Geoengineering proposals to deliberately alter the climate in response to the greenhouse effect have appeared in scientific advisory reports since at least the 1960s. Until recently, these proposals generally remained outside the mainstream discussions of climate policy, which focused either on strategies to reduce emissions or adapt to climate change impacts. However, there is growing concern among many scientists that the lack of progress on emissions reductions will lead to gradual increases in atmospheric concentrations of CO beyond a threshold that could prevent substantial impacts to human health and environmental systems. Furthermore, there is also concern about the existence of “tipping points,” where the earth’s climate system reaches a threshold that unexpectedly results in abrupt and severe changes. One example would be the rapid collapse of the West Antarctic Ice Sheet—which would lead to a large and sudden contribution to sea level rise. These concerns have led to increased interest in geoengineering as a potential tool to help reduce the impacts of climate change, although the NRC study noted that few, if any, individuals are promoting geoengineering as a near-term alternative to emissions reductions. While both CDR and SRM are intended to reduce global temperatures, there are substantial differences in how CDR and SRM operate on the climate system, the timescales required for results, and their associated risks and trade-offs. Consequently, CDR and SRM are often discussed separately. The Royal Society identified several CDR approaches that would directly remove CO in the ocean or on land. Examples of ocean-based CDR approaches include: Enhanced removal by physical processes. Enhanced upwelling/downwelling—altering ocean circulation patterns to bring deep, nutrient rich water to the ocean’s surface (upwelling), to promote phytoplankton growth—which removes CO-rich water from the surface of the ocean to the deep-sea (downwelling). Enhanced removal by biological processes. Ocean fertilization— introducing nutrients such as iron, phosphorus, or nitrogen to the ocean surface to promote phytoplankton growth. The phytoplankton remove CO is transported to the deep ocean as detritus. Enhanced removal by chemical processes. Ocean-based enhanced weathering— accelerating chemical reactions between certain minerals and CO to a nongaseous state. Methods include adding chemically reactive alkaline minerals, such as limestone or silicates, to the ocean to increase the ocean’s natural ability to absorb and store COExamples of land-based CDR approaches include: Physical removal by industrial processes. Direct air capture— technology-based processing of ambient air to remove CO from the atmosphere. The resulting stream of pure CO Biomass energy with CO capture and geological sequestration— harvesting vegetation and using it as a fuel source with capture and storage of the resulting emissions in geological formations (geological sequestration). Biomass for sequestration—harvesting of vegetation and sequestering it as organic material by burying trees or crop wastes, or as charcoal (biochar). Afforestation and land-use management—the planting of trees on lands that historically have not been forested, or otherwise managing vegetation cover to maximize CO, which convert the CO into a geological formation of reactive minerals. The Royal Society identified several SRM approaches that would reflect a small percentage of incoming sunlight back to space, as shown in figure 2. SRM approaches are generally discussed in terms of which sphere they would act upon—space, the atmosphere, or the earth’s surface. Examples of SRM approaches include: Space-based methods. Reflecting or deflecting incoming solar radiation using space-based shielding materials, such as mirrors. Atmosphere-based methods. Stratospheric aerosol injection—injecting reflective aerosol particles into the stratosphere to scatter sunlight back into space. Although it is possible that a wide range of particles could serve this purpose, most attention has been on sulfur particles—partly because temporary global cooling has been produced in the past by volcanic eruptions. Cloud-brightening—adding sea salt or other cloud condensation surfaces to low-level marine clouds to increase their ability to reflect sunlight before it reaches the earth’s surface. Surface-based methods. Increasing the reflectivity of the earth’s land or ocean surfaces through activities such as painting roofs white, planting more reflective crops or other vegetation, or covering desert or ocean surfaces with reflective materials. According to the NRC and Royal Society studies, geoengineering is one of several potential tools to limit the impact and consequences of climate change. However, these studies state that geoengineering is a potential complement to, rather than a substitute for, sharp reductions in greenhouse gas emissions. For example, while geoengineering includes a range of approaches with varying degrees of potential effectiveness and consequences—no geoengineering approach can provide an easy or risk- free alternative solution to the problem of climate change, according to the Royal Society study. For example, compared to current CDR proposals, using SRM to divert incoming sunlight would relatively quickly produce a cooling effect to counteract the warming influence of increased atmospheric concentrations of greenhouse gases. However, SRM does not address the rising atmospheric concentration