases the alkalinity
and pH of rivers and the ocean (values of which have been
lowered by the increase in atmospheric CO2). Additional
solutes, like Si, P, and K, as well as a suite of trace metals,
are also likely to be released, depending on the applied rock
sources. These have the capacity to act as nutrients for plant
and phytoplankton growth, potentially enhancing terrestrial
and oceanic net primary productivity (including crop yields).
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HARTMANN ET AL.: ENHANCED WEATHERING
This could further reduce the amount of CO2 in the atmosphere
by increasing terrestrial biomass (the amount of carbon held in
plant tissues and soil organic matter). Likewise, the export of
particulate organic carbon to the deep ocean and subsequent
sedimentation could be increased if diatom growth was
stimulated by the input of dissolved silicon to the coastal zone.
However, our ability to predict these effects is far from
definitive and impacts on marine ecosystems could be
significant.
[105] Because purposefully lowering atmospheric concentrations of CO2 would have worldwide impacts (just as our inadvertent increasing of them does), and because the areas where
Enhanced Weathering would be most successful are concentrated in the tropics, a globally relevant management plan would
be essential to applying Enhanced Weathering on the scale necessary to significantly influence atmospheric CO2 concentrations on short time scales (decades to centuries). This would
entail the creation of local and regional management structures
to oversee the implementation of Enhanced Weathering and to
monitor its effects carefully. Significant investment in
agricultural, mining, and transportation infrastructure will be
required, especially in lesser developed regions, to carry out
Enhanced Weathering on a large scale. Likewise, the mass
of rock powder annually requiring transportation from
mines and processing centers to the fields and forests of
application may require a significant increase in freight
capacity. As transportation is still powered by fossil fuels,
substantial transportation distances might reduce the effectiveness of Enhanced Weathering. The most appropriate
deployment of Enhanced Weathering may use and adapt
existing infrastructure from the agricultural industry.
[106] One of the major outcomes of this review is that we do
not currently know enough to be able to predict how much the
fluxes of carbon and nutrients between compartments in the
Earth System (soils, terrestrial biosphere, rivers, estuaries,
ocean, marine biosphere, marine sediments) would change
following geoengineering scale deployment of Enhanced
Weathering. While dissolution rates of many minerals have
been quantified for a wide range of environmental conditions,
it is difficult to extrapolate the laboratory results to the
much more complex and variable natural environment,
specifically if additional material is applied to the natural
systems. Nonetheless, we need to improve our ability to
do this, to understand the efficacy and consequences of
Enhanced Weathering.
[107] The potential negative environmental impact of
Enhanced Weathering is also important to consider and
investigate further (Table 6). Application of rock powder to
the land surface might increase the concentration of airborne
dust in the local environment. The potential risk to human and
animal health may limit the appropriate application sites (away
from human centres or sensitive ecosystems) or the severity of
comminution, depending on the techniques applied. This in
turn will limit the efficacy and effectiveness of Enhanced
Weathering. The mobilization of potentially toxic elements
contained in some silicate rocks may detrimentally effect primary production and/or accumulate in the food chain, both
of which could be harmful to human populations. Therefore
an assessment of usable rocks and their locations is needed.
[108] Studies that bridge the gap between laboratory
results, local field sites and regional/global biogeochemical fluxes are strongly needed before any large scale
Enhanced Weathering schemes could be undertaken
(Table 7). While the consequences of not taking action
against the current extremely rapid rise in CO2 may
become increasingly severe, research into Enhanced
Weathering, as with any geoengineering strategy (or,
more generally, any strategy to manage global biogeochemical cycles), needs to be conducted in a transparent, rigorous manner, involving not just researchers,
industry, and politicians, but the general public throughout the research process.
[109] On a final note, within the next few years to decades,
it may be inevitable that Enhanced Weathering is deployed,
if not for its potential to sequester CO2, then as a means of
bolstering crop growth. As human population numbers
continue to increase exponentially, there is increasing
demand to feed more people every year. The pressure to
do so may eventually result in increased application of
mineral powder as fertilizer and to increase the drought
tolerance and disease resistance of crops [van Straaten,
2002, and references provided above]. If this occurs as
projected, it will significantly change the fluxes of carbon,
silicon, and other biogeochemically active elements
between land and ocean within the next few decades
(although it would not be the first agricultural or industrial
practice to do so). For this reason, it would be useful to be
able to predict the ecological and biogeochemical impacts
of Enhanced Weathering regardless of whether it is ever
carried out for the purpose of CO2 sequestration.
