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The Science of Geoengineering
Ken Caldeira,1 Govindasamy Bala,2 and Long Cao3
1
Department of Global Ecology, Carnegie Institution for Science, Stanford, California 94305;
email: [email protected]
2
Center for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore 560 012,
India
3
Department of Earth Sciences, Zhejiang University, Hangzhou, Zhejiang 310027, China
Annu. Rev. Earth Planet. Sci. 2013. 41:231–56
Keywords
The Annual Review of Earth and Planetary Sciences is
online at earth.annualreviews.org
carbon dioxide removal, solar radiation management, climate,
environment, energy
This article’s doi:
10.1146/annurev-earth-042711-105548
c 2013 by Annual Reviews.
Copyright All rights reserved
Abstract
Carbon dioxide emissions from the burning of coal, oil, and gas are increasing atmospheric carbon dioxide concentrations. These increased concentrations cause additional energy to be retained in Earth’s climate system, thus
increasing Earth’s temperature. Various methods have been proposed to
prevent this temperature increase either by reflecting to space sunlight that
would otherwise warm Earth or by removing carbon dioxide from the atmosphere. Such intentional alteration of planetary-scale processes has been
termed geoengineering. The first category of geoengineering method, solar geoengineering (also known as solar radiation management, or SRM),
raises novel global-scale governance and environmental issues. Some SRM
approaches are thought to be low in cost, so the scale of SRM deployment
will likely depend primarily on considerations of risk. The second category of
geoengineering method, carbon dioxide removal (CDR), raises issues related
primarily to scale, cost, effectiveness, and local environmental consequences.
The scale of CDR deployment will likely depend primarily on cost.
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1. INTRODUCTION
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The term geoengineering as applied in its current context was introduced into the scientific
literature by Victor Marchetti in the title of his classic paper describing deep-sea disposal of carbon
dioxide (CO2 ) (Marchetti 1977). This term has come to refer to large-scale efforts to diminish
climate change resulting from greenhouse gases that have already been released to the atmosphere.
Such efforts include both solar geoengineering (also known as solar radiation management, or
SRM) and carbon dioxide removal (CDR) (R. Soc. 2009). SRM aims to diminish the amount of
climate change produced by high greenhouse gas concentrations, whereas CDR involves removing
CO2 and other greenhouse gases from the atmosphere.
These geoengineering approaches may complement other strategies to diminish risks posed
by climate change (Figure 1), including conservation (reducing demand for goods and services),
efficiency (producing goods and services with few energy inputs), low- or zero-carbon emission
energy technologies (producing that energy with sources that emit less CO2 ), and adaptation
(increasing resilience to effects of climate change that do occur). These various options are not
mutually exclusive, although decisions must be made regarding how much effort should be put
Desire for
improved
well-being
Impacts on
humans and
ecosystems
Conservation
Adaptation
Consumption
of goods and
services
Efficiency
Climate
change
Solar
geoengineering
Low-carbon
emission energy
technologies
Consumption
of energy
CO2
removal
CO2 in
atmosphere
CO2
emissions
Figure 1
Most geoengineering approaches fall into one of two categories: carbon dioxide removal or solar
geoengineering. These approaches can be viewed as part of a portfolio of strategies for diminishing climate
risk and damage. Carbon dioxide removal attempts to break the link between CO2 emissions and
accumulation of CO2 in the atmosphere. Solar geoengineering (also known as solar radiation management)
attempts to break the link between accumulation of CO2 in the atmosphere and the amount of climate
change that can result.
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into researching, developing, and implementing each approach. Such decisions can be improved
by careful scientific and technical analysis.
Geoengineering approaches have been the subject of previous reviews, including chapters in
1992 and 2011 US National Academy reports (Comm. Am. Clim. Choices Natl. Res. Counc. 2011,
Natl. Acad. Sci. 1992). Notably, David Keith contributed a review to a related Annual Reviews
journal more than a decade ago (Keith 2000). The UK Royal Society assembled a panel in 2009
that produced a good summary, including references to issues involving international governance
and ethics (R. Soc. 2009).
Proposals to consider the intentional alteration of climate have raised concerns related to politics, policy, governance, and ethics (Blackstock & Long 2010, Jamieson 1996). These discussions
often cite “the importance of democratic decision-making, the prohibition against irreversible
environmental changes, and the significance of learning to live with nature” ( Jamieson 1996,
p. 329). Here we focus on the physical science of geoengineering, dividing our discussion into two
major classes of activities: reflecting sunlight away from Earth (SRM/solar geoengineering) and
removing greenhouse gases from the atmosphere (CDR).
2. SOLAR GEOENGINEERING/SOLAR RADIATION MANAGEMENT
2.1. Overview
Increases in atmospheric CO2 and other greenhouse gases exert a radiative forcing on the climate system by making it more difficult for heat to escape to space. SRM/solar geoengineering
approaches aim to offset this warming influence by reducing the amount of sunlight absorbed by
Earth (R. Soc. 2009) (Table 1). This can be achieved by reflecting some sunlight away from Earth
(Figure 2).
On average, Earth absorbs approximately 240 W of sunlight per square meter. A doubling of
atmospheric CO2 causes a radiative forcing of ∼4 W m−2 . Therefore, to offset the 4 W m−2 forcing
requires reflection of approximately 4/240, or ∼1.7%, of incoming solar radiation (Caldeira &
Wood 2008, Govindasamy & Caldeira 2000, Govindasamy et al. 2002, Lunt et al. 2008). Precise
numbers depend on uncertain climate system feedbacks and differences in climate system response
to different types of radiative forcing (Hansen et al. 2005).
Some computer model studies simulated the effect of solar geoengineering approaches by
reducing solar intensity in the models (Govindasamy & Caldeira 2000, Govindasamy et al. 2003)
or by imposing specified aerosol distributions (Ban-Weiss & Caldeira 2010) or optical depths
(Ricke et al. 2010). More complete models considered processes affecting the size and transport
of stratospheric aerosols (Rasch et al. 2008a, Robock et al. 2008).
Model results indicate that measures to reflect incoming sunlight away from Earth could
potentially start cooling Earth within months and achieve several Kelvin of cooling within a decade
(Matthews & Caldeira 2007) (Figure 3). Such approaches may be able to prevent the collapse of
the Greenland ice sheet (Irvine et al. 2009) or other undesirable consequences of climate change.
However, the sudden failure of a solar geoengineering scheme could subject Earth to extremely
rapid warming—at a rate many times that of the current warming (Matthews & Caldeira 2007,
Robock et al. 2008) (Figure 3b). Whereas the nongeoengineered world warms relatively slowly
with relatively slow increases in atmospheric CO2 , in the case of a catastrophic failure of a solar
geoengineering system, Earth would experience a large climate forcing at the time of system failure
and would warm rapidly for several decades. Furthermore, compared with a climate that has a
higher temperature and a high CO2 level, much more carbon would be stored in the oceans and
land in a climate with low solar irradiance, low temperature, and high CO2 . In the case of a halt or
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Table 1 Summary of solar geoengineering proposals
Relative risk
to
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Solar
geoengineering
method
Maximum
cooling
potentiala
Attainable
speed of
deploymentb
Relative cost
per unit effectc
environment
per unit
effectd
Selected references
Space-based
schemes
High
Slow
High
Low
Angel 2006, Early 1989
Stratospheric
aerosols
High
Fast
Low
Medium
Budyko 1982; Rasch et al.
2008b, 2009; Robock et al.
2008
Whitening of clouds
Medium
Fast
Low
High
Latham et al. 2008, Rasch
et al. 2009
Whitening of the
ocean
Medium
???
