the science of geoengineering - homepages.ed.ac.uk review.pdf · ea41ch10-caldeira ari 19 april...

EA41CH10-Caldeira ARI 19 April 2013 15:34

The Science of GeoengineeringKen Caldeira,1 Govindasamy Bala,2 and Long Cao3

1Department of Global Ecology, Carnegie Institution for Science, Stanford, California 94305;email: [email protected] for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore 560 012,India3Department of Earth Sciences, Zhejiang University, Hangzhou, Zhejiang 310027, China

Annu. Rev. Earth Planet. Sci. 2013. 41:231–56

The Annual Review of Earth and Planetary Sciences isonline at earth.annualreviews.org

This article’s doi:10.1146/annurev-earth-042711-105548

Copyright c© 2013 by Annual Reviews.All rights reserved

Keywords

carbon dioxide removal, solar radiation management, climate,environment, energy

Abstract

Carbon dioxide emissions from the burning of coal, oil, and gas are increas-ing atmospheric carbon dioxide concentrations. These increased concentra-tions cause additional energy to be retained in Earth’s climate system, thusincreasing Earth’s temperature. Various methods have been proposed toprevent this temperature increase either by reflecting to space sunlight thatwould otherwise warm Earth or by removing carbon dioxide from the at-mosphere. Such intentional alteration of planetary-scale processes has beentermed geoengineering. The first category of geoengineering method, so-lar geoengineering (also known as solar radiation management, or SRM),raises novel global-scale governance and environmental issues. Some SRMapproaches are thought to be low in cost, so the scale of SRM deploymentwill likely depend primarily on considerations of risk. The second category ofgeoengineering method, carbon dioxide removal (CDR), raises issues relatedprimarily 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

The term geoengineering as applied in its current context was introduced into the scientificliterature by Victor Marchetti in the title of his classic paper describing deep-sea disposal of carbondioxide (CO2) (Marchetti 1977). This term has come to refer to large-scale efforts to diminishclimate 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, orSRM) and carbon dioxide removal (CDR) (R. Soc. 2009). SRM aims to diminish the amount ofclimate change produced by high greenhouse gas concentrations, whereas CDR involves removingCO2 and other greenhouse gases from the atmosphere.

These geoengineering approaches may complement other strategies to diminish risks posedby 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 emissionenergy 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 notmutually exclusive, although decisions must be made regarding how much effort should be put

Consumption

of goods and

services

Consumption

of energy

Climate

change

Impacts on

humans and

ecosystems

CO2removal

Desire for

improved

well-being

Adaptation

Solargeoengineering

Low-carbon

emission energy

technologies

Conservation

Efficiency

CO2

emissions

CO2 in

atmosphere

Figure 1Most geoengineering approaches fall into one of two categories: carbon dioxide removal or solargeoengineering. These approaches can be viewed as part of a portfolio of strategies for diminishing climaterisk and damage. Carbon dioxide removal attempts to break the link between CO2 emissions andaccumulation 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 climatechange that can result.

232 Caldeira · Bala · Cao

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into researching, developing, and implementing each approach. Such decisions can be improvedby careful scientific and technical analysis.

Geoengineering approaches have been the subject of previous reviews, including chapters in1992 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 Reviewsjournal more than a decade ago (Keith 2000). The UK Royal Society assembled a panel in 2009that produced a good summary, including references to issues involving international governanceand ethics (R. Soc. 2009).

Proposals to consider the intentional alteration of climate have raised concerns related to pol-itics, policy, governance, and ethics (Blackstock & Long 2010, Jamieson 1996). These discussionsoften cite “the importance of democratic decision-making, the prohibition against irreversibleenvironmental 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 twomajor classes of activities: reflecting sunlight away from Earth (SRM/solar geoengineering) andremoving 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 cli-mate system by making it more difficult for heat to escape to space. SRM/solar geoengineeringapproaches aim to offset this warming influence by reducing the amount of sunlight absorbed byEarth (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 ofatmospheric CO2 causes a radiative forcing of ∼4 W m−2. Therefore, to offset the 4 W m−2 forcingrequires 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). Precisenumbers depend on uncertain climate system feedbacks and differences in climate system responseto different types of radiative forcing (Hansen et al. 2005).

Some computer model studies simulated the effect of solar geoengineering approaches byreducing 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 transportof stratospheric aerosols (Rasch et al. 2008a, Robock et al. 2008).

Model results indicate that measures to reflect incoming sunlight away from Earth couldpotentially 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 ofthe 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 extremelyrapid 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 slowlywith relatively slow increases in atmospheric CO2, in the case of a catastrophic failure of a solargeoengineering system, Earth would experience a large climate forcing at the time of system failureand would warm rapidly for several decades. Furthermore, compared with a climate that has ahigher temperature and a high CO2 level, much more carbon would be stored in the oceans andland in a climate with low solar irradiance, low temperature, and high CO2. In the case of a halt or

www.annualreviews.org • The Science of Geoengineering 233

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Table 1 Summary of solar geoengineering proposals

Solargeoengineeringmethod

Maximumcooling

potentiala

Attainablespeed of

deploymentbRelative cost

per unit effectc

Relative riskto

environmentper uniteffectd Selected references

Space-basedschemes

High Slow High Low Angel 2006, Early 1989

Stratosphericaerosols

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, Raschet al. 2009

Whitening of theocean

Medium ??? ??? ??? Pres. Sci. Advis. Comm.Environ. Pollut. Panel1965, Seitz 2011

Plant reflectivity Low Medium Medium High Doughty et al. 2011,Ridgwell et al. 2009