APPENDIX A
A1. Management of Enhanced Weathering at a
Geoengineering Scale
[110] To sequester a significant amount of carbon dioxide
from the atmosphere, an Enhanced Weathering program
would need to process 1 Gt to 10 s of Gt of rock per year. This
would make it one of the largest global industries. Therefore,
as discussed in previous sections, it would require a wellorganized assessment of altered biogeochemical fluxes as well
as broader environmental and other consequences. In this case,
attention is focused on analysing the various benefits, costs
and risks of technologies and pathways, taking into account
multiple criteria of evaluation, including environmental,
agricultural, public health, and financial criteria as well as
the projected magnitude of CO2 sequestration. A comprehensive diagnostic process would develop a governance and
accounting structure that includes local, regional and global
stakeholders and decision makers, including mining corporations and the minerals transport industry, farmers and forest
businesses, and the civil society and local stakeholders such
as citizens in the vicinity of areas where Enhanced Weathering
is employed. Later phases would require an agency organizing
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HARTMANN ET AL.: ENHANCED WEATHERING
Figure A1. A proposed concept for multi-stakeholder assessment and management of Enhanced Weathering, adapted from
Scheffran [2006].
the global review and monitoring in light of data gathered
from the various regions of application.
[111] Stakeholder involvement in decision-making and
management can be described as a multi-step process that
includes the following phases (see Figure A1) [based on
Scheffran, 2006]:
[112] 1. Situation and context analysis: To assist decision
makers with proactive planning and management [c.f. van
Ast and Boot, 2003], various methods from system dynamics
and spatial modelling would be used to study the consequences of the application of Enhanced Weathering in time
and space, at local and global levels. Much of the scientific
analysis reviewed in this article falls into this analysis,
including the CO2-sequestration mechanisms and fluxes in
the respective natural systems. This analysis should also
include consideration of relevant social and economic factors.
[113] 2. Option identification and scenario generation:
In order to provide key information about the various possible options for implementation, computer simulations are
needed to explore selected scenarios and identify the most
relevant possibilities, and their consequences. These models
need to be able to allow comparison of greenhouse gas reductions under different actions. Spatio-temporal analyses can
determine the effectiveness of previous management decisions and provide projections for future management
choices. Modelling the alteration of material fluxes due to
Enhanced Weathering would demand a new generation of
Earth System Models capable of forecasting consequences
at the regional as well as at the global scale, learning from
more advanced modelling approaches for solar radiation
management (Kravitz et al., 2011; Schmidt et al., 2012).
[114] 3. Assessment of consequences and criteria-based
evaluation: A systemic approach is crucial for identifying the
plausible causal chains and consequences which could result
from the use of different Enhanced Weathering techniques in
particular environments, including ecological impacts and
other risks, as well as social effects including domestic protests
and international conflicts. This analysis is based on the plausibility, relevance and uncertainty of events, building on
principles, criteria and performance indicators defined by scientists, decision makers and other stakeholders (see, for
example, the criteria given in the following section). Possible
guardrails and critical limits could include thresholds for soil
chemistry or water pH, or limitations on the addition of specific elements into local aquatic-, soil- or ecosystems-systems.
[115] 4. Decision-making and negotiation: Without
agreement, disputes among interest groups could block
decision-making and problem solving. Integrated Enhanced
Weathering decision-making and risk management (building
on the experience from disaster risk management strategies
[e.g., Hartmann et al., 2006; Levy et al., 2007; Schneider,
2008] can use various tools, including optimal control,
multi-criteria methods, agent-based modeling and experimental games. Conflict resolution, participatory approaches
and mediation can help to balance different interests.
[116] 5. Planning and rules for action: To implement actions on Enhanced Weathering, concrete planning is required,
with specific rules and regulations that need to be followed
and verified. In an environment with high uncertainties and
rapidly changing conditions, planning needs to adapt to the
state of knowledge and capability. For international coordination of regional processes, institutional mechanisms, governance structures and legal approaches are required that involve
stakeholders [c.f. Shidawara, 1999]. One approach is to create
incentives to participate and coordinate, e.g. through a global
certification system and purposeful designed syndication of
the Earth system [Elliott and Hanson, 2003]. Legal instruments can apply at national and multi-national levels to
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HARTMANN ET AL.: ENHANCED WEATHERING
authorize or restrain certain Enhanced Weathering measures [UBA, 2011].
[117] 6. Monitoring and learning: Continuous monitoring of the outcomes of global Enhanced Weathering application is critical for learning and improvement. Key indicators
about anticipated consequences must be monitored, and the
initial proposed environmental indicators need to be continuously re-evaluated. Existing environmental monitoring
programs are restricted by the limited number of monitoring
stations and need to be accompanied by comprehensive
information sources, including land use and socio-economic
data [c.f. Plate, 2002].
