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 this order (Houghton 2008). Hence, this value may represent the maximum potential land carbon storage.
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.
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 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.
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 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.
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 becomesmore 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 + H2O + CO2 → Ca2+ + 2HCO3 − . 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 + H2O → Ca2+ + 2HCO3 − + SiO2. 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: 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 atmosphericCO2 (Kheshgi 1995). Alternatively, carbonateminerals 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 fertilization could acidify the deep ocean by storing more CO2 there (Cao & Caldeira 2010b) and could increase releases of the greenhouse gasN2O, 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).
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 individualCDRmethods have only marginal potential to affect atmosphericCO2 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 climatemitigation targets, such asCO2 stabilization below 400 ppm this century (Matthews 2010).
Table 3 CDR methods and their characteristics
Abbreviations: BECS, biomass energy with CO2 sequestration; CDR, carbon dioxide removal; N/A, not applicable; Pg C, petagrams of carbon.
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 dissolvedCO2 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 removeCO2 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).
CDRmethods 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 would be with higher CO2 concentrations. Themain 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 ofCO2 from the atmosphere is environmentally equivalent to the reduction of emissions. If used on a sufficiently large scale and if otherCO2 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
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 producingmajor 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.
The authors are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review. K.C.’s name is on several patents, some of which could conceivably be used for the purposes of intentional climate modification, but if any of these patents is ever used for the purposes of altering climate, any proceeds that accrue to K.C. for this use will be donated to nonprofit nongovernmental organizations and charities. K.C. has no expectation of or interest in developing a personal revenue stream based on the use of these patents for climate modification.
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