Whitening of builtstructures

Low Medium Medium High Akbari et al. 2009, Menonet al. 2010

Adapted from the Royal Society Report on geoengineering (R. Soc. 2009) and citations in text as noted.aHigh 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.bFast means deployable within a decade; medium means that deployment would take decades.cHigh means costlier than conventional mitigation approaches; medium means less costly than conventional approaches but costly enough for economicsto 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.dApproaches 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 storedin 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, itmay 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 atmosphericCO2 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 thefull amount of global-mean surface warming, would cause a reduction in global-mean precipitationdue to the precipitation-suppression property of CO2 forcing (Bala et al. 2008, Caldeira & Wood2008, Lunt et al. 2008). Alternatively, if solar geoengineering were implemented to counteractchanges in global-mean precipitation, there would be some residual surface warming.

Increasing atmospheric CO2 content also affects the climate system via its effect on plantstomata (Sellers et al. 1996). This effect, referred to as CO2-physiological forcing, increasesthe CO2-radiative warming by approximately 10% at the global scale and can account for upto 30% of the total warming at regional scales (Boucher et al. 2009, Cao et al. 2010). ThisCO2-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 reverseeffects of CO2 fertilization of plants (Govindasamy et al. 2002). Jones et al. (2011) suggested that


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b

d e f

a

c

Figure 2Solar geoengineering/solar radiation management approaches work by reflecting to space sunlight that would otherwise have beenabsorbed. 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 productivityowing to changes in regional precipitation. One key difference between the spaced-basedapproach and the stratospheric aerosol–based approach is that the scattering effect of sulfateaerosols increases the amount of diffuse solar radiation that reaches the land surface in spite ofthe reduction in total solar radiation. It is thought that increased diffuse solar radiation tendsto 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 notbeen considered in global modeling studies of stratospheric aerosol geoengineering.

The moderation of global-mean climate does not necessarily lead to a uniform moderation ofclimate in all regions (Ban-Weiss & Caldeira 2010, Jones et al. 2011, Ricke et al. 2010). Studies haveshown that solar geoengineering could diminish the amount of temperature change in all regionsbut would increase the magnitude of precipitation changes in some regions (Hegerl & Solomon2009). Ban-Weiss & Caldeira (2010) found that having a stratospheric aerosol loading that isweighted toward polar regions results in a temperature distribution more similar to the low-CO2


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Temperature with business-as-usual emissions

13

14

15

16

17

13

14

2000

a

b

2025

2050

2075

15

16

17

18

19

2000 2020 2040 2060 2080 2100

Su

rfa

ce a

ir t

em

pe

ratu

re (

°C)

Model year

Figure 3Model-simulated global and annual mean surface air temperature (red lines) for a business-as-usual CO2emission scenario (Matthews & Caldeira 2007). (a) Cases showing cooling when solar intensity is reduced inyears 2000, 2025, 2050, and 2075. (b) Cases in which solar intensity is decreased to compensate for increasingCO2 content and then returned rapidly to the full value. Simulations with doubled climate sensitivity areplotted 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. Reproducedfrom Matthews & Caldeira (2007) with permission.

climate than that yielded by a globally uniform aerosol loading. However, this polar weighting ofstratospheric 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 andAfrican summer monsoons. Lunt et al. (2008) reported that compared with the natural climate,a uniform reduction in solar radiation leads to reduced El Nino–related variability and increasedNorth 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

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

Precipitation change (m year–1)

Figure 4Model-simulated (Caldeira & Wood 2008) annual mean changes in temperature (left panels) and precipitation (right panels) for the caseof 2 × CO2 (top panels) and that of 2 × CO2 with a reduction in global-mean solar insolation of 1.84% (bottom panels). The changes arecalculated as the departure from the simulation with 1 × CO2. The idealized solar geoengineering scheme largely offsets most of theCO2-induced temperature and precipitation changes but leaves some residual warming at the poles and leads to an overall decrease inprecipitation. Reproduced from Caldeira & Wood (2008) with permission.

causes changes in El Nino and related climate teleconnection patterns. Moore et al. (2010) cal-culated that an aerosol injection delivering a constant 4 W m−2 in radiative forcing could delaysea-level rise by 40–80 years.

Solar geoengineering approaches do not directly alter atmospheric CO2 content and there-fore do not mitigate CO2-induced ocean acidification (Matthews et al. 2009). In addition, solargeoengineering approaches do not prevent CO2-induced changes in terrestrial carbon cycle, in-cluding biomass and net primary production (e.g., Govindasamy et al. 2002). Furthermore, solargeoengineering approaches would cool in the stratosphere (Bala et al. 2010, Govindasamy &Caldeira 2000, Govindasamy et al. 2003) and could aggravate changes to stratospheric chemistryand 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 sunlightabsorbed by Earth. This sunlight could potentially be deflected away from the Earth either inspace, 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 theamount of incoming solar radiation reaching Earth. Numerous techniques have been proposed toachieve this goal. Early (1989) proposed constructing a thin glass shield from lunar materials andplacing 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 objecttoward 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 Lagrangepoint. Other proposals include placing mirrors in orbit around Earth (Natl. Acad. Sci. 1992) andplacing rings around Earth that are composed of particles or constellations of spacecraft (Pearsonet 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 onesquare kilometer each hour (Govindasamy & Caldeira 2000). Such rates mean that large-scaledeployment is likely to be a long process and to remain infeasible for many decades (McInnes 2010).

