Climate Intervention: Reflecting Sunlight to Cool Earth (2015)

Summary

Our planet has entered a period in which its climate is changing more rapidly than ever experienced in recorded human history, primarily caused by the rapid buildup of carbon dioxide (CO₂) in the atmosphere from the burning of fossil fuels. Scientists have identified a number of risks from changing climate, including rising sea level, drought, heat waves, more severe storms, increasing precipitation intensity, and associated disruption of terrestrial and aquatic ecosystems. Additionally, elevated atmospheric CO₂ is diffusing into the ocean, measurably acidifying surface waters and affecting marine ecosystems. Natural processes currently remove about half of our emissions from the atmosphere each year. Once emissions cease, it will take thousands of years before those processes eventually return Earth to something like preindustrial levels of atmospheric CO₂.

The two main options for responding to the risks of climate change involve mitigation—reducing and eventually eliminating human-caused emissions of CO₂ and other greenhouse gases (GHGs)—and adaptation—reducing the vulnerability of human and natural systems to changes in climate. A third potentially viable option, currently under development but not yet widely deployed, is carbon dioxide removal (CDR) from the atmosphere accompanied by reliable sequestration. A fourth, more speculative family of approaches called albedo modification seeks to offset climate warming by greenhouse gases by increasing the amount of sunlight reflected back to space.¹ Albedo modification techniques mask the effects of greenhouse warming; they do not reduce greenhouse gas concentrations (see Box S.1 for definitions of key terms).

The Committee on Geoengineering Climate: Technical Evaluation and Discussion of Impacts was charged with conducting a technical evaluation of a limited number of “geoengineering” (also known as “climate engineering”) techniques that have been proposed so far and commenting generally on the potential impacts of deploying these technologies, including possible environmental, economic, and national security concerns. The committee prefers the term “climate intervention” because “geoengineering” has other meanings in the context of geological engineering. Furthermore, the term “engineering” implies a more precisely tailored and controllable process than might be the case for these climate interventions.

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¹ Another speculative approach that seeks to make cirrus clouds thinner to increase the infrared thermal energy returned to space is considered alongside albedo modification approaches.

Climate Intervention—purposeful actions intended to produce a targeted change in some aspect of the climate (e.g., global mean or regional temperature); includes actions designed to remove carbon dioxide or other greenhouse gases from the atmosphere or to change Earth’s radiation balance (referred to as “albedo modification”), but not efforts to limit emissions of greenhouse gases (i.e., climate mitigation).

Carbon Dioxide Removal—intentional efforts to remove carbon dioxide from the atmosphere, including land management strategies, accelerated weathering, ocean iron fertilization, bioenergy with carbon capture and sequestration, and direct air capture and sequestration. CDR techniques complement carbon capture and sequestration methods that primarily focus on reducing CO₂ emissions from point sources such as fossil fuel power plants.

Albedo Modification—intentional efforts to increase the amount of sunlight that is scattered or reflected back to space, thereby reducing the amount of sunlight absorbed by Earth, including injecting aerosols into the stratosphere, marine cloud brightening, and efforts to enhance surface reflectivity.

This study was supported by the National Academy of Sciences, the U.S. intelligence community, the National Oceanic and Atmospheric Administration, the National Aeronautics and Space Administration, and the Department of Energy (the statement of task for the committee can be found in Appendix A). This summary presents overarching conclusions from a pair of reports the committee authored in response to its charge. These reports are intended to provide a thoughtful, clear scientific foundation that informs ethical, legal, and political discussions surrounding these potentially controversial topics.

CARBON DIOXIDE REMOVAL AND ALBEDO MODIFICATION WITHIN A PORTFOLIO OF CLIMATE RESPONSES

There is no substitute for dramatic reductions in the emissions of CO₂ and other greenhouse gases to mitigate the negative consequences of climate change and, concurrently, to reduce ocean acidification. Mitigation, although technologically feasible, has been difficult to achieve for political, economic, and social reasons that may persist well into the future. Whatever we do as a society, some adaptation will be necessary, but the degree to which it is needed depends on the amount of climate change and the degree to which future emissions of CO₂ and other GHGs (henceforth in this context the committee often mentions only CO₂ as it has the largest climate impact) are reduced. Although there are ongoing efforts at climate adaptation in many

communities, both humans and ecosystems face substantial challenges in adapting to the varied impacts of climate change over the coming century. For that reason, it may be prudent to examine additional options for limiting the risks from climate change (namely CDR and albedo modification), which could contribute to a broader portfolio of responses, even as mitigation and adaptation remain the primary emphasis. The committee evaluated CDR and albedo modification within this broader portfolio of climate response.

