On June 30, Joe Biden’s administration endorsed a new program to study the feasibility and consequences of solar radiation modification (SRM). Also known as solar geoengineering, SRM refers to human efforts to reduce the warming effects of sunlight on accumulating greenhouse gases by reflecting a small percentage of that light back into space. While declaring that the United States has no current plans to develop and deploy such technologies, the report supports a five-year, U.S. government–wide research program to study SRM as a supplementary strategy to address climate change, alongside but subordinate to the existing strategies of emissions reductions, carbon dioxide removal (CDR), and adaptation.
SRM has long been the proverbial third rail of public discourse on climate change. It is untried and untested, and many critics oppose even its research for fear of normalizing it as a policy option. Beyond possible unintended consequences, some worry that it could introduce what economists call a moral hazard by giving governments, corporations, and consumers a get-of-jail-free card to continue their polluting ways, rather than doubling down on emissions reductions. Moreover, it would not be a solution to climate change, since it would not return the climate to its original, preindustrial state. Unless supplemented by emissions reductions and CDR, CO2 levels would continue to rise, guaranteeing continued ocean acidification.
The prospect of SRM also raises ethical dilemmas, including tricky questions of procedural, distributional, and intergenerational justice. If humanity proceeds down this route, for instance, who should have control over the global thermostat—and under what authority? In addition, even if it proved effective, SRM could have important distributional consequences, affecting precipitation patterns and ecosystems in some regions and communities more than others. Finally, SRM could lock humanity into decades of climate intervention, since ceasing it before greenhouse gases had been reduced would result in an abrupt spiking of temperatures, a phenomenon known as termination shock.
These concerns should be taken seriously and evaluated. At the same time, they need to be analyzed and assessed in the context of the dangers posed by the planet’s current climate trajectory, as well as the potential benefits of SRM. In principle, SRM could provide a low-cost and high-leverage way to hold global temperatures steady or even reduce them to their preindustrial norm while the world makes slow, steady progress on reducing emissions, developing and scaling up innovative technologies and nature-based approaches to sequester and store atmospheric carbon, and adapting to the direct and indirect impacts of climate change. In sum, it could buy humanity some time, shaving the peak of the warming that is otherwise inevitable over the next several decades. It thus merits consideration as a fourth strategy for managing climate risk, alongside emissions reductions, CDR, and adaptation.
In principle, SRM could be accomplished in various ways. These include dispersing tiny reflective particles of sulfate, calcite, or other materials in the upper atmosphere—a process known as stratospheric aerosol injection (SAI). Other possibilities include lofting sea salt particles from ships or ocean-based platforms into the lower atmosphere (or troposphere), an approach known as marine cloud brightening (MCB); deploying mirrors in space to reflect incoming sunlight; or brightening the Earth’s surface by planting particular crops, painting rooftops white, or other methods. Scientists estimate that humanity could hold temperatures steady by reflecting just 1 percent of incoming sunlight and that some methods could be relatively inexpensive. SAI, for instance, might be accomplished for as little as $20 billion to support a fleet of continually flying aircraft—a minute fraction of the hundreds of trillions of dollars it will cost to decarbonize the global economy to achieve net zero by 2050.
Both SAI and MCB, which are the focus of the White House report, would mimic processes that already occur, both natural and man-made. The 1991 eruption of Mount Pinatubo in the Philippines, for instance, ejected sulfur and other materials into the stratosphere, cooling the Earth by 0.6 degrees Celsius over fifteen months. Similarly, particulate pollution from economic activity currently makes the planet 0.5–1.1 degrees Celsius cooler than it would otherwise be. MCB, meanwhile, would mimic the brightening effect created by pollution from merchant vessels at sea—visible as ship tracks in satellite images.
Two things have been missing from the SRM debate. The first is an adequate understanding of its feasibility and the potential consequences for Earth’s natural systems and peoples’ well-being. In the absence of such knowledge, the world is flying blind. The second is an institutional framework to govern SRM research, to say nothing of any potential deployment.
These gaps are untenable. The world needs a solid empirical foundation—grounded in natural and social science, as well as ethics and law—to decide whether SRM makes sense. It also needs norms and rules to govern relevant research and any future deployment of SRM. In the absence of such rules, individual governments (or even private actors) could go rogue, launching unilateral or competing SRM programs, with destabilizing political and geopolitical consequences—a danger the U.S. intelligence community has highlighted.
Recognizing these gaps, Congress in its Consolidated Appropriations Act of 2022 directed the White House Office of Science and Technology Policy (OSTP) to work with U.S. agencies to develop a “research and governance framework to provide guidance on transparency, engagement, and risk management for publicly funded work in solar geoengineering research.”
Last week’s White House report is the result. It proposes a comprehensive, five-year federal program to support SRM research and expand international cooperation on its governance, to be undertaken under the auspices of the U.S. Global Change Research Program, comprised of more than a dozen U.S. government agencies. The most important in this context are the National Oceanic and Atmospheric Administration, the National Aeronautics and Space Administration, and the Department of Energy. All three already conduct valuable research on climate science related to SRM, though their budgets are small compared to what is required and their efforts are insufficiently aligned.
