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EA41CH10-Caldeira ARI ANNUAL REVIEWS 19 April 2013 15:34 Further 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. Click here for quick links to Annual Reviews content online, including: • Other articles in this volume • Top cited articles • Top downloaded articles • Our comprehensive search 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. 231 EA41CH10-Caldeira ARI 19 April 2013 15:34 1. INTRODUCTION 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. 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. 232 Caldeira · · Bala Cao 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. EA41CH10-Caldeira ARI 19 April 2013 15:34 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 www.annualreviews.org • The Science of Geoengineering 233 EA41CH10-Caldeira ARI 19 April 2013 15:34 Table 1 Summary of solar geoengineering proposals Relative risk to 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. 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 234 Caldeira · · Bala Cao EA41CH10-Caldeira ARI 19 April 2013 15:34 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. 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 www.annualreviews.org • The Science of Geoengineering 235 EA41CH10-Caldeira ARI 19 April 2013 15:34 17 a 16 Surface air temperature (°C) 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. 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 236 Caldeira · · Bala Cao EA41CH10-Caldeira ARI 19 April 2013 15:34 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. 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 www.annualreviews.org • The Science of Geoengineering 237 EA41CH10-Caldeira ARI 19 April 2013 15:34 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). 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. 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 238 Caldeira · · Bala Cao EA41CH10-Caldeira ARI 19 April 2013 15:34 3.30 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. 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 www.annualreviews.org • The Science of Geoengineering 239 ARI 19 April 2013 15:34 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. 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. EA41CH10-Caldeira 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. 240 Caldeira · · Bala Cao EA41CH10-Caldeira ARI 19 April 2013 15:34 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. 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 www.annualreviews.org • The Science of Geoengineering 241 EA41CH10-Caldeira ARI Ocean fertilization 19 April 2013 15:34 Ocean alkalinity addition Accelerated chemical weathering of rocks Manufacturing carbonate minerals using silicate rocks and CO2 from air Direct air CO2 capture Afforestation/ reforestation 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. 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 242 Caldeira · · Bala Cao EA41CH10-Caldeira ARI 19 April 2013 15:34 Table 2 Taxonomy of CDR approachesa 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. 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 www.annualreviews.org • The Science of Geoengineering 243 EA41CH10-Caldeira ARI 19 April 2013 15:34 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. 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 244 Caldeira · · Bala Cao 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. EA41CH10-Caldeira ARI 19 April 2013 15:34 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 www.annualreviews.org • The Science of Geoengineering 245 EA41CH10-Caldeira ARI 19 April 2013 15:34 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. 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. 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 . 246 Caldeira · · Bala Cao (2) EA41CH10-Caldeira ARI 19 April 2013 15:34 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: 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. 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 www.annualreviews.org • The Science of Geoengineering 247 ARI 19 April 2013 15:34 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). 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. EA41CH10-Caldeira 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 248 Caldeira · · Bala Cao EA41CH10-Caldeira ARI 19 April 2013 15:34 Table 3 CDR methods and their characteristics 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. 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 www.annualreviews.org • The Science of Geoengineering 249 EA41CH10-Caldeira ARI 19 April 2013 15:34 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 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. 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. 250 Caldeira · · Bala Cao EA41CH10-Caldeira ARI 19 April 2013 15:34 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. LITERATURE CITED Ackerman TP, Flynn DM, Marchand RT. 2003. 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Feasibility of ocean fertilization and its impact on future atmospheric CO2 levels. Geophys. Res. Lett. 32:L09703 256 Caldeira · · Bala Cao EA41-FrontMatter ARI 7 May 2013 Annual Review of Earth and Planetary Sciences 7:19 Contents 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 EA41-FrontMatter ARI 7 May 2013 7:19 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 ix