???
???
Pres. Sci. Advis. Comm.
Environ. Pollut. Panel
1965, Seitz 2011
Plant reflectivity
Low
Medium
Medium
High
Doughty et al. 2011,
Ridgwell et al. 2009
Whitening of built
structures
Low
Medium
Medium
High
Akbari et al. 2009, Menon
et al. 2010
Adapted from the Royal Society Report on geoengineering (R. Soc. 2009) and citations in text as noted.
a
High means able to offset warming from all future fossil-fuel emissions; medium means able to offset at least 10% of emissions projected for this century;
low means able to offset less than 10% of cumulative century-scale emissions.
b
Fast means deployable within a decade; medium means that deployment would take decades.
c
High means costlier than conventional mitigation approaches; medium means less costly than conventional approaches but costly enough for economics
to be a significant issue; low means that direct costs are unlikely to be a significant factor in the decision whether or not to deploy this option.
d
Approaches that produce patchy influences on the climate system are deemed riskier than approaches capable of more uniformly distributed influences.
failure of the solar geoengineering approaches, a sudden warming would cause the carbon stored
in the land and ocean reservoir to be released into the atmosphere, triggering further warming
(Matthews & Caldeira 2007).
Models indicate that reflection of additional sunlight away from Earth would cause a high-CO2
climate to become more similar to a low-CO2 climate (Ban-Weiss & Caldeira 2010). However, it
may not be possible to simultaneously restore all climatic fields (e.g., temperature and precipitation)
close to the natural state (Figure 4). In the absence of surface warming, increased atmospheric
CO2 reduces both evaporation and precipitation by stabilizing the atmosphere (Andrews et al.
2009, Bala et al. 2008). Therefore, solar geoengineering approaches, if implemented to offset the
full amount of global-mean surface warming, would cause a reduction in global-mean precipitation
due to the precipitation-suppression property of CO2 forcing (Bala et al. 2008, Caldeira & Wood
2008, Lunt et al. 2008). Alternatively, if solar geoengineering were implemented to counteract
changes in global-mean precipitation, there would be some residual surface warming.
Increasing atmospheric CO2 content also affects the climate system via its effect on plant
stomata (Sellers et al. 1996). This effect, referred to as CO2 -physiological forcing, increases
the CO2 -radiative warming by approximately 10% at the global scale and can account for up
to 30% of the total warming at regional scales (Boucher et al. 2009, Cao et al. 2010). This
CO2 -physiological forcing reduces evapotranspiration and thus precipitation (Betts et al. 2007,
Cao et al. 2010). Reflection of sunlight offsets the CO2 -induced warming but cannot reverse
effects of CO2 fertilization of plants (Govindasamy et al. 2002). Jones et al. (2011) suggested that
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a
b
c
d
e
f
Figure 2
Solar geoengineering/solar radiation management approaches work by reflecting to space sunlight that would otherwise have been
absorbed. Illustrated methods are (a) using satellites in space, (b) injecting aerosols into the stratosphere, (c) brightening marine clouds,
(d ) making the ocean surface more reflective, (e) growing more reflective plants, and ( f ) whitening roofs and other built structures.
stratospheric aerosol injection could have consequences for regional net primary productivity
owing to changes in regional precipitation. One key difference between the spaced-based
approach and the stratospheric aerosol–based approach is that the scattering effect of sulfate
aerosols increases the amount of diffuse solar radiation that reaches the land surface in spite of
the reduction in total solar radiation. It is thought that increased diffuse solar radiation tends
to increase plant photosynthesis and therefore the land carbon sink (Knohl & Baldocchi 2008,
Mercado et al. 2009), but this effect is not universally accepted (Angert et al. 2004) and has not
been considered in global modeling studies of stratospheric aerosol geoengineering.
The moderation of global-mean climate does not necessarily lead to a uniform moderation of
climate in all regions (Ban-Weiss & Caldeira 2010, Jones et al. 2011, Ricke et al. 2010). Studies have
shown that solar geoengineering could diminish the amount of temperature change in all regions
but would increase the magnitude of precipitation changes in some regions (Hegerl & Solomon
2009). Ban-Weiss & Caldeira (2010) found that having a stratospheric aerosol loading that is
weighted toward polar regions results in a temperature distribution more similar to the low-CO2
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17
a
16
Surface air temperature (°C)
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15
r
atu
per
m
e
T
e
h
w it
sin
bu
al e
usu
s
-a
ess
o
ssi
mi
ns
2075
14
2050
2000
2025
13
19
b
18
17
16
15
14
13
2000
2020
2040
2060
2080
2100
Model year
Figure 3
Model-simulated global and annual mean surface air temperature (red lines) for a business-as-usual CO2
emission scenario (Matthews & Caldeira 2007). (a) Cases showing cooling when solar intensity is reduced in
years 2000, 2025, 2050, and 2075. (b) Cases in which solar intensity is decreased to compensate for increasing
CO2 content and then returned rapidly to the full value. Simulations with doubled climate sensitivity are
plotted as dashed lines. Abrupt deployment of a solar geoengineering scheme can produce a rapid cooling,
and an abrupt failure of a solar geoengineering scheme could cause a rapid rebound warming. Reproduced
from Matthews & Caldeira (2007) with permission.
climate than that yielded by a globally uniform aerosol loading. However, this polar weighting of
stratospheric sulfate tended to degrade the degree to which the hydrological cycle is restored.
Robock et al. (2008) found that both tropical and Arctic SO2 injection disrupt the Asian and
African summer monsoons. Lunt et al. (2008) reported that compared with the natural climate,
a uniform reduction in solar radiation leads to reduced El Niño–related variability and increased
North Atlantic overturning. Braesicke et al. (2011) found that a large reduction in solar radiation
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2 × CO2
2 × CO2
with
1.84%
solar
reduction
Precipitation change (m year–1)
Temperature change (°C)
–1
0
1
2
3
4
5
6
7
–1.3
–0.9
–0.5
–0.1 0.1
0.5
0.9
1.3
Figure 4
Model-simulated (Caldeira & Wood 2008) annual mean changes in temperature (left panels) and precipitation (right panels) for the case
of 2 × CO2 (top panels) and that of 2 × CO2 with a reduction in global-mean solar insolation of 1.84% (bottom panels). The changes are
calculated as the departure from the simulation with 1 × CO2 . The idealized solar geoengineering scheme largely offsets most of the
CO2 -induced temperature and precipitation changes but leaves some residual warming at the poles and leads to an overall decrease in
precipitation. Reproduced from Caldeira & Wood (2008) with permission.
causes changes in El Niño and related climate teleconnection patterns. Moore et al. (2010) calculated that an aerosol injection delivering a constant 4 W m−2 in radiative forcing could delay
sea-level rise by 40–80 years.
Solar geoengineering approaches do not directly alter atmospheric CO2 content and therefore do not mitigate CO2 -induced ocean acidification (Matthews et al. 2009). In addition, solar
geoengineering approaches do not prevent CO2 -induced changes in terrestrial carbon cycle, including biomass and net primary production (e.g., Govindasamy et al. 2002). Furthermore, solar
geoengineering approaches would cool in the stratosphere (Bala et al. 2010, Govindasamy &
Caldeira 2000, Govindasamy et al. 2003) and could aggravate changes to stratospheric chemistry
and ozone depletion (Tilmes et al. 2008, 2009).
2.2. Solar Geoengineering Approaches
All solar geoengineering approaches aim to influence climate by reducing the amount of sunlight
absorbed by Earth. This sunlight could potentially be deflected away from the Earth either in
space, in the stratosphere, in the lower atmosphere, or at Earth’s surface (Figure 2).