2.2.2. Stratospheric aerosol–based approaches. The injection of sulfate aerosols into thelower stratosphere would cool Earth by scattering the solar radiation back to space. Studies ofclimate geoengineering using sulfate aerosols have concluded that stratospheric aerosols couldreduce global-mean temperatures, but concerns remain regarding many issues, including effectson regional climate, precipitation, and ozone loss (Rasch et al. 2008b).

Insight into the potential for injecting sulfate aerosols into the stratosphere to cool Earthhas been demonstrated from the cooling observed after large volcanic eruptions such as MountPinatubo in 1991 (Crutzen 2006, Soden et al. 2002), although the volcanic eruption is an imperfectanalog of sulfate aerosol injection. The Mount Pinatubo eruption placed enough material in theatmosphere to offset approximately 4 W m−2 of radiative forcing, i.e., approximately enoughmaterial 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 thesame order of magnitude as a full-scale solar geoengineering deployment. However, the solargeoengineering deployment would involve replenishment of these aerosols as they were removedfrom the atmosphere by natural processes. The aerosols injected into the stratosphere by MountPinatubo settled and were transported out of the stratosphere on the timescale of approximatelyone year. Earth’s surface cooled by ∼0.5 K within the year following the eruption. Had the aerosollayer been maintained in the stratosphere, it would have cooled Earth’s surface by perhaps 3 K. Inaddition, following the volcanic eruption of Mount Pinatubo, investigators observed a substantialdecrease 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 highin the atmosphere to reflect solar radiation away from Earth, but most studies have focused onsulfate particles. Various techniques have been proposed for delivering the sulfate aerosol and/or itsprecursor gases (H2S 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 systemneed to be fully evaluated (Robock et al. 2009). The amount of warming that would be offset bya given injection of aerosol precursors is difficult to predict precisely because it can be affected bynonlinear feedbacks involving the delivery mechanisms, particle size and distribution, microphysicsof aerosol formation and growth, and climate change. Smaller particles (radius of ∼0.1 μm) aremore effective at scattering incoming energy and have no impact on longwave radiation, whereaslarger particles such as those following volcanic eruptions are less effective at scattering shortwaveradiation and absorb and emit in the longwave spectrum (Stenchikov et al. 1998). Rasch andcolleagues (Rasch et al. 2008a) found that approximately 1.5 Tg S year−1 of sulfate aerosols wouldbalance a doubling of CO2 if the particles were small, whereas perhaps double that amount may


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1.08

1.12

1.16

1.20

1.24

1986 1990 1994 1998

Co

nti

ne

nta

l d

isch

arg

e (

Sv

)

La

nd

pre

cipita

tion

(Sv

)

Year

3.00

3.10

3.20

3.30

Figure 5Time 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 periodclearly influenced by the Mount Pinatubo eruption is indicated by gray shading. Reproduced fromTrenberth & Dai (2007) with permission.

be needed if the particles were to reach the size seen following volcanic eruptions. There is stilluncertainty 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 affectthe amount of aerosol precursors that would need to be injected to counteract CO2 warming (Raschet al. 2008b). The altitude, location, and mode of injection into the stratosphere also influenceefficacy, 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 concentra-tions. An injection of particles into the stratosphere has the potential to provide surfaces that leadto efficient chlorine activation, which could approximately double the ozone-destroying potentialof 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 the2 × CO2 surface warming would cause a 30- to 70-year delay in the expected recovery of theAntarctic ozone hole. Heckendorn et al. (2009) found that sulfate aerosol geoengineering accel-erates 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 bright-ening is to increase the reflectivity of low-level marine stratocumulus clouds by increasing thenumber of cloud condensation nuclei (CCN). More CCN increase the number of cloud dropletswhile reducing the droplet size, thus increasing the total droplet surface area of the cloud and thecloud reflectivity (Twomey 1977). Extensive areas of marine stratocumulus clouds off the westcoasts of North and South America and the west coast of Africa have been identified as regionswhere marine cloud brightening approaches would be effective (Latham et al. 2008). The moststudied method of increasing CCN is spraying a fine seawater mist into the remote marine atmo-spheric boundary layer by conventional ocean-going vessels, by aircraft, or by specially designedunmanned remotely controlled sea craft (Salter et al. 2008). Calculations show that the change incloud 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 backgroundaerosol concentrations (Bower et al. 2006). Although in many climate modeling studies theaddition of CCN is implicitly assumed to increase cloud albedo, predicting how changes in cloudmicrophysical properties would affect cloud planetary albedo is difficult. Reduced droplet size maysuppress precipitation and further increase cloud cover (Albrecht 1989). In contrast, in some sit-uations 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 alwaysbrighten them” (p. 3983). It may be possible to increase CCN by fertilizing the Southern Oceanwith iron to stimulate phytoplankton growth and increase the phytoplankton emission of dimethylsulfide (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 theunderlying 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 clouddroplet concentrations in regions of low-level maritime clouds could roughly offset the radiativeeffect from a doubling of atmospheric CO2. Owing to the spatial inhomogeneity of cloud-albedoforcing, 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 todifferent seeding strategies and different model physics. Bala et al. (2010) simulated an idealizedscenario in which the cloud droplet size of all marine clouds is reduced to offset the global-meansurface temperature change due to a doubling of atmospheric CO2. They found a decrease inglobal-mean precipitation and evaporation but an increase in runoff over land. By seeding large-scale 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 yearsbut would cause a sharp decrease in precipitation over the Amazon basin. Rasch et al. (2009), byseeding a much larger portion of the ocean than that seeded by Jones et al. (2009, 2011), foundthat 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 significantlocal departure from the present-day level.