of greenhouse gases produced by human activity, and therefore would not reduce other serious climate change impacts such as ocean acidification. Furthermore, according to these studies, both CDR and SRM involve additional environmental risks or other trade-offs. For example, ocean- based CDR approaches, such as ocean fertilization, could have unanticipated negative impacts on ocean ecosystems. Additionally, the large-scale deployment of certain land-based CDR approaches, such as afforestation, land-use management for sequestration, and biomass for energy or burial, create trade-offs for land use. In general, the Royal Society study found that compared to CDR, most SRM approaches are associated with a higher risk of negative environmental effects, such as negative impacts on regional temperature or precipitation. For example, one study found that combining a reduction of incoming solar radiation with high levels of atmospheric CO could have substantial impacts on regional precipitation—potentially leading to reductions that could create droughts in some areas. Additionally, the Royal Society study said that increasing the reflectivity of desert or ocean surfaces could have major impacts on desert or ocean ecosystems. Moreover, this study indicated that the artificial balance between increased greenhouse gas concentrations and reduced solar radiation created by large-scale deployment of an SRM approach would need to be maintained over decades and possibly centuries or longer. Experts said that geoengineering is an emerging field, with relatively few experiments or other studies conducted and with major uncertainties remaining. We found that more is known about certain CDR approaches, since related laboratory and field experiments have been conducted, whereas there is limited understanding of other CDR approaches and SRM. Moreover, major uncertainties remain regarding the scientific, legal, political, economic, and ethical implications of researching or deploying geoengineering. : Update and Recommendations (Paris: 2007). energy requirements, and scalability. Similarly, the Royal Society study found that both land- and ocean-based enhanced weathering CDR approaches could potentially store a large amount of carbon, but face barriers to deployment such as scale, cost, and possible environmental consequences. This report also found that while some other land-based CDR approaches—such as afforestation, land-use management techniques, and biomass for energy or burial—can remove CO on a global scale is low. Other CDR approaches have been the focus of relatively few laboratory and field experiments, and fundamental questions remain about their potential efficacy. For example, according to the Royal Society and NRC studies, while ocean fertilization has received some sustained research activity, its potential to remove CO For example, according to researchers who designed a 2009 joint German and Indian iron fertilization experiment, their experiment was designed to test a range of scientific hypotheses pertaining to the structure and functioning of Southern Ocean ecosystems and their potential impact on global cycles of biologically-generated elements, such as carbon and nitrogen. Furthermore, these researchers noted that future long-term experiments to study phytoplankton blooms and their effect on the deep ocean and underlying sediments would have to be much larger than experiments to date. According to our review of relevant studies and expert interviews, understanding of SRM is more limited than that of CDR because there have been few laboratory experiments, field experiments, or computer modeling efforts. Two of the most frequently discussed SRM approaches are stratospheric aerosol injection and cloud-brightening, according to many of the scientific experts we spoke with. For stratospheric aerosol injection, some of the experts said that research to date consisted primarily of a few modeling analyses. They also said that more work would need to be done to assess whether this approach could reduce incoming solar radiation without serious consequences. For example, one study identified the potential for regional impacts on precipitation— potentially leading to drought in some areas. Based on our literature review and interviews with experts, to date only one study has been published for a field experiment related to SRM technologies—a 2009 Russian experiment that injected aerosols into the middle troposphere to measure their reflectivity. Similarly, in the case of cloud-brightening, several experts said that there currently is not enough research to assess its effectiveness or impacts. According to the 2010 NRC study, other methods for SRM, including using space-based reflectors and increasing the solar reflectivity of buildings or plants, have limited potential, either due to the cost of deployment or the limited potential to affect the climate. Experts we interviewed and relevant studies identified several major uncertainties in the field of geoengineering that are in need of further investigation. These uncertainties ranged from important scientific questions for CDR and SRM, to political, ethical, and regulatory issues. Areas that merit further investigation include: Technical feasibility and effectiveness of SRM and certain CDR approaches. Experts we interviewed and the Royal Society and NRC studies agreed that SRM approaches generally were not researched sufficiently to be considered well-understood or technically feasible.