2.2.1. Space-based approaches. Space-based solar geoengineering approaches aim to reduce the
amount of incoming solar radiation reaching Earth. Numerous techniques have been proposed to
achieve this goal. Early (1989) proposed constructing a thin glass shield from lunar materials and
placing it near the first Lagrange point of the Earth-Sun system. The first Lagrange point, L1, is
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a neutrally stable point on the axis between Earth and the Sun where the forces pulling an object
toward the Sun are exactly balanced by the forces pulling an object toward Earth.
Angel (2006) proposed placing a sunshade consisting of multiple “flyers” at the L1 Lagrange
point. Other proposals include placing mirrors in orbit around Earth (Natl. Acad. Sci. 1992) and
placing rings around Earth that are composed of particles or constellations of spacecraft (Pearson
et al. 2006).
To offset just for the annual increase in radiative forcing from anthropogenic CO2 emissions,
more than 10,000 km2 of reflection area would need to be deployed each year—more than one
square kilometer each hour (Govindasamy & Caldeira 2000). Such rates mean that large-scale
deployment is likely to be a long process and to remain infeasible for many decades (McInnes 2010).
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2.2.2. Stratospheric aerosol–based approaches. The injection of sulfate aerosols into the
lower stratosphere would cool Earth by scattering the solar radiation back to space. Studies of
climate geoengineering using sulfate aerosols have concluded that stratospheric aerosols could
reduce global-mean temperatures, but concerns remain regarding many issues, including effects
on regional climate, precipitation, and ozone loss (Rasch et al. 2008b).
Insight into the potential for injecting sulfate aerosols into the stratosphere to cool Earth
has been demonstrated from the cooling observed after large volcanic eruptions such as Mount
Pinatubo in 1991 (Crutzen 2006, Soden et al. 2002), although the volcanic eruption is an imperfect
analog of sulfate aerosol injection. The Mount Pinatubo eruption placed enough material in the
atmosphere to offset approximately 4 W m−2 of radiative forcing, i.e., approximately enough
material to offset the global-mean radiative forcing from a doubling of atmospheric CO2 content.
Therefore, the 1991 Mount Pinatubo eruption represents a stratospheric aerosol injection of the
same order of magnitude as a full-scale solar geoengineering deployment. However, the solar
geoengineering deployment would involve replenishment of these aerosols as they were removed
from the atmosphere by natural processes. The aerosols injected into the stratosphere by Mount
Pinatubo settled and were transported out of the stratosphere on the timescale of approximately
one year. Earth’s surface cooled by ∼0.5 K within the year following the eruption. Had the aerosol
layer been maintained in the stratosphere, it would have cooled Earth’s surface by perhaps 3 K. In
addition, following the volcanic eruption of Mount Pinatubo, investigators observed a substantial
decrease in precipitation over land and a record decrease in runoff (Trenberth & Dai 2007)
(Figure 5).
A range of substances, including black carbon (Ban-Weiss et al. 2012, Kravitz et al. 2012)
and special engineered particles (Keith 2010, Teller et al. 1997), could potentially be placed high
in the atmosphere to reflect solar radiation away from Earth, but most studies have focused on
sulfate particles. Various techniques have been proposed for delivering the sulfate aerosol and/or its
precursor gases (H2 S and SO2 ), including high-altitude balloons, artillery guns, high-level aircraft,
tall towers, and space elevators (Crutzen 2006, Rasch et al. 2008b, Robock et al. 2009, Teller et al.
1997). The associated technical implementation, benefit, risk, and cost of each delivering system
need to be fully evaluated (Robock et al. 2009). The amount of warming that would be offset by
a given injection of aerosol precursors is difficult to predict precisely because it can be affected by
nonlinear feedbacks involving the delivery mechanisms, particle size and distribution, microphysics
of aerosol formation and growth, and climate change. Smaller particles (radius of ∼0.1 μm) are
more effective at scattering incoming energy and have no impact on longwave radiation, whereas
larger particles such as those following volcanic eruptions are less effective at scattering shortwave
radiation and absorb and emit in the longwave spectrum (Stenchikov et al. 1998). Rasch and
colleagues (Rasch et al. 2008a) found that approximately 1.5 Tg S year−1 of sulfate aerosols would
balance a doubling of CO2 if the particles were small, whereas perhaps double that amount may
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3.30
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1.20
3.20
1.16
3.10
1.12
3.00
Land precipitation (Sv)
Continental discharge (Sv)
1.24
1.08
1986
1990
Year
1994
1998
Figure 5
Time series of estimated annual continental freshwater discharge into the oceans (1 Sv = 106 m3 s−1 )
(Trenberth & Dai 2007). Also shown is observed precipitation integrated over global land areas. The period
clearly influenced by the Mount Pinatubo eruption is indicated by gray shading. Reproduced from
Trenberth & Dai (2007) with permission.
be needed if the particles were to reach the size seen following volcanic eruptions. There is still
uncertainty regarding the size distribution and lifetime of stratospheric sulfate aerosols; thus,
it is possible that considerably more sulfate particles would be needed (Heckendorn et al. 2009,
Niemeier et al. 2011). Induced changes in stratosphere-troposphere-exchange processes can affect
the amount of aerosol precursors that would need to be injected to counteract CO2 warming (Rasch
et al. 2008b). The altitude, location, and mode of injection into the stratosphere also influence
efficacy, and this is an area of active investigation (Heckendorn et al. 2009, Niemeier et al. 2011,
Robock et al. 2008).
Sulfate aerosol geoengineering can affect stratosphere chemistry, including ozone concentrations. An injection of particles into the stratosphere has the potential to provide surfaces that lead
to efficient chlorine activation, which could approximately double the ozone-destroying potential
of chlorofluorocarbon-derived chlorine in polar regions (Tilmes et al. 2008, 2009). Tilmes et al.
(2008, 2009) showed that an injection of stratospheric sulfate aerosols large enough to offset the
2 × CO2 surface warming would cause a 30- to 70-year delay in the expected recovery of the
Antarctic ozone hole. Heckendorn et al. (2009) found that sulfate aerosol geoengineering accelerates the hydroxyl-catalyzed ozone destruction cycles and would cause some ozone depletion.
2.2.3. Marine cloud brightening. The basic principle behind the idea of marine cloud brightening is to increase the reflectivity of low-level marine stratocumulus clouds by increasing the
number of cloud condensation nuclei (CCN). More CCN increase the number of cloud droplets
while reducing the droplet size, thus increasing the total droplet surface area of the cloud and the
cloud reflectivity (Twomey 1977). Extensive areas of marine stratocumulus clouds off the west
coasts of North and South America and the west coast of Africa have been identified as regions
where marine cloud brightening approaches would be effective (Latham et al. 2008). The most
studied method of increasing CCN is spraying a fine seawater mist into the remote marine atmospheric boundary layer by conventional ocean-going vessels, by aircraft, or by specially designed
unmanned remotely controlled sea craft (Salter et al. 2008). Calculations show that the change in
cloud albedo is sensitive to the droplet number concentration and that marine cloud brightening
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would be most effective in clean-air regions and least effective in regions with high background
aerosol concentrations (Bower et al. 2006). Although in many climate modeling studies the
addition of CCN is implicitly assumed to increase cloud albedo, predicting how changes in cloud
microphysical properties would affect cloud planetary albedo is difficult. Reduced droplet size may
suppress precipitation and further increase cloud cover (Albrecht 1989). In contrast, in some situations the aerosol indirect effect could reduce cloud albedo (Ackerman et al. 2003, Wood 2007).