2.2.4. Surface-albedo enhancement. Numerous methods to increase the reflectivity of Earth’ssurface 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-thirdof the planetary surface and approximately half of the land surface is cloud covered, ∼10% ofradiation incident on the global land surface would need to be reflected to offset the radiativeforcing from a doubling of atmospheric CO2 content. Thus, achieving substantial global-meantemperature 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 thisincrease 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 incrop albedo is achievable. Akbari et al. (2009) estimated that increasing the worldwide albedos ofurban roofs and paved surfaces would induce a radiative forcing of −0.044 W m−2, assuming anet albedo increase of 0.1 for urban areas. Seitz (2011) proposed that ocean albedo can be increasedsubstantially by having a fleet of ships inject an abundance of very small bubbles over vast oceanareas. If this method could increase ocean albedo globally by ∼0.05 from its present-day valueof ∼0.06, it would produce a global temperature decrease that is of the same magnitude as thetemperature increase caused by a doubling of atmospheric CO2 content.


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Surface-based albedo modification approaches introduce large spatial heterogeneity in radiativeforcing (Irvine et al. 2011). Ridgwell et al. (2009) simulated the climate effect of a 0.04 increasein crop albedo and found a summertime cooling of up to 1◦C in much of North America andCentral Europe. A modeling study by Doughty et al. (2011) found that planting brighter cropsmight decrease the maximum daily air temperature (measured 2 m above the surface) by 0.25◦Cper 0.01 increase in surface albedo at high latitudes (>30◦) but that planting brighter crops atlow latitudes (<30◦) is less effective at diminishing temperatures. Oleson et al. (2010) simulatedthe effects of white roofs that are installed globally and found that daily maximum and minimumtemperatures 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 wouldoffset many effects of increased greenhouse gas concentrations. However, this offsetting would beimperfect, and climatic conditions might deteriorate in some regions as a result. Whereas theseapproaches are aimed at reducing climate risk, deployment of such systems would introduce arange of new risks.

Some consider solar geoengineering as one element in a portfolio of responses to risks posedby climate change (Wigley 2006). In other words, solar geoengineering is considered an approachthat can be implemented jointly with efforts to reduce greenhouse gas emissions and increaseadaptive resilience. All these approaches might be combined in ways to produce the maximumamount of risk reduction at the lowest cost.

Some consider solar geoengineering research as an insurance policy should global warmingimpacts 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 stim-ulated by a publication by Paul Crutzen (Crutzen 2006). Computer model simulations indicatedthat solar geoengineering has the potential to greatly cool planetary temperatures within years(Matthews & Caldeira 2007), lending technical credence to the idea that such geoengineeringmight be deployable in the context of an imminent or ongoing climate emergency.

If atmospheric greenhouse gas concentrations continue to increase alongside a solar geo-engineering deployment aimed at offsetting the effects of those greenhouse gases, then theamount of solar geoengineering would need to increase with time, masking ever greater amountsof greenhouse-gas-induced warming. Should the deployment fail or for some other reason beabruptly terminated, rapid warming could ensue (Matthews & Caldeira 2007). Thus, deploymentof 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 deploy-ment might be localized. A study in which reflection of sunlight was limited to the Arctic regionsfound cooling that extended throughout the Northern Hemisphere (Caldeira & Wood 2008),but that simulation did not consider dispersal of the aerosols themselves. Because stratosphericaerosols cannot easily be confined to polar regions, climate effects of large polar aerosol injectionswould 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 atmo-sphere via fossil-fuel emissions and land-use change. Currently, anthropogenic CO2 emission is


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Oceanfertilization

Oceanalkalinity addition

Acceleratedchemical weathering

of rocks

Manufacturingcarbonate mineralsusing silicate rocks

and CO2 from air

Direct air CO2capture

Afforestation/reforestation

Biomass energywith carbon

capture/storage

Figure 6Diagram illustrating carbon dioxide removal approaches: ocean fertilization, ocean alkalinity addition, accelerated chemical weatheringof rocks, manufacture of products using silicate rocks and carbon from the air, direct capture of CO2 from the air, biomass energy withcarbon 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 emissionsabsorbed 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 oftimescales. Whereas the airborne fraction remaining at any given time is sensitive to backgroundconditions, climatically significant quantities of CO2 can persist in the atmosphere for thousandsof years. Eventually, most human-caused CO2 emissions to the atmosphere will be absorbed bythe 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 emissionslikely 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 ondecadal to centennial timescales by employing strategies that use natural processes to accelerateor augment the slow removal of anthropogenic CO2 from the atmosphere. Some such carbondioxide removal (CDR) methods (e.g., reforestation) have already been considered in negotiationsunder the United Nations Framework Convention on Climate Change (http://unfccc.int/; seealso 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 excessCO2 from the atmosphere and store the carbon in the land biosphere, ocean, or deep geologicalreservoirs (Figure 6 and Table 2).