The deployment of any climate response strategy requires consideration of many factors: How effective is the strategy at achieving predictable and desirable outcomes? How much does the strategy cost to implement at a scale that matters? What are the risks for unintended consequences and opportunities for co-benefits? What governance mechanisms are in place or are needed to ensure that safety, equity, and other ethical aspects are considered (e.g., intergenerational implications)?

As the committee analyzed these factors for specific CDR and albedo modification strategies, it became apparent that there are vast differences in the inherent characteristics of the two approaches. CDR seeks to mitigate the primary causes of present climate change by reducing the amount of CO₂ in the atmosphere. Albedo modification seeks to offset some of the climatic effects of high greenhouse gas concentrations but does not address the greenhouse gas concentrations themselves. The research needs, environmental risks, and political ramifications associated with albedo modification are dramatically different from those associated with carbon dioxide removal (see Table S.1).

Recommendation 1: Efforts to address climate change should continue to focus most heavily on mitigating greenhouse gas emissions in combination with adapting to the impacts of climate change because these approaches do not present poorly defined and poorly quantified risks and are at a greater state of technological readiness.

CARBON DIOXIDE REMOVAL READY FOR INCREASED RESEARCH AND DEVELOPMENT

Some CDR strategies seek to sequester carbon in the terrestrial biosphere or the ocean by accelerating processes that are already occurring as part of the natural carbon cycle and which already remove significant quantities of CO₂ from the atmosphere. These approaches have challenges and risks that need to be assessed, including verifying and monitoring the amount of carbon removed, incomplete understanding

TABLE S.1 Overview of General Differences between Carbon Dioxide Removal Proposals and Albedo Modification Proposals

NOTE: GHG stands for greenhouse gases released by human activities and natural processes and includes carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and others. The committee intends to limit discussion to proposals that raise the fewest problematic issues, thus excluding ocean iron fertilization from the CDR list. Each statement may not be true of some proposals within each category.

of how long carbon may be sequestered before possible rerelease to the atmosphere, unintended effects such as the release of other greenhouse gases that can partially offset or even cancel out the climate benefits from carbon sequestration, and expanded competition for resources such as land and freshwater. In general, published estimates show that land management and reforestation can remove significant

amounts of CO₂ from the atmosphere and can often generate substantial co-benefits. On the other hand, previous studies nearly all agree that deploying ocean iron fertilization at climatically relevant levels poses risks that outweigh potential benefits. However, there may be other methods to enhance uptake of CO₂ through accelerated weathering cycles on land and in the ocean that are more environmentally benign and thus worth pursuing.

Other CDR approaches involve capturing CO₂ from the atmosphere and disposing of it by pumping it underground at high pressure. These include bioenergy with carbon capture and sequestration (BECCS), which uses plants to remove the CO₂ from the air, and direct air capture and sequestration (DACS), which includes various techniques to scrub CO₂ directly from ambient air. Proposals to capture CO₂ from the atmosphere have challenges and uncertainties including cost and maximum scale of feasible deployment. Removing CO₂ from ambient air is more difficult than removing CO₂ from the stack gas of power plants that burn conventional fuel or biomass because of its much lower concentration in ambient air; thus, it will involve higher costs in most circumstances. CDR approaches such as DACS and BECCS require reliable long-term disposal or sequestration of carbon to prevent its return to the atmosphere. Reliable disposal has challenges, environmental risks, and uncertainties, including cost, long-term monitoring, potential induced seismicity, and leakage.

The barriers to deployment of CDR approaches are largely related to slow implementation, limited capacity, policy considerations, and high costs of presently available technologies. Additional research and analysis will provide information to help address those challenges. For these reasons, if carbon removal technologies are to be widely deployed, it is critical to embark now on a research program to lower the technical barriers to efficacy and affordability. In the end, any actions to decrease the excess burden of atmospheric CO₂ serve to decrease, or at least slow the onset of, the risks posed by climate change. Environmental risks vary among CDR approaches but are generally much lower than the risks associated with albedo modification approaches. However, it is also less risky environmentally to avoid a given CO₂ emission to the atmosphere than to emit it with the expectation that it will be purposefully removed from the atmosphere at some later time. Developing the ability to capture and reliably and safely dispose of climatically important amounts of atmospheric CO₂ requires research into how to make the more promising options more effective, more environmentally friendly, and less costly. Such research investments would accelerate this development and could help avoid some of the greatest climate risks that the current carbon emission trajectory poses.