[118] The information gathered throughout the monitoring
and learning process can then be an input for further situation and context analysis (Step 1, above). These phases form
a repeated cycle of multi-stakeholder participation, assessment and management (see Figure 1). As a result of learning
and repetition, the assessment and management process can
be made more effective. Connecting activities in this cycle
are the extraction of essential information, the simulation
of scenarios, the data-based validation of results, the evaluation of consequences, the communication of decisions and
outcomes, and the capacity-building of involved actors. In
each of these phases, particular stakeholders can participate
according to their qualification, and various tools can be
used for supporting the respective stages as well as the interaction of stakeholders throughout the process.
A2. Legal Framework and Recent Developments on
Climate Engineering With Relevance for Enhanced
Weathering
[119] Legal instruments to regulate Enhanced Weathering
can apply at different levels. Individual states can promote
a variety of national policies and regulations to authorize
or prohibit certain Enhanced Weathering measures. In accordance with customary international law, states have to
ensure that activities within their own territory do not generate substantial adverse consequences for the environment
beyond their own borders. Substantial adverse effects on
the environment are not permitted in areas such as the high
seas, the Antarctic and outer space. At present, there are no
effective geoengineering technologies, nore binding international regulations. The proposed geoengineering technique
of Enhanced Weathering would also change global biogeochemical cycles, not only climate, and is at a smaller scale
already in practice due the unregulated changes in land use
or agricultural practices. While some treaties cover partial
aspects, they would need to be extended to be effective
[see the reviews in: Proelss and Güssow, 2011; Rickels
et al., 2011; UBA, 2011].
[120] In particular, the UN Framework Convention on
Climate Change (UNFCCC) precludes dangerous anthropogenic
interference with the climate system, which would apply to
dangerous climate change as well dangerous climate engineering. To be more binding, the term “dangerous interference”
would have to be specified. Similarly, the 1978 Convention
on the Prohibition of Military or Any Other Hostile Use of
Environmental Modification Techniques (ENMOD) restrains
the military or other hostile modification of the Earth’s environment, but it does not prohibit the peaceful use of environmental
modification. One example for which already some treaties
could be applied is the proposed use of sulphur aerosols as a
geoengineering technique [Crutzen, 2006]: Since the release
of sulphur aerosols could damage the ozone layer, it could be
contrary to the 1987 Montreal Protocol on Substances that
Deplete the Ozone Layer.
[121] Research into ocean fertilization is basically allowed
by regulations, if it is ensured that hazards for humans and
the environment are avoided. Of particular relevance are
the 1982 United Nations Convention on the Law of the
Sea, the 1972 Convention on the Prevention of Marine
Pollution through Dumping of Wastes and other Matter,
and the corresponding London Protocol. These should
equally apply to Enhanced Weathering, if minerals are to
be spread near or along coastlines, or in the open ocean.
[122] It remains to be decided whether there should be
a standardized, broad regulation on geoengineering, or
whether the respective international conventions should be
supplemented with specific provisions on geoengineering
measures such as Enhanced Weathering, e.g. within the
UNFCCC. Regulatory questions pertain not just to the implementation of geoengineering, but also to research at the
field scale. Political debates are gaining in importance, with
first steps being taken in the US and Great Britain toward defined national research strategies and regulation requirements. The progressive development, including scientific
research, of any geoengineering technology needs clear
political regulation and coordination. This may come from
a number of different directions:
[123] 1. While the UNFCCC parties have not yet taken a
stand on climate engineering, the Parties to the 2008 Convention on Biological Diversity (CBD) formulated a resolution
whose content was virtually identical to that of the 2008 London
Convention and London Protocol [IMO, 1996]. The tenth
CBD on 29th of October 2010 in Nagoya recommended a
moratorium on climate engineering activities. Accordingly,
only scientific studies small in extent and which fulfill four
criteria are granted/permitted. Most of the recent climate
engineering research is allowed (e.g., that based on computer
modeling), but open field experiments are prohibited.
[124] 2. In preparation for its Fifth Assessment Report planned for 2014 - the IPPC has decided to discuss the
meaning of climate engineering as a possible reaction to
climate change and the possible results and risks for nature
and society. The IPCC arranged an expert’s meeting on the
subject in June, 2011 in Lima, Peru [IPCC, 2011].
[125] 3. In a 2010 report for the U.S. House of Representatives, strategies for international coordination were discussed
[CST, 2010]. In this context the Congressional Research
Service started a study on the applicability of existing US laws
and international arrangements on possible geoengineering
tests as well as larger-scale implementation [Bracmort et al.,
2010]. In this context, the Government Accountability Office
compiled an overview on climate engineering research activities of US-federal institutions [GAO, 2010].
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HARTMANN ET AL.: ENHANCED WEATHERING
[126] 4. In cooperation with the initiative of the U.S. House
of Representatives, the Science and Technology Committee of
the British parliament compiled a report for a possible international regulation of climate engineering [STC, 2009]. Because
the technologies and procedures greatly differ between different potential strategies for geoengineering, they concluded that
regulation measures would need to be explicitly adapted to the
specific character of each approach.
