The Future of Solar Geoengineering Research

Solar geoengineering, also known as solar radiation modification (SRM), encompasses a set of risky, untested, and yet potentially globally beneficial approaches that might be used in the future to help address the growing risks of climate change—especially if paired with more aggressive efforts toward mitigating greenhouse gas emissions, carbon dioxide removal, and adaptation.

The most well-studied of these SRM approaches is stratospheric aerosol injection, which would involve emitting millions of tons of sulfates into the stratosphere from specially designed airplanes, thus reflecting a small percentage of incoming sunlight and cooling the Earth. Stratospheric aerosol injection is notable, because it would be extremely cheap relative to current strategies for mitigating climate change as well as quickly effective at slowing, halting, or even reversing warming. However, the method comes with both biophysical and social risks that are not fully understood.

Interest in SRM is growing as climate impacts become increasingly dire. Yet, much about the approach is uncertain; thus, much more research in climate science and the social sciences is needed before policymakers should ever consider developing an SRM deployment capability. Ongoing and robust public engagement with a diverse set of global stakeholders and communities will be crucial, especially given that the global effects of SRM may be felt differently across different regions. The development of transparent, democratic, and inclusive global governance will be necessary to promote good decisionmaking and just outcomes from SRM. However, such governance structures have not yet been created.

As part of an ongoing exploration of these and related issues surrounding SRM, Resources for the Future (RFF) hosted a two-day gathering of international experts from a range of disciplines. At the event, “Social Geoengineering Futures: Interdisciplinary Research to Inform Decisionmaking,” participants explored the implications of SRM as a set of emerging technologies. Seven themes emerged from the discussions, all of which are ripe for further research.

The Role of New Information in Making Decisions

At a minimum, when decisionmakers engage with this issue, they likely will want to start with well-documented assessments of SRM. With ongoing but disparate research efforts around the world, the time may be right for such a comprehensive international assessment of SRM. If and when an assessment is warranted, the Intergovernmental Panel on Climate Change may be the best candidate organization and bring legitimacy to the effort. When this first assessment does happen, the conclusion merely may be that more research is needed. That is fine. This field is still young, and it is probably too early to make definitive conclusions about any key questions.

What new information about SRM can best help future decisionmakers with their choices about potential SRM development or deployment? SRM research in the natural, physical, and social sciences includes modeling, lab experiments, small-scale outdoor field experiments, and eventually larger-scale outdoor field experiments. Related social science research includes political and economic analysis, game-theory simulation, expert elicitations and surveys, and other approaches.

If SRM ever is deployed, it likely would occur within a portfolio of other ongoing strategies to reduce the risks of climate change, which would include emissions mitigation, carbon dioxide removal, and adaptation. But what is the best timing of these different policies?

Is SRM a tool that arises only if the world fails to achieve the goal of the Paris Agreement to keep the global increase in temperature below 1.5°C? Or might SRM arise as an option once net-zero emissions are achieved, but as warming continues? (That is, a full phase-out in greenhouse gas emissions at some point in the future may still lead to decades of continued warming.)

Alternatively, SRM might be tied to carbon dioxide removal. In an overshoot scenario, negative emissions will be necessary to limit warming to a lower equilibrium temperature. SRM could be used in the meantime as a complementary tool to alleviate damaging impacts as the climate equilibrates.

Or, carbon dioxide removal may be tied to SRM in a different way: if obstacles arise in the large-scale deployment of carbon dioxide removal, then SRM may end up as a substitute rather than a complement.

These hypotheticals all echo the conclusions of the Sixth Assessment Report from the Intergovernmental Panel on Climate Change, which suggest that, despite its essential and dominant role in addressing climate change, mitigation is likely only one part of the larger long-term solution to climate change that also includes carbon dioxide removal.

The False Hope of Using Tipping Points in Climate Decisionmaking

An argument sometimes is made that climatic “tipping points” (such as the disintegration of the West Antarctic Ice Sheet) can be used as decision thresholds, marking when global SRM would be deployed justifiably. Two problems arise with this argument.