The nonlinear dynamic response of cloud physics to increasing aerosols led Latham et al. (2008)
to argue that “it is unjustifiably simplistic to assume that adding CCN to the clouds will always
brighten them” (p. 3983). It may be possible to increase CCN by fertilizing the Southern Ocean
with iron to stimulate phytoplankton growth and increase the phytoplankton emission of dimethyl
sulfide (DMS), which oxidizes in the atmosphere to create sulfate aerosols (Wingenter et al.
2007). However, the effectiveness of such a geoengineering approach is highly uncertain; even the
underlying assumption that iron fertilization increases DMS emission is questioned (Bopp et al.
2008).
Latham et al. (2008) reported that the net radiative forcing from a doubling of the natural cloud
droplet concentrations in regions of low-level maritime clouds could roughly offset the radiative
effect from a doubling of atmospheric CO2 . Owing to the spatial inhomogeneity of cloud-albedo
forcing, climate response to marine cloud brightening is expected to show large regional variations.
Simulated climate effects from marine cloud brightening vary greatly among models owing to
different seeding strategies and different model physics. Bala et al. (2010) simulated an idealized
scenario in which the cloud droplet size of all marine clouds is reduced to offset the global-mean
surface temperature change due to a doubling of atmospheric CO2 . They found a decrease in
global-mean precipitation and evaporation but an increase in runoff over land. By seeding largescale stratocumulus clouds in the North Pacific, South Pacific, and South Atlantic, Jones et al.
(2009, 2011) found that cloud seeding could delay global warming for approximately 25 years
but would cause a sharp decrease in precipitation over the Amazon basin. Rasch et al. (2009), by
seeding a much larger portion of the ocean than that seeded by Jones et al. (2009, 2011), found
that cloud seeding cannot result in a simultaneous return of global-mean surface temperature,
precipitation, and sea ice to the present-day level and observed in these climatic fields a significant
local departure from the present-day level.
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2.2.4. Surface-albedo enhancement. Numerous methods to increase the reflectivity of Earth’s
surface have been proposed; these include modifying the reflectivities of rural areas, urban areas,
deserts, and the ocean surface. However, because land represents somewhat less than one-third
of the planetary surface and approximately half of the land surface is cloud covered, ∼10% of
radiation incident on the global land surface would need to be reflected to offset the radiative
forcing from a doubling of atmospheric CO2 content. Thus, achieving substantial global-mean
temperature reductions through altering land-surface albedo represents a daunting challenge.
Ridgwell et al. (2009) argued that a 0.08 increase in crop albedo (from 0.2) is feasible, and this
increase has been estimated to yield an upper-limit radiative forcing of −0.35 W m−2 (Lenton
& Vaughan 2009). However, there is no convincing evidence that this global 40% increase in
crop albedo is achievable. Akbari et al. (2009) estimated that increasing the worldwide albedos of
urban roofs and paved surfaces would induce a radiative forcing of −0.044 W m−2 , assuming a
net albedo increase of 0.1 for urban areas. Seitz (2011) proposed that ocean albedo can be increased
substantially by having a fleet of ships inject an abundance of very small bubbles over vast ocean
areas. If this method could increase ocean albedo globally by ∼0.05 from its present-day value
of ∼0.06, it would produce a global temperature decrease that is of the same magnitude as the
temperature increase caused by a doubling of atmospheric CO2 content.
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Surface-based albedo modification approaches introduce large spatial heterogeneity in radiative
forcing (Irvine et al. 2011). Ridgwell et al. (2009) simulated the climate effect of a 0.04 increase
in crop albedo and found a summertime cooling of up to 1◦ C in much of North America and
Central Europe. A modeling study by Doughty et al. (2011) found that planting brighter crops
might decrease the maximum daily air temperature (measured 2 m above the surface) by 0.25◦ C
per 0.01 increase in surface albedo at high latitudes (>30◦ ) but that planting brighter crops at
low latitudes (<30◦ ) is less effective at diminishing temperatures. Oleson et al. (2010) simulated
the effects of white roofs that are installed globally and found that daily maximum and minimum
temperatures averaged over all urban areas decreased by 0.6◦ C and 0.3◦ C, respectively.
2.3. Solar Geoengineering Discussion
The studies reviewed above indicate that reflecting incoming sunlight away from Earth would
offset many effects of increased greenhouse gas concentrations. However, this offsetting would be
imperfect, and climatic conditions might deteriorate in some regions as a result. Whereas these
approaches are aimed at reducing climate risk, deployment of such systems would introduce a
range of new risks.
Some consider solar geoengineering as one element in a portfolio of responses to risks posed
by climate change (Wigley 2006). In other words, solar geoengineering is considered an approach
that can be implemented jointly with efforts to reduce greenhouse gas emissions and increase
adaptive resilience. All these approaches might be combined in ways to produce the maximum
amount of risk reduction at the lowest cost.
Some consider solar geoengineering research as an insurance policy should global warming
impacts prove worse than anticipated and other measures fail or prove too costly (Hoffert et al.
2002). Interest in the potential for using sulfate aerosols as a response to climate change was stimulated by a publication by Paul Crutzen (Crutzen 2006). Computer model simulations indicated
that solar geoengineering has the potential to greatly cool planetary temperatures within years
(Matthews & Caldeira 2007), lending technical credence to the idea that such geoengineering
might be deployable in the context of an imminent or ongoing climate emergency.
If atmospheric greenhouse gas concentrations continue to increase alongside a solar geoengineering deployment aimed at offsetting the effects of those greenhouse gases, then the
amount of solar geoengineering would need to increase with time, masking ever greater amounts
of greenhouse-gas-induced warming. Should the deployment fail or for some other reason be
abruptly terminated, rapid warming could ensue (Matthews & Caldeira 2007). Thus, deployment
of such a system could be viewed as an intergenerational transfer of the risk of abrupt termination.
Several studies have addressed the extent to which the effects of a solar geoengineering deployment might be localized. A study in which reflection of sunlight was limited to the Arctic regions
found cooling that extended throughout the Northern Hemisphere (Caldeira & Wood 2008),
but that simulation did not consider dispersal of the aerosols themselves. Because stratospheric
aerosols cannot easily be confined to polar regions, climate effects of large polar aerosol injections
would likely be detectable at the hemispheric scale (Robock et al. 2008).
3. CARBON DIOXIDE REMOVAL
3.1. Introduction
Human activities perturb the natural carbon cycle by emitting excess CO2 into the atmosphere via fossil-fuel emissions and land-use change. Currently, anthropogenic CO2 emission is
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Ocean
fertilization
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Ocean
alkalinity addition
Accelerated
chemical weathering
of rocks
Manufacturing
carbonate minerals
using silicate rocks
and CO2 from air
Direct
air CO2
capture
Afforestation/
reforestation
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Biomass energy
with carbon
capture/storage
Figure 6
Diagram illustrating carbon dioxide removal approaches: ocean fertilization, ocean alkalinity addition, accelerated chemical weathering
of rocks, manufacture of products using silicate rocks and carbon from the air, direct capture of CO2 from the air, biomass energy with
carbon capture and storage, and afforestation or reforestation.
∼10 petagrams of carbon (Pg C) per year; nearly half is absorbed by the land biosphere and ocean,
and the rest accumulates in the atmosphere (Peters et al. 2012). The fraction of CO2 emissions
absorbed by the land biosphere and ocean is expected to decrease in the future.