Because CO2 emissions have climate consequences lasting many thousands of years (Archeret al. 2009), such emissions have been considered to cause climate change on timescales thatare relevant to most human activities. The prospect of capturing CO2 from the air presentsthe possibility of reversing anthropogenic CO2 emissions. If in the future CO2 emissions arediscovered to be damaging, we (or more likely our descendants) could pay to remove this excess


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http://unfccc.int/;


Table 2 Taxonomy of CDR approachesa

Biological ChemicalLand surface Afforestation/reforestation

Improved forest managementSequestration in buildingsBiomass burialNo-till agricultureBiocharConservation agricultureFertilization of land plantsCreation of wetlandsBECCS

Enhanced weathering

Ocean surface Ocean fertilizationAlgae farming and burialBlue carbon (mangrove, kelp farming)Modification of ocean upwellingEnhanced weatheringOcean pipes

Ocean alkalinity addition

Industrial Direct air capture with CCSCarbon-absorbing cement

aCDR approaches can be categorized according to whether they use biological or chemical engineering methods to remove carbon dioxide from theatmosphere. They can also be categorized according to whether they require large areas of land or ocean surface or whether the process can be containedin 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 geoengineeringmethods that can mitigate global warming quickly by directly counteracting greenhouse radiativeforcing, CDR approaches will not have an appreciable effect on global climate for decades. Anidealized study that investigated the climate effect of an extreme CDR scenario (Cao & Caldeira2010a) found that, on the centennial timescale, a one-time removal of all anthropogenic CO2 fromthe 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 removedand the atmosphere maintained with preindustrial concentrations, substantial amounts of climatechange would persist for decades (Cao & Caldeira 2010a). Therefore, CDR methods do notprovide an opportunity for rapid reduction of global temperatures. However, with a concertedeffort over many decades of implementation, these methods could significantly reduce futureatmospheric CO2 concentrations. Because of the thermal inertia of the ocean, the decrease insurface temperature would lag the decreases in CO2 forcing.

CDR methods remove atmospheric CO2 and store it in vegetation, soil, oceans, or geologicalreservoirs. They would need to remove several Pg C per year from the atmosphere for at least sev-eral decades to have a discernible climate effect, and their effectiveness at decreasing atmosphericCO2 will depend on storage capacity and storage lifetime. Geological reservoirs are believed tohave a capacity of several thousand Pg C (Metz et al. 2005), and oceans may be able to store afew 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 accompaniedby an addition of alkalinity (Caldeira & Rau 2000). In contrast, the terrestrial biosphere may beable to store only ∼150 Pg C because the cumulative land-use flux in the past 200 years is of


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200

300

400

500

600

0

0.5

1.0

1.5

2.0

1800 2000 2200 2400

Zero emissions

One-time removal

Sustained removal

Year

Δ t

em

pe

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re (

ºC)

Atm

osp

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ric pC

O2 (

pp

m)

Figure 7Effects (Cao & Caldeira 2010a) of an instantaneous cessation of CO2 emissions in 2050 (red line), one-timeremoval of excess atmospheric CO2 (blue line), and removal of excess atmospheric CO2 followed bycontinued removal of CO2 that degasses from the atmosphere and ocean ( green line). To a firstapproximation, a cessation of emissions prevents further warming but does not lead to significant coolingon the centennial timescale. A one-time removal of excess atmospheric CO2 eliminates approximately half ofthe warming experienced at the time of the removal. To cool the planet back to preindustrial levels requiresthe removal of all previously emitted CO2, an amount equivalent to approximately twice the amount ofexcess CO2 in the atmosphere.

this order (Houghton 2008). Hence, this value may represent the maximum potential land carbonstorage.

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 orships and injected directly into the deep ocean or ocean floor. Most authors at this time do notconsider CO2 captured at power plants to be a form of geoengineering. A review and assessmentof 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 pro-posed 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 theatmosphere 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 effluxof 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 onlydoes anthropogenic CO2 in the atmosphere need to be removed, but anthropogenic CO2 storedin 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 wouldrequire permanently sequestering an amount of carbon equal to the total amount of historicalCO2 emissions (Cao & Caldeira 2010a, Lenton & Vaughan 2009, Matthews 2010). This effect ofrelease or decreased uptake of carbon by land and oceans because of CDR methods is termed therebound effect (Kirschbaum 2003, 2006). CDR methods could reduce plant productivity from thelevels associated with a high CO2 concentration. This diminished plant productivity could resultin 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 potentialto remove large quantities of CO2 from the atmosphere can be considered geoengineering meth-ods; 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 topolicies 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 climatesystem, yet each approach differs with regard to its efficacy, state of development, potential scaleof application, cost, and risks (R. Soc. 2009). To contribute substantially to climate change pre-vention, these approaches must be applied at a scale that is comparable to the scale of the energysystem that is releasing CO2 into the atmosphere.

3.2.1. Afforestation/reforestation. Afforestation is the direct human-induced growth of foreston land that has not historically been forested. Reforestation is the direct human-induced conver-sion of nonforested land to forested land on land that had been previously converted from forestto other uses.