[127] 5. Non-governmental organizations are mostly critical about the idea of climate engineering, because they argue
that it manifests the conditions which led to the climate crises
in the first place. For instance, the Action Group on Erosion,
Technology and Concentration [ETC, 2011] pursues a public
anti-GE campaign against “geopiracy”.
[128] 6. On the 29th September 2011, the European parliament considered but dismissed a resolution in which GE
measures on a large scale would be opposed [EU, 2011].
[129] Set within the context of this emerging regulatory
debate, the various scientific-technical, economic, legal and
political aspects of climate engineering are increasingly the
focus of academic attention [c.f. the overview in Kintisch,
2010]. Following the Royal Society report [UK-RoyalSociety, 1999], related issues and questions were discussed
in a number of workshops, conferences and studies:
[130] 1. In 2009, the International Risk Governance
Council (IRGC) and other organizations conducted a workshop in Lisbon. The resulting working paper supports research
on climate engineering and outlines a framework for international regulation, including the definition “of permissible
ranges/permitted areas” of research [Morgan and Ricke,
2009].
[131] 2. In the context of a report by the British House of
Commons, the “Oxford Principles” were formulated, to
address research regulations on climate engineering [Rayner
et al., 2009]. The five principles suggest: (i) regulating
geoengineering as a public good, (ii) letting the public take
part in the decision-making process, (iii) disclosing the
results of research, (iv) having an independent impact
assessment conducted, and (v) only beginning with any
implementation after a governance process is completed.
[132] 3. Corresponding proposals and recommendations
were also made at the Asilomar Conference in 2010, which
pleas for a responsible handling of climate engineering. Voluntary guidelines were discussed, which include cooperative
research that involves the public. An international climate
engineering regulation would be based on five principles,
namely that research in this field: (i) should benefit humanity
and the environment, (ii) be conducted openly and cooperatively, (iii) permit an independent technical assessment,
(iv) define the limits of accountability in the context of new
governance and monitoring mechanisms, and (v) involve
the public during the complete process.
[133] 4. In the US, the “Bipartisan Policy Center” [BPC,
2011] formed a task force that presented a report in October
2011 on the possible effectiveness, feasibility and the consequences of climate engineering technologies, as well as
options for risk management. Leading experts from different
areas are developing recommendations for the US
government with regard to the research in geoengineering
and supervision strategies.
[134] 5. In the UK, a 2010 joint initiative of the British
Engineering and Physical Sciences Research Council
(NERC) and the Natural Environment Research Council
(NERC) was launched to perform research and an impact
analysis of geoengineering measures [EPSRC/NERC, 2010].
Potential means, efficiencies and consequences of procedures for Solar Radiation Management are being examined,
while maintaining the involvement of the public. NERC
supports other public discourse activities about
geoengineering to explore public assessments and to communicate better future research possibilities [NERC, 2011].
However, efforts focused on Enhanced Weathering have
yet to emerge.
[135] 6. Several studies and conferences in Germany in
2011 show a growing interest of scientists and politicians
in the subject of climate engineering, for example the
study of the German Federal Environmental Agency
[UBA, 2011] and a scoping study by the Kiel Earth Institute
in October 2011, focused on assessing the application and
the regulation of climate engineering [Rickels et al., 2011].
[136] These various discussions provide valuable input
toward formulating strategies for research and possible implementation of any geoengineering scheme. The majority of
geoengineering governance discussions have focused on solar
radiation management, with particular emphasis on preventing
adverse environmental effects. As the effectiveness of carbon
dioxide removal technologies is dependent on the practices
used for deployment (material processing, transport, application), and the size of the industry is several orders of magnitude larger than SRM, the regulatory framework is likely to
be substantially different. Continued efforts to understand the
scope for Enhanced Weathering will need to carefully adjust
to this changing regulatory and ethical environment, in order
to legitimize the results from the ongoing research.
½137) ACKNOWLEDGMENTS. This review was stimulated by the workshop of the Institute for Advanced Sustainability
Studies on the recarbonization of the biosphere, which resulted in a
publication about the recarbonization of the biosphere [Lal et al.,
2012]. Jens Hartmann was supported through the German Science
Foundation (DFG-project HA 4472/6-1) and the Cluster of
Excellence “CliSAP” (EXC177), Universität Hamburg. Phil
Renforth acknowledges funding from the Oxford Martin School
(University of Oxford) and A. Joshua West from the U.S. National
Science Foundation (EAR). Hans Dürr received funding from
NSERC (Canada Excellence Research Chair in Ecohydrology –
Philippe van Cappellen). Helpful comments were provided by
Mark Torres. The Editor on this paper was Eelco Rohling. He
thanks Yves Godderis and two anonymous reviewers for their
review assistance on this manuscript.
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