The White House report breaks new ground in several ways. Most significantly, it adopts an explicit “risk versus risk” framing to the question of SRM. That is, it insists that any decisions about research and ultimate deployment must be considered not in isolation but against the catastrophic and growing dangers inherent in the unintentional experiment that humanity is already running by pumping massive quantities of greenhouse gases into the atmosphere. These dangers are rising as critical components of the Earth system—from Antarctic ice shelves and Arctic permafrost to the Amazon rainforest and coral reefs—appear to be approaching critical tipping points. Transgressing these thresholds could bring about abrupt, nonlinear changes that themselves exacerbate global warming.
The document is cautious and measured. The authors underscore that the report “should not be interpreted as an endorsement of implementation of SRM,” and they are forthright in enumerating its uncertainties. At the same time, they insist that the potential dangers of SRM must be studied and weighed against those posed by climate change. The only route to informed and ethical decisionmaking is through greater scientific knowledge, accompanied by a responsible governance structure for such research. These conclusions are broadly consistent with the findings of the influential 2021 report from the National Academies of Science, Engineering and Medicine, “Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance.”
The new report identifies several priorities for any U.S.-government-funded SRM program. These include improving scientific understanding of the relevant physical science, developing scenarios for the impact of alternative SRM approaches, examining the potential socioeconomic and ecological implications of their implementation, and enhancing international cooperation on SRM research efforts.
Each element is important. Scientists currently lack adequate understanding of basic natural science relevant to SRM, including about the composition of the atmosphere and the dynamics of cloud-aerosol interactions. To fill these lacunae, the plan calls for enhanced observation capabilities, including from satellites and aircraft platforms; expanded computer modeling efforts; and augmented systems to monitor and detect potential SRM deployments by other governments. Of note, the report endorses limited outdoor experiments—a politically contentious issue—to better understand the processes that would be involved in actual SRM deployment. The White House also calls for an “international scientific assessment of the state of understanding of SRM methods,” although it disappointingly stops short of identifying any authoritative multilateral body—such as the Intergovernmental Panel on Climate Change (IPCC)—that should assume this responsibility.
In parallel, the report calls for the U.S. government to develop “a standard set of SRM scenarios.” This would allow U.S. agencies and funded partners to consider the costs and benefits of alternative types of solar climate interventions and to compare these with the “risks associated with plausible climate change scenarios not involving SRM.”
Importantly, the report insists that U.S.-supported research should consider not simply the scientific feasibility of SRM technologies but also their potential impacts on human well-being and Earth’s natural ecosystems. Like climate change itself, the authors note, any solar climate intervention is likely to raise “cultural, moral, and ethical considerations” that cannot be captured by scientific modeling, such as the impact of SRM on many different types of communities and future generations. Accordingly, the research program must engage social scientists and philosophers alongside physical scientists and engineers. The report likewise acknowledges that SRM will likely have tangible environmental impacts, including on biodiversity, rainfall, food and water security, and many other ecosystem services that nature provides. Research efforts should aim to predict and compare those effects with the anticipated ecological consequences of climate change.
Finally, the White House report calls for “international cooperation on solar radiation modification research.” This is vital on several grounds. Among other benefits, a collaborative approach could help defuse any global suspicions that the United States is planning to impose SRM on the world; facilitate the global sharing of scientific knowledge; and nurture partnerships with scientists in developing countries, including fragile states most vulnerable to climate change and, potentially, most affected by future climate interventions.
While the administration should be commended for its broad commitment to international cooperation, the White House report provides few details about how this collaboration should unfold or what multilateral venues might be most promising. It is silent on existing frameworks for scientific collaboration (beyond a brief mention of the World Climate Research Program of the World Meteorological Organization). And it stops short of identifying any institution—such as the IPCC or the Montreal Protocol’s assessment panels—that might produce an authoritative report on the current state of SRM science. Nor does the report mention recent efforts within other multilateral frameworks, such as the United Nations Environment Assembly, to weigh in on this topic.
This caution is unsurprising given the political and diplomatic sensitivity surrounding SRM, but it is also a missed opportunity. Equally disappointing, the report does not address the need for an anticipatory international framework to govern any future deployment of solar climate intervention. As I have written elsewhere, the growing risk of unilateral action and the potential geopolitical fallout from such decisions necessitate international discussions today on the precise multilateral norms and rules of the road that should govern any planetary-scale SRM interventions in the future.
Yet the White House report is an invaluable contribution to the ongoing debate on SRM and the principles that should inform both scientific research and international outreach. One useful suggestion is that the U.S. government may wish to limit cooperation on SRM research to international partners that have “a strong commitment to acting transparently” and are committed to emissions reductions, “lest it appear that research on SRM would somehow be at the expense of mitigation.”
By coincidence, the White House report came just two days after the European Commission released its own report on the “climate and security nexus.” While the latter’s discussion of SRM is limited to a single paragraph, the commission’s endorsement of research collaboration and exploratory discussions of its international governance have attracted considerablemedia attention. Given the near-simultaneous appearance of these two documents, the timing could be propitious for a transatlantic—and ideally global—dialogue on multilateral frameworks needed to advance scientific research and global governance on this high-stakes response to climate change.