Forests affect surface properties such as albedo, evapotranspiration, and surface roughness, allof which can have climate consequences (Bonan 2008). Many studies have shown that afforestationin seasonally snow-covered boreal and temperate regions could reduce surface albedo and result innet warming despite increased carbon storage. In contrast, afforestation in tropical regions couldproduce an additional cooling effect due to increased latent heat flux from evapotranspirationand increased formation of low clouds that would add to the cooling effect of increased carbonstorage (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 withoutmuch 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 thussurface 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 canbe affected by climate change. Reforestation and afforestation would tend to increase the changein 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 perhapsrestore 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 sequesteredcarbon susceptible to rerelease, although some forms of storage may prove long lasting.

3.2.2. Biomass energy with CO2 sequestration. It is possible to capture CO2 from electricpower plants and pump it underground for long-term storage in a deep geologic formation (Metzet al. 2005). If this CO2 capture and storage technology were used at an electric power plantfueled with biomass, it would serve as a method to remove CO2 from the atmosphere and storeit permanently underground (Keith et al. 2006, Metz et al. 2005). The deep ocean could alsopotentially be used as a long-term carbon storage site (Metz et al. 2005). This approach allowsrepeated use of the same land in that plants can be farmed and used for biofuels, and this processcan be repeated. Application of carbon capture and storage to biomass energy sources could resultin the net removal of CO2 from the atmosphere (often referred to as negative emissions) providedthe biomass is not harvested at an unsustainable rate (Metz et al. 2005). Furthermore, the useof 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% ofthe global land area (approximately one-fourth of the global agricultural land area) were used toremove atmospheric CO2 using biomass energy with carbon capture and storage, approximately1 Pg C per year could be removed, or approximately 100 Pg C in this century. Optimisticeconomic analysis suggests that this method could be roughly cost competitive with moreconventional methods of achieving deep reductions in CO2 emissions from electric power plants(Rhodes & Keith 2005). Biomass energy with carbon capture and storage becomes more attractiveif society chooses to pursue low atmospheric CO2 stabilization targets that would require negativenet 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 slowrelative to the rate at which fossil fuel is being burned (Kelemen et al. 2011). Natural chemicalweathering 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 wouldtake tens of thousands of years or more for natural processes to remove the amount of CO2 thatwe may emit in this century. It has been suggested that this removal rate could be accelerated byintentional 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 thousandsto tens of thousands of years by processes involving the weathering or dissolution of carbonateminerals (Archer et al. 2009). This weathering reaction can be typified by:

CaCO3 + H2O + CO2 → Ca2+ + 2HCO3−. (1)

Over hundreds of thousands of years, additional net transfer of CO2 to the ocean is effected byreactions typified by this silicate-mineral weathering reaction:

CaSiO3 + 2CO2 + H2O → Ca2+ + 2HCO3− + SiO2. (2)


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In the case of silicate weathering, there can be net transfer from atmospheric reservoirs to solidform. Reaction (2) followed by Reaction (1) operating in the reverse direction yields the followingnet reaction:

CaSiO3 + CO2 → CaCO3 + SiO2. (3)

The goal of accelerated weathering approaches is either to effect Reactions (1) and (2) with storageof CO2 in dissolved form in the ocean (mostly as bicarbonate, HCO−

3 ) or to use Reaction (3) toproduce 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 theatmospheric CO2 will be stored as a component of carbonate minerals or as bicarbonate ionstransported to the oceans (Schuiling & Krijgsman 2006). Crushing the minerals increases reactivesurface areas, thus increasing reaction rates. Reaction rates could also be increased by exposing theminerals to high CO2 concentrations (Kelemen & Matter 2008). Weathering of silicate mineralswould increase the pH and carbonate mineral saturation of soils and ocean surface waters. There-fore, 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 silicaterocks, could be dissolved in the oceans (House et al. 2007), causing the oceans to absorb additionalCO2. Carbonate minerals such as limestone could be heated to produce lime [Ca(OH)2], whichcould be added to the oceans to increase their alkalinity and thereby promote ocean uptake ofatmospheric CO2 (Kheshgi 1995). Alternatively, carbonate minerals could be directly released intothe oceans (Harvey 2008, Kheshgi 1995). In another ocean-based weathering proposal, carbonaterocks would be ground and reacted with concentrated CO2 captured at power plants to producebicarbonate solution, which would be released to the oceans (Rau 2008, Rau & Caldeira 1999). Thestorage of carbon, along with alkaline minerals, in the ocean appears to be effectively permanenton 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 theproduction of organic carbon molecules. Microscopic photosynthetic organisms in surface oceanwaters (i.e., phytoplankton) produce organic carbon compounds from inorganic carbon that isdissolved in sea water. Some of this organic matter sinks into the deep ocean. Thus, phytoplanktoneffectively remove dissolved inorganic carbon from the near-surface ocean and transport organiccarbon to the deep ocean. The removal of inorganic carbon from the near-surface ocean reducesthe partial pressure of CO2 at the ocean surface, resulting in a flux of CO2 from the atmosphereto the ocean ( Jin et al. 2008). In this way, phytoplankton cause CO2 to be taken up from theatmosphere and cause the carbon in that CO2 to be transported to the deep ocean as organiccarbon. The basic concept of ocean fertilization as a climate change mitigation strategy is to addnutrients to the ocean to increase planktonic productivity and thereby increase both the uptakeof atmospheric CO2 and the downward flux of carbon out of the ocean’s near-surface layers. Ironhas been the most widely discussed fertilizer, but other nutrients such as phosphate and nitrogenhave been considered. The addition of iron has been suggested as a possible means of improvingthe biological pump in deep waters (Lampitt et al. 2008, Martin 1990, Smetacek & Naqvi 2008).