Recommendation 2: The committee recommends research and development investment to improve methods of carbon dioxide removal and disposal at scales that would have a global impact on reducing greenhouse warming, in particular to minimize energy and materials consumption, identify and quantify risks, lower costs, and develop reliable sequestration and monitoring.

• It is increasingly likely that, as a society, we will need to deploy some forms of CDR to avoid the worst impacts of climate change, but without research investment now such attempts at climate mitigation are likely to fall well short of needed targets.
• Many CDR strategies provide viable and reasonably low-risk approaches to reducing atmospheric concentrations of CO₂. Because the rate of CO₂ removal is inherently slow, CDR must be sustained at large scales over very long periods of time to have a significant effect on CO₂ concentrations and the associated risks of climate change.
• Absent some new technological innovation, large-scale CDR techniques have costs comparable to or exceeding those of avoiding carbon dioxide emissions by replacing fossil fuels with low-carbon energy sources. Widespread CDR deployment would likely occur in a policy environment in which there are limits or a price is imposed on emissions of carbon dioxide, and in that case CDR will compete directly with mitigation on a cost basis (i.e., cost per ton of CO₂ removed versus cost per ton of CO₂ emission avoided).
• Decisions regarding deployment of CDR will be largely based on cost and scalability. Carbon dioxide removal strategies might entail some local or even regional environmental risk, but in some cases, CDR strategies may have also substantial co-benefits.
• Several federal agencies should have a role in defining and supporting CDR research and development. The committee recommends a coordinated approach that draws upon the historical strength of the various agencies involved and uses existing coordination mechanisms, such as the U.S. Global Change Research Program, to the extent possible.

ALBEDO MODIFICATION PRESENTS POORLY UNDERSTOOD RISKS

Proposed albedo modification approaches introduce environmental, ethical, social, political, economic, and legal risks associated with intended and unintended consequences. However, there are both theoretical and observational reasons to believe that albedo modification has the potential to rapidly offset some of the consequences of global warming at an affordable cost. If less energy from the Sun is absorbed by the

Earth system, the surface of Earth will cool on average. This is clearly demonstrated by the history of past volcanic eruptions. For example, the eruption of Mount Pinatubo in the Philippines in June of 1991 injected 20 million tons of sulfur dioxide into the stratosphere, which increased Earth’s reflectivity (albedo) and decreased the amount of sunlight absorbed, causing globally averaged surface air temperatures to cool an estimated 0.3°C for a period of 3 years. Such cooling can take place rapidly, within a year of the change in albedo, but only lasts for a few years unless additional material is injected. Increasing the reflectivity of low clouds is another strategy that might be able to cool the planet within a year or two from the onset of the intervention.

Modeling studies indicate that significant cooling, equivalent in amplitude to the warming produced by doubling the CO₂ concentration in the atmosphere, can be produced by the introduction of tens of millions of tons of aerosol-forming gases into the stratosphere. Although there are many reasons to be cautious in interpreting model results, climate simulations can extend scientific understanding of albedo modification to timescales beyond those observed with volcanic eruptions. Modeling results also suggest that the benefits and risks will not be uniformly distributed around the globe.

Feasibility studies (based on models, as yet untested in the field) suggest that it may be possible to introduce aerosols into the stratosphere that can produce significant reduction in incoming sunlight (1 W/m² or more) with few if any major technological innovations required. Direct costs of deployment of a stratospheric aerosol layer of sufficient magnitude to offset global mean radiative forcing of CO₂ have been estimated to be at least an order of magnitude less than the cost of decarbonizing the world’s economy. Although these cost estimates do not include an appropriate monitoring system or indemnification for damages from albedo modification actions, they are small enough that decisions are likely to be based primarily on considerations of potential benefits and risks, and not primarily on the basis of direct cost.