Atmospheric CO2 concentrations adjust to CO2 additions or subtractions on a range of
timescales. Whereas the airborne fraction remaining at any given time is sensitive to background
conditions, climatically significant quantities of CO2 can persist in the atmosphere for thousands
of years. Eventually, most human-caused CO2 emissions to the atmosphere will be absorbed by
the oceans, but this process will take many centuries (Archer et al. 2009, Broecker et al. 1979,
Solomon et al. 2009). Consequently, the impacts of continued anthropogenic CO2 emissions
likely will be felt for millennia (Archer et al. 2009, Hegerl & Solomon 2009, Lowe et al. 2009,
Matthews & Caldeira 2008). It has been proposed that we could slow or reverse climate change on
decadal to centennial timescales by employing strategies that use natural processes to accelerate
or augment the slow removal of anthropogenic CO2 from the atmosphere. Some such carbon
dioxide removal (CDR) methods (e.g., reforestation) have already been considered in negotiations
under the United Nations Framework Convention on Climate Change (http://unfccc.int/; see
also Reyer et al. 2009, Streck & Scholz 2006).
CDR approaches aim to tackle the climate problem by addressing the root cause of the problem:
increasing atmospheric greenhouse gas concentrations. These approaches aim to remove excess
CO2 from the atmosphere and store the carbon in the land biosphere, ocean, or deep geological
reservoirs (Figure 6 and Table 2).
Because CO2 emissions have climate consequences lasting many thousands of years (Archer
et al. 2009), such emissions have been considered to cause climate change on timescales that
are relevant to most human activities. The prospect of capturing CO2 from the air presents
the possibility of reversing anthropogenic CO2 emissions. If in the future CO2 emissions are
discovered to be damaging, we (or more likely our descendants) could pay to remove this excess
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Table 2 Taxonomy of CDR approachesa
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Biological
Chemical
Land surface
Afforestation/reforestation
Improved forest management
Sequestration in buildings
Biomass burial
No-till agriculture
Biochar
Conservation agriculture
Fertilization of land plants
Creation of wetlands
BECCS
Enhanced weathering
Ocean surface
Ocean fertilization
Algae farming and burial
Blue carbon (mangrove, kelp farming)
Modification of ocean upwelling
Enhanced weathering
Ocean pipes
Ocean alkalinity addition
Industrial
Direct air capture with CCS
Carbon-absorbing cement
a
CDR approaches can be categorized according to whether they use biological or chemical engineering methods to remove carbon dioxide from the
atmosphere. They can also be categorized according to whether they require large areas of land or ocean surface or whether the process can be contained
in relatively small industrial facilities.
Abbreviations: BECCS, biomass energy with carbon capture and storage; CCS, carbon capture and storage; CDR, carbon dioxide removal.
CO2 from the atmosphere. However, CDR methods could be costly if implemented at scale,
and their effects on the climate system are slow (R. Soc. 2009). Unlike the solar geoengineering
methods that can mitigate global warming quickly by directly counteracting greenhouse radiative
forcing, CDR approaches will not have an appreciable effect on global climate for decades. An
idealized study that investigated the climate effect of an extreme CDR scenario (Cao & Caldeira
2010a) found that, on the centennial timescale, a one-time removal of all anthropogenic CO2 from
the atmosphere would offset less than 50% of the warming experienced at the time of CO2 removal
(Figure 7). Furthermore, even if all excess atmospheric CO2 could be instantaneously removed
and the atmosphere maintained with preindustrial concentrations, substantial amounts of climate
change would persist for decades (Cao & Caldeira 2010a). Therefore, CDR methods do not
provide an opportunity for rapid reduction of global temperatures. However, with a concerted
effort over many decades of implementation, these methods could significantly reduce future
atmospheric CO2 concentrations. Because of the thermal inertia of the ocean, the decrease in
surface temperature would lag the decreases in CO2 forcing.
CDR methods remove atmospheric CO2 and store it in vegetation, soil, oceans, or geological
reservoirs. They would need to remove several Pg C per year from the atmosphere for at least several decades to have a discernible climate effect, and their effectiveness at decreasing atmospheric
CO2 will depend on storage capacity and storage lifetime. Geological reservoirs are believed to
have a capacity of several thousand Pg C (Metz et al. 2005), and oceans may be able to store a
few thousand Pg C in the form of dissolved inorganic carbon for several centuries (Caldeira et al.
2005). This retention could be increased greatly if the addition of carbon were to be accompanied
by an addition of alkalinity (Caldeira & Rau 2000). In contrast, the terrestrial biosphere may be
able to store only ∼150 Pg C because the cumulative land-use flux in the past 200 years is of
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Atmospheric pCO2 (ppm)
600
500
Zero emissions
400
One-time removal
300
Sustained removal
200
Δ temperature (ºC)
2.0
1.5
1.0
0.5
0
1800
2000
2200
2400
Year
Figure 7
Effects (Cao & Caldeira 2010a) of an instantaneous cessation of CO2 emissions in 2050 (red line), one-time
removal of excess atmospheric CO2 (blue line), and removal of excess atmospheric CO2 followed by
continued removal of CO2 that degasses from the atmosphere and ocean ( green line). To a first
approximation, a cessation of emissions prevents further warming but does not lead to significant cooling
on the centennial timescale. A one-time removal of excess atmospheric CO2 eliminates approximately half of
the warming experienced at the time of the removal. To cool the planet back to preindustrial levels requires
the removal of all previously emitted CO2 , an amount equivalent to approximately twice the amount of
excess CO2 in the atmosphere.
this order (Houghton 2008). Hence, this value may represent the maximum potential land carbon
storage.
The first carbon cycle geoengineering proposal was to inject CO2 into the deep ocean
(Marchetti 1977). CO2 captured at power plants or by air capture can be transported via pipes or
ships and injected directly into the deep ocean or ocean floor. Most authors at this time do not
consider CO2 captured at power plants to be a form of geoengineering. A review and assessment
of deep-ocean injection was made by the Intergovernmental Panel on Climate Change in 2005
(Caldeira et al. 2005).
Physical leakage of carbon from its storage reservoir is a concern associated with many proposed CDR techniques, as temporary storage is largely equivalent to a delayed release of carbon
(Herzog et al. 2003). For example, most carbon stored on land in reduced form is not permanently
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stored because future land-use change, fires, or decay can rerelease the stored carbon back to the
atmosphere on timescales that are relevant to human decision making.
CO2 removed from the atmosphere by CDR approaches will cause a reduction in the CO2
gradient between atmosphere and land/ocean sinks. This decline in gradient will result in an efflux
of carbon from the land and ocean to the atmosphere or a decline in carbon uptake by these sinks
(Kirschbaum 2003). Therefore, if atmospheric CO2 is to be maintained at low levels, not only
does anthropogenic CO2 in the atmosphere need to be removed, but anthropogenic CO2 stored
in the ocean and on land needs to be removed as well when it outgasses to the atmosphere (Cao &
Caldeira 2010a). Consequently, decreasing atmospheric CO2 to preindustrial CO2 levels would
require permanently sequestering an amount of carbon equal to the total amount of historical
CO2 emissions (Cao & Caldeira 2010a, Lenton & Vaughan 2009, Matthews 2010). This effect of
release or decreased uptake of carbon by land and oceans because of CDR methods is termed the
rebound effect (Kirschbaum 2003, 2006). CDR methods could reduce plant productivity from the
levels associated with a high CO2 concentration. This diminished plant productivity could result
in less biosphere carbon uptake than otherwise would occur (Cao & Caldeira 2010a).
Only CDR methods that remove CO2 from a large area and methods that have the potential
to remove large quantities of CO2 from the atmosphere can be considered geoengineering methods; these include afforestation/reforestation, biomass energy with CO2 sequestration (BECS),
accelerated weathering over land, ocean fertilization, direct injection of CO2 into deep oceans,
ocean-based enhanced weathering, and direct air capture (Table 2).