Modeling and experimental investigation of ocean iron fertilization indicate limited potentialfor carbon sequestration (Cao & Caldeira 2010b, Jin et al. 2008, Joos et al. 1991, Peng & Broecker1991, Watson et al. 1994). Global model studies show that atmospheric CO2 concentrationscould 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) andcould increase releases of the greenhouse gas N2O, which could offset climate benefits of increasedCO2 storage in the oceans ( Jin & Gruber 2003).

The effectiveness of ocean iron fertilization depends both on the amount of carbon fixed in theocean’s surface layers and on the ultimate fate of this carbon. Most of the carbon that is reducedthrough photosynthesis in the ocean’s surface layers is oxidized (respired, remineralized) in thesesame 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 Oceanshowed that iron addition can stimulate planktonic productivity; however, there was relatively littleincrease in the amount of carbon exported to the deep ocean (Buesseler et al. 2004). In contrast, ina 2004 experiment, more than half of the increase in phytoplankton biomass sank below 1,000 mdepth (Smetacek et al. 2012). In addition, the utilization of macronutrients such as N and P inthe 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 carbonstorage (Gnanadesikan & Marinov 2008, Gnanadesikan et al. 2003, Watson et al. 2008).

3.2.6. Direct capture from air. Direct air capture refers to the capture of CO2 that is producedfrom the ambient air; the method typically employs chemical processes to separate the CO2 fromthe rest of the atmosphere (Metz et al. 2005). The captured CO2 would be transported and usedfor commercial purposes or stored underground in geological reservoirs. Carbon storage in well-chosen 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% ofpower plant flue gases, it is generally thought that direct air capture would not be able to competeeconomically with capture from power plants in most circumstances. Nevertheless, there may besome niche applications (e.g., commercial demand for CO2, stranded energy sources) in whichdirect air capture would be economically justifiable. Direct air capture is important because itsuggests that if the effects of climate change prove particularly dire, there are potential means toreverse them (Keith et al. 2006).

The potential for direct air capture of CO2 changes climate policy in several ways (Keithet al. 2006). Because CO2 captured directly from the air has essentially the same climate effectsregardless of where it was captured, the cost of this method sets a globally uniform upper boundon the cost of CO2 emissions abatement (i.e., if an emissions reduction strategy costs more thandirect air capture, then the latter could be deployed instead). Because the air capture technologyneed not be closely integrated with our existing energy system, direct air capture presents theprospect 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 energytechnologies 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

CDR method

Carbon storage(type of

reservoir)Timescale of

carbon storage

Potential amountof atmospheric

carbon removedby the year 2100 Reference(s)

Afforestation/reforestation

Land Decades 80–140 Pg C Canadell & Raupach 2008

48 Pg C Sitch et al. 2005BECS Ocean /Geological Centuries to

millennia100 Pg C Our estimate

Accelerated weatheringover land

Ocean Centuries tomillennia

N/A Kelemen & Matter 2008, Schuiling& Krijgsman 2006

Ocean fertilization Ocean Centuries tomillennia

30–66 Pg C Aumont & Bopp 2006, Zeebe &Archer 2005

200 Pg C Cao & Caldeira 2010aDirect CO2 injection Ocean Centuries to

millenniaNo obvious limit Caldeira et al. 2005, Shaffer 2010

Ocean-based weathering Ocean Centuries tomillennia

N/A Kheshgi 1995, Rau 2008

Direct air capture Ocean/Geological Centuries tomillennia

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 capacityto remove a climatically important amount of CO2 from the atmosphere, although the cost ofdeployment at the required scale might be considered prohibitive.

The large-scale deployment of some CDR techniques could have unintended environmentalconsequences. For example, ocean fertilization increases the amount of dissolved CO2 in the ocean(Cao & Caldeira 2010b), and this could have significant adverse environmental consequences forcoral reefs and other ecosystems in which calcifying organisms play a major role (Hoegh-Guldberget al. 2007). All biologically based carbon storage options require the involvement of large spatialareas 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 carbonstorage in living biomass (e.g., afforestation) or to the use of biomass as a fuel with carbon captureand storage. In addition, some ocean-based carbon storage options (e.g., application of lime orcarbonate minerals to the sea surface to stimulate carbon dissolution) require both large areas andsignificant mining activity. Any large-scale application of these strategies to remove CO2 couldresult 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 geologicalreservoirs. However, most of these options are either limited in their capacity or expensive todeploy at the scale of global fossil-fuel CO2 emission. Important considerations for evaluatingCDR methods include the permanence of the storage, the speed at which the system can bedeployed, 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 theyare slow acting in the elimination of atmospheric CO2 and they tend to be costly or impossibleto apply at the scale of global fossil-fuel CO2 emissions. However, if applied on a large scale andfor 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 reductionof 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 ofreducing not only CO2 emissions but also atmospheric CO2 concentrations.