Albedo modification presents a number of risks and expected repercussions. Observed effects from volcanic eruptions include stratospheric ozone loss, changes to precipitation (both amounts and patterns), and likely increased growth rates of forests caused by an increase in diffuse solar radiation. Large volcanic eruptions are by their nature uncontrolled and short lived, and have in rare cases led to widespread crop failure and famine (e.g., the Tambora eruption in 1815). However, effects of a sustained albedo modification by introduction of aerosol particles may differ substantially from effects of a brief volcanic eruption. Models also indicate that there would be consequences of concern, such as some ozone depletion or a reduction in global precipitation associated with sustained albedo modification. Furthermore, albedo modification

does nothing to reduce the buildup of atmospheric CO₂, which is already changing the makeup of terrestrial ecosystems and causing ocean acidification and associated impacts on oceanic ecosystems.

Another risk is that the success of albedo modification could reduce the incentive to curb anthropogenic CO₂ emissions and that albedo modification would instead be deployed with ever increasing intensity. The committee considers it to be irrational and irresponsible to implement sustained albedo modification without also pursuing emissions mitigation, carbon dioxide removal, or both. Climate models indicate that the combination of large-scale albedo modification with large-scale CO₂ increases could lead to a climate with different characteristics than the current climate. Without reductions in CO₂ levels in the atmosphere, the amount of albedo modification required to offset the greenhouse warming would continue to escalate for millennia, generating greater risks of negative consequences if it is terminated for any reason (e.g., undesirable side effects, political unrest, and cost), because the effects of the forcing from the CO₂ concentrations present at the time of termination will be rapidly revealed.

It is not possible to quantify or even identify other environmental, social, political, legal, and economic risks at this time, given the current state of knowledge about this complex system. The uncertainties in modeling of both climate change and the consequences of albedo modification make it impossible today to provide reliable, quantitative statements about relative risks, consequences, and benefits of albedo modification to the Earth system as a whole, let alone benefits and risks to specific regions of the planet. To provide such statements, scientists would need to understand the influence of various possible activities on both clouds and aerosols, which are among the most difficult components of the climate system to model and monitor. Introducing albedo modification at scales capable of substantial reductions in climate impacts of future higher CO₂ concentrations would be introducing a novel situation into the Earth system, with consequences that are poorly constrained at present.

Gaps in our observational system also present a critical barrier to responsible deployment of albedo modification strategies. Currently, observational capabilities lack the capacity to monitor the evolution of an albedo modification deployment (e.g., the fate of the aerosols and secondary chemical reactions), its effect on albedo, or its environmental effects on climate or other important Earth systems. Finally, an international forum for cooperation and coordination on any sort of climate intervention discussion and planning is lacking.

Recommendation 3: Albedo modification at scales sufficient to alter climate should not be deployed at this time.

• Albedo modification strategies for offsetting climate impacts of high CO₂ concentrations carry risks that are poorly identified in their nature and unquantified.
• Deployment at climate-altering amplitudes should only be contemplated armed with a quantitative and accurate understanding of the processes that participate in albedo modification. This understanding should be demonstrated at smaller scales after intended and unintended impacts to the Earth system have been explicitly documented, both of which are lacking.
• There is significant potential for unanticipated, unmanageable, and regrettable consequences in multiple human dimensions from albedo modification at climate-altering scales, including political, social, legal, economic, and ethical dimensions.
• Current observing systems are insufficient to quantify the effects of any intervention. If albedo modification at climate-altering scales were ever to occur, it should be accompanied by an observing system that is appropriate for assessing the impacts of the deployment and informing subsequent actions.
• If research and development on albedo modification were to be done at climate-altering scales, it should be carried out only as part of coordinated national or international planning, proceeding from smaller, less risky to larger, more risky projects; more risky projects should be undertaken only as information is collected to quantify the risks at each stage.

THE NEED FOR MORE RESEARCH ON ALBEDO MODIFICATION

There are many research opportunities that would allow the scientific community to learn more about the risks and benefits of albedo modification, knowledge which could better inform societal decisions without imposing the risks associated with large-scale deployment. There are several hypothetical, but plausible, scenarios under which this information would be useful. For example:

• If, despite mitigation and adaptation, the impacts of climate change still become intolerable (e.g., massive crop failures throughout the tropics), society would face very tough choices regarding whether and how to deploy albedo modification until such time as mitigation, carbon dioxide removal, and adaptation actions could significantly reduce the impacts of climate change.
• The international community might consider a gradual phase-in of albedo modification to a level expected to create a detectable modification of Earth’s

• climate, as a large-scale field trial aimed at gaining experience with albedo modification in case it needs to be scaled up in response to a climate emergency. This might be considered as part of a portfolio of actions to reduce the risks of climate change.