The Intergovernmental Panel on Climate Change (IPCC) uses the term mitigation to refer to
policies to reduce CO2 emissions to the atmosphere or enhance carbon sinks (Metz et al. 2005).
Because CDR methods remove CO2 from the atmosphere and enhance its storage in land, ocean,
or geological reservoirs, they can be considered climate change mitigation activities.
3.2. Carbon Dioxide Removal Approaches
CDR approaches (Figure 6) share the goal of diminishing human intervention in the climate
system, yet each approach differs with regard to its efficacy, state of development, potential scale
of application, cost, and risks (R. Soc. 2009). To contribute substantially to climate change prevention, these approaches must be applied at a scale that is comparable to the scale of the energy
system that is releasing CO2 into the atmosphere.
3.2.1. Afforestation/reforestation. Afforestation is the direct human-induced growth of forest
on land that has not historically been forested. Reforestation is the direct human-induced conversion of nonforested land to forested land on land that had been previously converted from forest
to other uses.
Forests affect surface properties such as albedo, evapotranspiration, and surface roughness, all
of which can have climate consequences (Bonan 2008). Many studies have shown that afforestation
in seasonally snow-covered boreal and temperate regions could reduce surface albedo and result in
net warming despite increased carbon storage. In contrast, afforestation in tropical regions could
produce an additional cooling effect due to increased latent heat flux from evapotranspiration
and increased formation of low clouds that would add to the cooling effect of increased carbon
storage (Bala et al. 2007, Bathiany et al. 2010, Betts 2000, Bonan et al. 1992). However, one study
(Pongratz et al. 2011) shows that, because of farmers’ past preference for productive land without
much snow, reforestation in boreal regions typically would have a cooling influence on climate.
Changes in evapotranspiration have the potential to affect humidity and cloud cover and thus
surface temperature, especially in tropical regions (Bala et al. 2007). Land-cover change can affect
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climate in locations that are distant from the site of the change (Bala et al. 2007). Furthermore,
forests are subject to intermittent events such as forest fires, and the frequency of these events can
be affected by climate change. Reforestation and afforestation would tend to increase the change
in carbon storage that would occur as a result of CO2 fertilization or climate change (Bala et al.
2007, Kirschbaum 2003). An ambitious program of reforestation and afforestation could perhaps
restore to the land biosphere all of the carbon lost through historical deforestation. In this case,
atmospheric CO2 concentration could potentially be decreased by 40 to 70 ppm by the year 2100
(House et al. 2002). The storage of carbon in the terrestrial biosphere makes the sequestered
carbon susceptible to rerelease, although some forms of storage may prove long lasting.
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3.2.2. Biomass energy with CO2 sequestration. It is possible to capture CO2 from electric
power plants and pump it underground for long-term storage in a deep geologic formation (Metz
et al. 2005). If this CO2 capture and storage technology were used at an electric power plant
fueled with biomass, it would serve as a method to remove CO2 from the atmosphere and store
it permanently underground (Keith et al. 2006, Metz et al. 2005). The deep ocean could also
potentially be used as a long-term carbon storage site (Metz et al. 2005). This approach allows
repeated use of the same land in that plants can be farmed and used for biofuels, and this process
can be repeated. Application of carbon capture and storage to biomass energy sources could result
in the net removal of CO2 from the atmosphere (often referred to as negative emissions) provided
the biomass is not harvested at an unsustainable rate (Metz et al. 2005). Furthermore, the use
of biomass energy could supplant some use of fossil fuels. Some estimates (Kraxner et al. 2003)
show that a typical temperate forest in combination with capturing and long-term storage can,
on a sustainable basis, permanently remove ∼2.5 tons of carbon per year per hectare. If 3% of
the global land area (approximately one-fourth of the global agricultural land area) were used to
remove atmospheric CO2 using biomass energy with carbon capture and storage, approximately
1 Pg C per year could be removed, or approximately 100 Pg C in this century. Optimistic
economic analysis suggests that this method could be roughly cost competitive with more
conventional methods of achieving deep reductions in CO2 emissions from electric power plants
(Rhodes & Keith 2005). Biomass energy with carbon capture and storage becomes more attractive
if society chooses to pursue low atmospheric CO2 stabilization targets that would require negative
net CO2 emissions to the atmosphere (Azar et al. 2006).
3.2.3. Land-based Weathering. Weathering reactions typically take place at a rate that is slow
relative to the rate at which fossil fuel is being burned (Kelemen et al. 2011). Natural chemical
weathering reactions consume on the order of 0.1 Pg C per year of CO2 from the atmosphere—
approximately 1% of the rate of current anthropogenic emissions (Peters et al. 2012). It would
take tens of thousands of years or more for natural processes to remove the amount of CO2 that
we may emit in this century. It has been suggested that this removal rate could be accelerated by
intentional efforts to increase the rate of some or all of these weathering reactions.
There is net removal of CO2 from the atmosphere and transfer to the oceans over thousands
to tens of thousands of years by processes involving the weathering or dissolution of carbonate
minerals (Archer et al. 2009). This weathering reaction can be typified by:
CaCO3 + H2 O + CO2 → Ca2+ + 2HCO3 − .
(1)
Over hundreds of thousands of years, additional net transfer of CO2 to the ocean is effected by
reactions typified by this silicate-mineral weathering reaction:
CaSiO3 + 2CO2 + H2 O → Ca2+ + 2HCO3 − + SiO2 .
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In the case of silicate weathering, there can be net transfer from atmospheric reservoirs to solid
form. Reaction (2) followed by Reaction (1) operating in the reverse direction yields the following
net reaction:
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CaSiO3 + CO2 → CaCO3 + SiO2 .
(3)
The goal of accelerated weathering approaches is either to effect Reactions (1) and (2) with storage
of CO2 in dissolved form in the ocean (mostly as bicarbonate, HCO−
3 ) or to use Reaction (3) to
produce solid carbon-containing minerals.
It has been proposed that large amounts of silicate minerals such as olivine could be mined,
crushed, transported to, and distributed on agricultural land, with the intent that some of the
atmospheric CO2 will be stored as a component of carbonate minerals or as bicarbonate ions
transported to the oceans (Schuiling & Krijgsman 2006). Crushing the minerals increases reactive
surface areas, thus increasing reaction rates. Reaction rates could also be increased by exposing the
minerals to high CO2 concentrations (Kelemen & Matter 2008). Weathering of silicate minerals
would increase the pH and carbonate mineral saturation of soils and ocean surface waters. Therefore, weathering of silicate minerals could be applied to counteract effects of ocean acidification
(Caldeira & Wickett 2005).
3.2.4. Ocean-based weathering. It has been proposed that strong bases, derived from silicate
rocks, could be dissolved in the oceans (House et al. 2007), causing the oceans to absorb additional
CO2 . Carbonate minerals such as limestone could be heated to produce lime [Ca(OH)2 ], which
could be added to the oceans to increase their alkalinity and thereby promote ocean uptake of
atmospheric CO2 (Kheshgi 1995). Alternatively, carbonate minerals could be directly released into
the oceans (Harvey 2008, Kheshgi 1995). In another ocean-based weathering proposal, carbonate
rocks would be ground and reacted with concentrated CO2 captured at power plants to produce
bicarbonate solution, which would be released to the oceans (Rau 2008, Rau & Caldeira 1999). The
storage of carbon, along with alkaline minerals, in the ocean appears to be effectively permanent
on human timescales (Caldeira et al. 2005, Caldeira & Rau 2000, Kheshgi 1995).