4. DISCUSSION AND CONCLUSIONS

This review describes some of the many creative proposals to diminish risk from anthropogenicclimate change. There are other proposals that have not been discussed here; a review such as thismust focus on proposals for which there is some supporting peer-reviewed literature.

Most proposed solar geoengineering approaches are controversial and raise a range of impor-tant issues regarding governance, equity, and ethics (R. Soc. 2009) that are beyond the scope of thisreview of the basic science. Most of these approaches present new and novel risks that are difficultto quantify or even identify. Nevertheless, several solar geoengineering approaches may be ableto cool Earth rapidly and reduce the amount of climate change caused by increased atmosphericgreenhouse gas concentrations, and such approaches could prove important should a profoundclimate crisis develop (or threaten to develop). More research could help narrow, but could noteliminate, 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 routinelyconsidered in discussions of climate change mitigation. The primary questions relate to the abilityof various options to store carbon effectively and affordably at large scale without producing majoradverse local environmental consequences. For example, if industrialized air capture with geologicstorage 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 todeploy that option.

This review discusses no option that can completely offset the effects of today’s fossil-fuelCO2 emissions. No such option is expected to arise. Solar geoengineering proposals raise theprospect 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 optionscannot be deployed at the scale of our fossil-fuel emissions, and the scalable options appear to beexpensive relative to the cost of other mitigation options. Thus, neither solar geoengineering norCDR can provide the certain reduction in environmental risk that is offered by cuts in greenhousegas emissions.

DISCLOSURE STATEMENT

The authors are not aware of any affiliations, memberships, funding, or financial holdings thatmight 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, butif any of these patents is ever used for the purposes of altering climate, any proceeds that accrueto 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 useof these patents for climate modification.


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EA41-FrontMatter ARI 7 May 2013 7:19

Annual Reviewof Earth andPlanetary Sciences

Volume 41, 2013 Contents

On EscalationGeerat J. Vermeij � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 1

The Meaning of StromatolitesTanja Bosak, Andrew H. Knoll, and Alexander P. Petroff � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �21

The AnthropoceneWilliam F. Ruddiman � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �45

Global Cooling by Grassland Soils of the Geological Pastand Near FutureGregory J. Retallack � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �69

PsychrophilesKhawar S. Siddiqui, Timothy J. Williams, David Wilkins, Sheree Yau,

Michelle A. Allen, Mark V. Brown, Federico M. Lauro, and Ricardo Cavicchioli � � � � � �87

Initiation and Evolution of Plate Tectonics on Earth:Theories and ObservationsJun Korenaga � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 117

Experimental Dynamos and the Dynamics of Planetary CoresPeter Olson � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 153

Extracting Earth’s Elastic Wave Response from Noise MeasurementsRoel Snieder and Eric Larose � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 183

Miller-Urey and Beyond: What Have We Learned About PrebioticOrganic Synthesis Reactions in the Past 60 Years?Thomas M. McCollom � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 207

The Science of GeoengineeringKen Caldeira, Govindasamy Bala, and Long Cao � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 231

Shock Events in the Solar System: The Message from Minerals inTerrestrial Planets and AsteroidsPhilippe Gillet and Ahmed El Goresy � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 257

The Fossil Record of Plant-Insect DynamicsConrad C. Labandeira and Ellen D. Currano � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 287

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EA41-FrontMatter ARI 7 May 2013 7:19

The Betic-Rif Arc and Its Orogenic Hinterland: A ReviewJohn P. Platt, Whitney M. Behr, Katherine Johanesen,

and Jason R. Williams � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 313

Assessing the Use of Archaeal Lipids as Marine Environmental ProxiesAnn Pearson and Anitra E. Ingalls � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 359

Heat Flow, Heat Generation, and the Thermal Stateof the LithosphereKevin P. Furlong and David S. Chapman � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 385

The Isotopic Anatomies of Molecules and MineralsJohn M. Eiler � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 411

The Behavior of the Lithosphere on Seismic to Geologic TimescalesA.B. Watts, S.J. Zhong, and J. Hunter � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 443

The Formation and Dynamics of Super-Earth PlanetsNader Haghighipour � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 469

Kimberlite VolcanismR.S.J. Sparks � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 497

Differentiated Planetesimals and the Parent Bodies of ChondritesBenjamin P. Weiss and Linda T. Elkins-Tanton � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 529

Splendid and Seldom Isolated: The Paleobiogeography of PatagoniaPeter Wilf, N. Ruben Cuneo, Ignacio H. Escapa, Diego Pol,

and Michael O. Woodburne � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 561

Electrical Conductivity of Mantle Minerals: Role of Waterin Conductivity AnomaliesTakashi Yoshino and Tomoo Katsura � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 605

The Late Paleozoic Ice Age: An Evolving ParadigmIsabel P. Montanez and Christopher J. Poulsen � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 629

Composition and State of the CoreKei Hirose, Stephane Labrosse, and John Hernlund � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 657

Enceladus: An Active Ice World in the Saturn SystemJohn R. Spencer and Francis Nimmo � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 693

Earth’s Background Free OscillationsKiwamu Nishida � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 719

Global Warming and Neotropical Rainforests: A Historical PerspectiveCarlos Jaramillo and Andres Cardenas � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 741

The Scotia Arc: Genesis, Evolution, Global SignificanceIan W.D. Dalziel, Lawrence A. Lawver, Ian O. Norton,

and Lisa M. Gahagan � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 767

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