• If an unsanctioned act of albedo modification were to occur, scientific research would be needed to understand how best to detect and quantify the act and its consequences and impacts.

In any of these scenarios, better understanding of the feasibility, verifiability, consequences (intended and unintended), and efficacy of proposed albedo modification strategies would be critical. Indeed, current implementation options are clearly crude and developing better methods in advance of any future development would provide less risky options for society and state actors to consider. There is a risk that research on albedo modification could distract from efforts to mitigate greenhouse gas emissions. This “moral hazard” risk may have kept more albedo modification research from being done up to now. The committee argues that, as a society, we have reached a point where the severity of the potential risks from climate change appears to outweigh the potential risks from the moral hazard associated with a suitably designed and governed research program. Hence, it is important to understand whether and to what extent albedo modification techniques are viable.

Much of the required research on albedo modification overlaps considerably with the basic scientific research that is needed to improve understanding of the climate system. Examples of such “multiple benefit research”—research that can contribute to a better understanding of the viability of albedo modification techniques and also a better understanding of basic climate science—include conducting research on clouds and aerosols, maintaining the continuity of measurement of the top-of-atmosphere radiation budget, and monitoring ocean-atmosphere energy exchange through programs such as the Argo float system. Of necessity, much of this multiple-benefit research would be part of a comprehensive climate research portfolio or research program aimed at other purposes (e.g., effect of volcanic eruptions on aerosols). In addition, the committee argues that research topics specific to albedo modification should also be identified and prioritized as part of a larger research effort and tasked to the relevant federal agencies for possible support within existing or expanded research programs.

Recommendation 4: The committee recommends an albedo modification research program be developed and implemented that emphasizes multiple-benefit research that also furthers basic understanding of the climate system and its human dimensions.

• If future decision makers reach a point that they are contemplating adopting albedo modification, or assessing such an adoption by others, they will need to assess a wide range of factors, both technical and social, to compare the potential benefits and risks of an albedo modification deployment. These factors would include an assessment of the expected climate with only emissions reductions and CDR (including risks from continued greenhouse gas emissions with no intervention), the expected effects from starting albedo modification, the expected effects from terminating albedo modification, ethical issues, and social responses.
• The goal of the research program should be to improve understanding of the range of climate and other environmental effects of albedo modification, as well as understanding of unintended impacts.
• U.S. research on albedo modification should be supported by a number of scientific research agencies in a coordinated manner. The U.S. Global Change Research Program could provide valuable oversight and coordination to ensure that the aspects of the research that are of benefit to both basic climate science and understanding of albedo modification are taken into account.
• Small-scale field experiments with controlled emissions may for some situations with some forms of intervention be helpful in reducing model uncertainties, validating theory, and verifying model simulations in different conditions. Experiments that involve release of gases or particles into the atmosphere (or other controlled perturbations) should be well-enough understood to be benign to the larger environment, should be conducted at the smallest practical scales, should be designed so as to pose no significant risk, and should be planned subject to the deliberative process outlined in Recommendation 6.

Recommendation 5: The committee recommends that the United States improve its capacity to detect and measure changes in radiative forcing and associated changes in climate.

• A new generation of short-wavelength (albedo) and long-wavelength (outgoing infrared) space-based instruments should be developed and deployed that can measure radiative forcing with an accuracy of better than 1 W/m², including hyperspectral instruments that could improve discrimination of the processes that cause changes in radiative forcing. Such instruments would significantly improve understanding of the effects of clouds and stratospheric aerosols on climate, improve the ability to predict the effects of albedo modification, and provide an ability to detect large-scale albedo modification by unilateral and uncoordinated actors.

• An observational capability should be developed to make better use of future major volcanic eruptions to improve understanding of the effects of stratospheric aerosols on climate. This would involve space-based sensors and rapidly deployable ground-based and airborne sensors for monitoring stratospheric aerosols.

GOVERNANCE CONSIDERATIONS

Some types of research into intentional albedo modification will likely have legal, ethical, social, political, economic, and other important ramifications. Albedo modification research must abide by existing laws, regulations, and policies that apply to research broadly and its impacts on worker safety, the environment, and human and animal welfare. However, such research is not specifically addressed by any federal laws or regulations.