3.2.5. Ocean fertilization. The process of photosynthesis involves the uptake of CO2 and the
production of organic carbon molecules. Microscopic photosynthetic organisms in surface ocean
waters (i.e., phytoplankton) produce organic carbon compounds from inorganic carbon that is
dissolved in sea water. Some of this organic matter sinks into the deep ocean. Thus, phytoplankton
effectively remove dissolved inorganic carbon from the near-surface ocean and transport organic
carbon to the deep ocean. The removal of inorganic carbon from the near-surface ocean reduces
the partial pressure of CO2 at the ocean surface, resulting in a flux of CO2 from the atmosphere
to the ocean ( Jin et al. 2008). In this way, phytoplankton cause CO2 to be taken up from the
atmosphere and cause the carbon in that CO2 to be transported to the deep ocean as organic
carbon. The basic concept of ocean fertilization as a climate change mitigation strategy is to add
nutrients to the ocean to increase planktonic productivity and thereby increase both the uptake
of atmospheric CO2 and the downward flux of carbon out of the ocean’s near-surface layers. Iron
has been the most widely discussed fertilizer, but other nutrients such as phosphate and nitrogen
have been considered. The addition of iron has been suggested as a possible means of improving
the biological pump in deep waters (Lampitt et al. 2008, Martin 1990, Smetacek & Naqvi 2008).
Modeling and experimental investigation of ocean iron fertilization indicate limited potential
for carbon sequestration (Cao & Caldeira 2010b, Jin et al. 2008, Joos et al. 1991, Peng & Broecker
1991, Watson et al. 1994). Global model studies show that atmospheric CO2 concentrations
could be reduced by only 10%, even under highly optimistic assumptions. Furthermore, ocean
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fertilization could acidify the deep ocean by storing more CO2 there (Cao & Caldeira 2010b) and
could increase releases of the greenhouse gas N2 O, which could offset climate benefits of increased
CO2 storage in the oceans ( Jin & Gruber 2003).
The effectiveness of ocean iron fertilization depends both on the amount of carbon fixed in the
ocean’s surface layers and on the ultimate fate of this carbon. Most of the carbon that is reduced
through photosynthesis in the ocean’s surface layers is oxidized (respired, remineralized) in these
same layers, and in most cases only a small fraction is ultimately transported into the deep sea
(Lampitt et al. 2008, Lutz et al. 2002). For example, a 2002 experiment in the Southern Ocean
showed that iron addition can stimulate planktonic productivity; however, there was relatively little
increase in the amount of carbon exported to the deep ocean (Buesseler et al. 2004). In contrast, in
a 2004 experiment, more than half of the increase in phytoplankton biomass sank below 1,000 m
depth (Smetacek et al. 2012). In addition, the utilization of macronutrients such as N and P in
the fertilized region can lead to a decrease in production downstream from the fertilized region;
therefore, measurements in the fertilized field are insufficient to determine net additional carbon
storage (Gnanadesikan & Marinov 2008, Gnanadesikan et al. 2003, Watson et al. 2008).
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3.2.6. Direct capture from air. Direct air capture refers to the capture of CO2 that is produced
from the ambient air; the method typically employs chemical processes to separate the CO2 from
the rest of the atmosphere (Metz et al. 2005). The captured CO2 would be transported and used
for commercial purposes or stored underground in geological reservoirs. Carbon storage in wellchosen geological reservoirs appears to be effectively permanent on human timescales (Metz et al.
2005). Because CO2 makes up approximately 0.04% of the atmosphere and approximately 10% of
power plant flue gases, it is generally thought that direct air capture would not be able to compete
economically with capture from power plants in most circumstances. Nevertheless, there may be
some niche applications (e.g., commercial demand for CO2 , stranded energy sources) in which
direct air capture would be economically justifiable. Direct air capture is important because it
suggests that if the effects of climate change prove particularly dire, there are potential means to
reverse them (Keith et al. 2006).
The potential for direct air capture of CO2 changes climate policy in several ways (Keith
et al. 2006). Because CO2 captured directly from the air has essentially the same climate effects
regardless of where it was captured, the cost of this method sets a globally uniform upper bound
on the cost of CO2 emissions abatement (i.e., if an emissions reduction strategy costs more than
direct air capture, then the latter could be deployed instead). Because the air capture technology
need not be closely integrated with our existing energy system, direct air capture presents the
prospect for net emissions reduction without requiring a transformation of our energy system.
At least three methods have been proposed to capture CO2 from the atmosphere:
1. Adsorption on solids (Gray et al. 2008; Lackner 2009, 2010).
2. Absorption into highly alkaline solutions (Mahmoudkhani & Keith 2009, Stolaroff et al.
2008).
3. Absorption into moderately alkaline solutions with a catalyst (Bao & Trachtenberg 2006).
3.3. Discussion of Carbon Dioxide Removal Approaches
Most individual CDR methods have only marginal potential to affect atmospheric CO2 this century
(Table 3). In principle, the large-scale application of several approaches could remove up to
∼150 ppm of CO2 from the atmosphere. If combined with widespread deployment of energy
technologies that could reduce emissions and increase efficiency of energy use (e.g., Hoffert et al.
2002), this multipronged CDR approach may have the potential to enable otherwise unachievable
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Table 3 CDR methods and their characteristics
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CDR method
Carbon storage
(type of
Timescale of
reservoir)
carbon storage
Afforestation/
reforestation
Land
Decades
BECS
Ocean /Geological
Centuries to
millennia
Accelerated weathering
over land
Ocean
Ocean fertilization
Ocean
Potential amount
of atmospheric
carbon removed
by the year 2100
Reference(s)
80–140 Pg C
Canadell & Raupach 2008
48 Pg C
Sitch et al. 2005
100 Pg C
Our estimate
Centuries to
millennia
N/A
Kelemen & Matter 2008, Schuiling
& Krijgsman 2006
Centuries to
millennia
30–66 Pg C
Aumont & Bopp 2006, Zeebe &
Archer 2005
200 Pg C
Cao & Caldeira 2010a
Direct CO2 injection
Ocean
Centuries to
millennia
No obvious limit
Caldeira et al. 2005, Shaffer 2010
Ocean-based weathering
Ocean
Centuries to
millennia
N/A
Kheshgi 1995, Rau 2008
Direct air capture
Ocean/Geological
Centuries to
millennia
No obvious limit
Keith et al. 2006, Shaffer 2010
Abbreviations: BECS, biomass energy with CO2 sequestration; CDR, carbon dioxide removal; N/A, not applicable; Pg C, petagrams of carbon.
climate mitigation targets, such as CO2 stabilization below 400 ppm this century (Matthews 2010).
Only direct air capture in combination with storage in geological reservoirs has the capacity
to remove a climatically important amount of CO2 from the atmosphere, although the cost of
deployment at the required scale might be considered prohibitive.
The large-scale deployment of some CDR techniques could have unintended environmental
consequences. For example, ocean fertilization increases the amount of dissolved CO2 in the ocean
(Cao & Caldeira 2010b), and this could have significant adverse environmental consequences for
coral reefs and other ecosystems in which calcifying organisms play a major role (Hoegh-Guldberg
et al. 2007). All biologically based carbon storage options require the involvement of large spatial
areas owing to low efficiencies at the scale of the ecosystem (Drolet et al. 2008, Yuan et al.
2010). This requirement applies to large-scale forest management for the purposes of carbon
storage in living biomass (e.g., afforestation) or to the use of biomass as a fuel with carbon capture
and storage. In addition, some ocean-based carbon storage options (e.g., application of lime or
carbonate minerals to the sea surface to stimulate carbon dissolution) require both large areas and
significant mining activity. Any large-scale application of these strategies to remove CO2 could
result in conflicts with other land uses (Matthews 2010, R. Soc. 2009).