First, such thresholds probably won’t come into focus until they are actually happening (and perhaps only in hindsight). Moreover, well-managed SRM likely would take a couple of decades to develop and deploy, making it impossible to quickly launch an SRM response to avoid passing an impending threshold. Second, once any threshold is passed, recovery to the earlier state of the world will not be possible: SRM can’t regenerate the ice sheets once they melt.

This role of timing highlights the potential danger of delaying research on SRM. If we assume that dangerous climatic thresholds exist, then we already need to be moving faster on SRM research.

Among the top concerns about SRM is the risk of “moral hazard,” meaning that SRM research and/or increased talk of possible SRM deployment could weaken the motivation to reduce greenhouse gas emissions aggressively. This risk is potentially large and should not be underestimated. However, the prospects of this risk remain inconclusive, based on some initial research on the magnitude of such an effect.

Workshop participants also considered whether carbon dioxide removal—a more prevalent research topic for actual projects and current implementation—might cause a greater moral hazard than research on future SRM, given that SRM remains hypothetical.

Most who are paying attention to climate change policy would agree that greenhouse gas emissions must come down to net zero. The following question then could be posed: If SRM is used at some point in the future, how might that deployment bring the most benefit with the least amount of risk, while maximizing emissions mitigation? Even without SRM, encouraging the needed level of emissions mitigation remains an unsolved challenge in global policy.

Another Big Risk: Uncoordinated Deployment Scenarios

How might SRM be deployed? One possibility would be a chaotic, unmanaged, uncoordinated, or otherwise “nonoptimal” deployment. Despite earlier speculation about the ease with which any country (or, indeed, any high-net-worth individual) could implement a meaningful global SRM deployment, such a roll-out is not so easy to do. Only a very few nation-states have the technological and financial capacity to go forward with such an effort.

Multiple geopolitical reasons also could explain why a country may want to avoid unilateral deployment of SRM, even if the nation has the capacity and stands to benefit from reduced climate impacts. For example, states could be constrained by a desire to maintain good relations with allies or avoid conflict with adversaries.

A crucial area of research aims to understand how governance of SRM at the global level could be designed in a way that is both functional and just. For governance to function well, it should be able to make decisions as well as update those decisions based on new information. For SRM governance to be just, it should be broadly and globally representative, especially for those who may be adversely affected.

Could the United Nations Framework Convention on Climate Change assume this governance role? Or do we need a new framework, given that this existing climate treaty does not mention SRM? Other international agreements and treaties have been considered, and may likewise need revision, such as the Montreal Protocol to the Vienna Convention, which aims to protect stratospheric ozone, or the Convention on Environmental Modification, which prohibits hostile environmental modification.

In the absence of a specific geoengineering treaty, one idea for the near term proposed by the Climate Overshoot Commission is the development of an international moratorium on SRM deployment that is paired with higher levels of research into SRM. How would this work? Even if such a moratorium is not enforceable, would it at least be a good first step as part of a larger effort to encourage international norms around SRM? How would the moratorium be retired if SRM is needed at some point in the future? Would a moratorium present a danger of taking too strong a hold and actually delaying action on SRM, even in cases when we should instead be pursuing SRM development?

Most work on SRM to date has occurred in the developed countries of the Global North. As the discussion around SRM evolves, who will be part of relevant future decisionmaking processes? What is the best way to involve other groups, perspectives, and academic fields of study (such as the humanities) more fully into SRM research and related conversations? What level of oversight (and by whom) could best manage the possible risks of research without stifling the research?

Next Steps for Research on Solar Radiation Modification

Though full certainty about any emerging technology can never be achieved, an extensive “wish list” of questions could be developed for answering in this decade. These questions fall under three broad categories.

First, what near-term research steps are needed to increase confidence that an SRM deployment might work as envisioned, with the expected net climatic benefits globally, limited adverse effects regionally, and minimal ancillary biophysical risks? How feasible are the related steps, and what timeline is required for the needed laboratory and field research?