Given the perceived and real risks associated with some types of albedo modification research, open conversations about the governance of such research, beyond the more general research governance requirements, could encourage civil society engagement in the process of deciding the appropriateness of any research efforts undertaken.

“Governance” is not a synonym for “regulation.” Depending on the types and scale of the research undertaken, appropriate governance of albedo modification research could take a wide variety of forms ranging from the direct application of existing scientific research norms, to the development of new norms, to mechanisms that are highly structured and extensive. The most appropriate type of governance structures for albedo modification research will potentially depend on the nature and scale of that research. It is not the purview of the committee to make an assessment or recommendation of the appropriate structure. However, the committee does believe that governance considerations should be targeted at ensuring civil society involvement in decision making through a transparent and open process. It should focus on enabling safe and useful research on the viability and impacts of albedo modification strategies. Ultimately, the goal is to ensure that the benefits of the research are realized to inform civil society decision making, the associated challenges are well understood, and risks are kept small.

Recommendation 6: The committee recommends the initiation of a serious deliberative process to examine (a) what types of research governance, beyond those that already exist, may be needed for albedo modification research, and (b) the types of research that would require such governance, potentially based on the magnitude

of their expected impact on radiative forcing, their potential for detrimental direct and indirect effects, and other considerations.

• If a new governance structure is determined to be needed based on deliberations among governance experts and civil society representatives, the development of the governance structure should consider the importance of being transparent and having input from a broad set of stakeholders to ensure trust among the stakeholders and appropriate consideration of all dimensions.
• Such a governance structure should consider setting clear and quantitative guidelines for experimentation and be responsive to domestic and international laws and treaties.
• The deliberative process should consider focusing on research activities that involve injecting material into the atmosphere, for example aerosol-producing substances injected into the upper atmosphere or cloud-brightening substances injected near the surface.
• If a program of research in albedo modification includes controlled-emission experiments, it should provide for a sufficiently specific governance regime to at least define the scale of experiments at which oversight begins.
• The approach to governance should consider the need for increasing supervision as the scope and scale of the research and its potential implications increase, including the amount of material emitted, the area affected, and the length of time over which emission continues.
• The goal of the governance should be to maximize the benefits of research while minimizing risks.
• The United States should help lead the development of best practices or specific norms that could serve as a model for researchers and funding agencies in other countries and could lower the risks associated with albedo modification research.

CONCLUDING THOUGHTS

Addressing the challenges of climate change requires a portfolio of actions that carry varying degrees of risk and efficacy. CDR strategies and other technologies and approaches that reduce net emissions (e.g., carbon capture and sequestration, non-carbon-based energy, and energy efficiency improvements) offer the potential to slow the growth and reverse the increase of CO₂ concentrations in the atmosphere. The lowest-risk CDR strategies are currently limited by cost and at present cannot achieve the desired result of removing climatically important amounts of CO₂ beyond the significant removal already performed by natural processes. However, with declining

costs and stronger regulatory commitment, atmospheric CO₂ removal could become a valuable component of the portfolio of long-term approaches to reducing CO₂ concentrations in the atmosphere and associated impacts. Overall, there is much to be gained and very low risk in pursuing multiple parts of a portfolio of CDR strategies that demonstrate practical solutions over the short term and develop more cost-effective, regional-scale and larger solutions for the long term.

In contrast, even the best albedo modification strategies are currently limited by unfamiliar and unquantifiable risks and governance issues rather than direct costs. The committee reiterates that it is opposed to climate-altering deployment of albedo modification techniques, but it does recommend further research, particularly multiple-benefit research that furthers the basic understanding of the climate system and seeks to quantify the potential costs, consequences (intended and unintended), and risks from these proposed albedo modification techniques.

Climate change is a global challenge that will require complex and comprehensive solutions, which in turn will require that people of many nations work together toward common objectives. For the outcome to be as successful as possible, any climate intervention research should be robust, open, likely to yield valuable scientific information, and international in nature. The impacts of any potential future climate interventions should be honestly acknowledged and fairly considered. The committee firmly believes that there is no substitute for dramatic reductions in CO₂ emissions to mitigate the negative consequences of climate change at the lowest probability of risk to humanity. However, if society ultimately decides to intervene in Earth’s climate, the committee most strongly recommends any such actions be informed by a far more substantive body of scientific research than is available at present.