It appears feasible to remove CO2 from the atmosphere and store it in land, oceans, or geological
reservoirs. However, most of these options are either limited in their capacity or expensive to
deploy at the scale of global fossil-fuel CO2 emission. Important considerations for evaluating
CDR methods include the permanence of the storage, the speed at which the system can be
deployed, storage capacity, and potential adverse side effects (R. Soc. 2009).
CDR methods address the cause of climate change as well as the problem of ocean acidification.
As mentioned in Section 3.1, CDR methods could reduce plant productivity relative to what it
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would be with higher CO2 concentrations. The main disadvantages of these methods are that they
are slow acting in the elimination of atmospheric CO2 and they tend to be costly or impossible
to apply at the scale of global fossil-fuel CO2 emissions. However, if applied on a large scale and
for a long enough period, they could potentially contribute to the reduction of atmospheric CO2
content. The removal of CO2 from the atmosphere is environmentally equivalent to the reduction
of emissions. If used on a sufficiently large scale and if other CO2 emissions are sufficiently curtailed,
CDR options create the possibility of negative global net emissions and thus the possibility of
reducing not only CO2 emissions but also atmospheric CO2 concentrations.
4. DISCUSSION AND CONCLUSIONS
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This review describes some of the many creative proposals to diminish risk from anthropogenic
climate change. There are other proposals that have not been discussed here; a review such as this
must focus on proposals for which there is some supporting peer-reviewed literature.
Most proposed solar geoengineering approaches are controversial and raise a range of important issues regarding governance, equity, and ethics (R. Soc. 2009) that are beyond the scope of this
review of the basic science. Most of these approaches present new and novel risks that are difficult
to quantify or even identify. Nevertheless, several solar geoengineering approaches may be able
to cool Earth rapidly and reduce the amount of climate change caused by increased atmospheric
greenhouse gas concentrations, and such approaches could prove important should a profound
climate crisis develop (or threaten to develop). More research could help narrow, but could not
eliminate, outstanding uncertainties.
In contrast, most proposed CDR options, with the notable exception of ocean fertilization,
have been relatively uncontroversial. Some of these options, such as reforestation, are routinely
considered in discussions of climate change mitigation. The primary questions relate to the ability
of various options to store carbon effectively and affordably at large scale without producing major
adverse local environmental consequences. For example, if industrialized air capture with geologic
storage could be made to work without incurring significant local environmental consequences,
then the cost relative to other options would likely be the primary factor determining whether to
deploy that option.
This review discusses no option that can completely offset the effects of today’s fossil-fuel
CO2 emissions. No such option is expected to arise. Solar geoengineering proposals raise the
prospect of rapidly cooling the climate, but they introduce a whole new set of risks and challenges.
CDR proposals raise the prospect of removing some CO2 from the atmosphere, but most options
cannot be deployed at the scale of our fossil-fuel emissions, and the scalable options appear to be
expensive relative to the cost of other mitigation options. Thus, neither solar geoengineering nor
CDR can provide the certain reduction in environmental risk that is offered by cuts in greenhouse
gas emissions.
DISCLOSURE STATEMENT
The authors are not aware of any affiliations, memberships, funding, or financial holdings that
might be perceived as affecting the objectivity of this review. K.C.’s name is on several patents,
some of which could conceivably be used for the purposes of intentional climate modification, but
if any of these patents is ever used for the purposes of altering climate, any proceeds that accrue
to K.C. for this use will be donated to nonprofit nongovernmental organizations and charities.
K.C. has no expectation of or interest in developing a personal revenue stream based on the use
of these patents for climate modification.
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Volume 41, 2013
Annu. Rev. Earth Planet. Sci. 2013.41:231-256. Downloaded from www.annualreviews.org
by Stanford University - Main Campus - Lane Medical Library on 07/24/13. For personal use only.
On Escalation
Geerat J. Vermeij p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 1
The Meaning of Stromatolites
Tanja Bosak, Andrew H. Knoll, and Alexander P. Petroff p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p21
The Anthropocene
William F. Ruddiman p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p45
Global Cooling by Grassland Soils of the Geological Past
and Near Future
Gregory J. Retallack p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p69
Psychrophiles
Khawar S. Siddiqui, Timothy J. Williams, David Wilkins, Sheree Yau,
Michelle A. Allen, Mark V. Brown, Federico M. Lauro, and Ricardo Cavicchioli p p p p p p87
Initiation and Evolution of Plate Tectonics on Earth:
Theories and Observations
Jun Korenaga p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 117
Experimental Dynamos and the Dynamics of Planetary Cores
Peter Olson p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 153
Extracting Earth’s Elastic Wave Response from Noise Measurements
Roel Snieder and Eric Larose p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 183
Miller-Urey and Beyond: What Have We Learned About Prebiotic
Organic Synthesis Reactions in the Past 60 Years?
Thomas M. McCollom p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 207
The Science of Geoengineering
Ken Caldeira, Govindasamy Bala, and Long Cao p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 231
Shock Events in the Solar System: The Message from Minerals in
Terrestrial Planets and Asteroids
Philippe Gillet and Ahmed El Goresy p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 257
The Fossil Record of Plant-Insect Dynamics
Conrad C. Labandeira and Ellen D. Currano p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 287
viii
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The Betic-Rif Arc and Its Orogenic Hinterland: A Review
John P. Platt, Whitney M. Behr, Katherine Johanesen,
and Jason R. Williams p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 313
Assessing the Use of Archaeal Lipids as Marine Environmental Proxies
Ann Pearson and Anitra E. Ingalls p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 359
Annu. Rev. Earth Planet. Sci. 2013.41:231-256. Downloaded from www.annualreviews.org
by Stanford University - Main Campus - Lane Medical Library on 07/24/13. For personal use only.
Heat Flow, Heat Generation, and the Thermal State
of the Lithosphere
Kevin P. Furlong and David S. Chapman p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 385
The Isotopic Anatomies of Molecules and Minerals
John M. Eiler p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 411
The Behavior of the Lithosphere on Seismic to Geologic Timescales
A.B. Watts, S.J. Zhong, and J. Hunter p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 443
The Formation and Dynamics of Super-Earth Planets
Nader Haghighipour p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 469
Kimberlite Volcanism
R.S.J. Sparks p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 497
Differentiated Planetesimals and the Parent Bodies of Chondrites
Benjamin P. Weiss and Linda T. Elkins-Tanton p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 529
Splendid and Seldom Isolated: The Paleobiogeography of Patagonia
Peter Wilf, N. Rubén Cúneo, Ignacio H. Escapa, Diego Pol,
and Michael O. Woodburne p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 561
Electrical Conductivity of Mantle Minerals: Role of Water
in Conductivity Anomalies
Takashi Yoshino and Tomoo Katsura p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 605
The Late Paleozoic Ice Age: An Evolving Paradigm
Isabel P. Montañez and Christopher J. Poulsen p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 629
Composition and State of the Core
Kei Hirose, Stéphane Labrosse, and John Hernlund p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 657
Enceladus: An Active Ice World in the Saturn System
John R. Spencer and Francis Nimmo p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 693
Earth’s Background Free Oscillations
Kiwamu Nishida p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 719
Global Warming and Neotropical Rainforests: A Historical Perspective
Carlos Jaramillo and Andrés Cárdenas p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 741
The Scotia Arc: Genesis, Evolution, Global Significance
Ian W.D. Dalziel, Lawrence A. Lawver, Ian O. Norton,
and Lisa M. Gahagan p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 767
Contents
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