Would solar radiation modification increase or decrease overall risk?

Jesse ReynoldsJesse ReynoldsWienerJesse Reynolds

Guest post by Tyler Felgenhauer, Govindasamy Bala, Mark Borsuk, Matthew Brune, Inés Camilloni, Jonathan Wiener, and Jianhua Xu 

[The views of guest post authors are their own and do not necessarily reflect any official positions of the authors’ organisations or C2G. We encourage a constructive conversation involving multiple viewpoints and voices] 

This blog is based on a new report published by C2G: Solar Radiation Modification: A Risk-Risk Analysiswhich has summary versions available in English, 中文, Français and Español.

A key consideration in deciding whether to pursue solar radiation modification (SRM) to offset global warming should be a comparison of the extent of climate risk that the technology is able to reduce against the severity of any countervailing risks that it may engender.

SRM as a potential risk reduction strategy  

It may not be widely appreciated that even if humanity were to achieve significant reductions in greenhouse gas (GHG) emissions atmospheric concentrations would continue to rise. Only net-zero emissions (NZE) would flatten GHG concentration trajectories. Even after NZE were to be achieved, however, a changed climate would persist for decades to centuries because of the long lifetime of carbon dioxide in the atmosphere. Thus, in addition to aggressive emissions reductions, carbon dioxide removal (CDR) will need to be a key piece of a broader response portfolio intended to bring down global mean surface temperature. Further, adaptation will be necessary to mitigate damages from residual warming. Some climate scientists have suggested that it may also be worth considering some form of solar radiation modification (SRM) as a means for reducing some climate risks while societies pursue these other measures.   Many analyses of GHG emissions reductions, CDR, and SRM tend to focus on their effects on global temperature, but a broader “risk-risk analysis” can help to identify and assess a wider array of relevant risks. 

What is SRM? 

SRM refers to a range of large-scale approaches that increase the amount of sunlight that is reflected back to space. For example, a method called stratospheric aerosol injection (SAI) would involve the intentional release of highly reflective fine particles, such as sulfate aerosols, into the stratosphere. The idea is that the resulting reduction in solar radiation reaching the Earth’s surface would offset some or all of the warming caused by GHG concentrations. Thus, rather than addressing the root cause of climate change by attempting to slow or reverse GHG accumulation in the atmosphere, SRM is intentional anthropogenic climate change of another form. Once the necessary technology and infrastructure are developed, SAI in particular could be a fairly inexpensive means of cooling the Earth relatively quickly and at a global scale. (Other proposed SRM options include brightening of low-level marine clouds through injection of sea-salt aerosols, increasing the reflectivity of the Earth’s surface through various means, or even placing mirrors in space.) SAI would not completely offset climate change in all regions and all seasons and would need to be deployed continuously, as its effects are only temporary. The approach may also be risky because of the potential for unexpected interactions with the climate and the possible introduction of new biophysical and societal risks. Decisions regarding the possible development or deployment of SAI or other forms of SRM may thus come down to the question of whether these approaches increase or decrease overall risk, and for whom. 

Risk vs. risk 

Our research team recently presented an analytical framework for comparing the climate change risks in a future world without SAI against the aggregate of residual and newly generated risks in a future world that includes SAI as part of a portfolio of climate risk mitigation approaches. Such a framework was explicitly called for by the recent U.S. National Academies of Sciences, Engineering, and Medicine report, “Reflecting Sunlight: Recommendations for Solar Geoengineering and Research Governance.” The framework’s further development and use would enable more comprehensive evaluation and enactment of various possible climate change management strategies. By identifying and comparing the extent to which these strategies impact climate as the target risk, introduce novel countervailing risks of their own, and potentially generate other co-benefits, the risk-risk framework can aid in policy deliberation and minimize the heuristics and biases that can cloud sound decision making. 

Our implementation of the risk-risk framework demonstrated the value of this approach for assessing SRM, and laid the groundwork for more detailed further analyses; still, we offered the following preliminary findings: 

  • As a supplement to GHG emissions reductions, CDR, and adaptation, SRM has the potential to yield large direct benefits to humans and natural ecosystems by lessening the near-term damages of climate change and lowering the chances of crossing catastrophic climate tipping points. 
  • SRM could pose countervailing risks to biophysical systems, including changes in stratospheric ozone and surface UV radiation, acid rain, and unintended changes in temperature and precipitation patterns. The extent of these risks could be controlled to some degree by appropriate design and governance of implementation. 
  • SRM may also pose countervailing risks to societal systems, including the risk of international conflict, the risk of rapid climate change resulting from unplanned sudden termination, and the risk of delaying or discouraging GHG emissions mitigation. Here too, the extent of these risks would depend on the design and governance of implementation. 
  • SRM could provide co-benefits, including an increase in diffuse sunlight, possibly benefitting some crops and ecosystems, and slightly reduced tropospheric ozone in the mid and high latitudes. However, these effects are likely to be small, uncertain, and unlikely to play a significant role in risk-risk tradeoffs. 
  • More extensive use of SRM may yield greater reductions in many temperature-related climate risks but is also likely to increase the level of countervailing risks. The sooner and greater we make GHG emissions reductions, the less need there may be for SRM, thereby likely reducing overall risk exposure (subject to any countervailing risks of emission reduction).  
  • Risk-risk analysis can help focus climate change risk management on broader societal objectives, rather than on temperature goals alone. This can be important, as many climate impacts do not scale directly with temperature. 
  • Existing international governance aimed at addressing climate change and its impacts appears to be inadequately designed for addressing SRM and its distinctive characteristics. Governance of SRM may need to restrain the imposition of global risks through hasty or unwise unilateral action, which is a different challenge than mobilizing collective action to reduce GHG emissions. Thus, it may be necessary to draw on lessons from a wider array of international agreements and institutions. 

The specific benefits and risks of SRM implementation would depend on a number of other decisions, including the particular objectives being pursued, the emission pathway and adaptation plans being followed, and the governance framework in place. Conceptually, the emissions pathway and anticipated adaptive capacity would determine the level of residual climate risk that might be addressed by SRM. Decisions and governance on SRM should then seek to simultaneously minimize the combination of these climate risks and the climate and countervailing risks posed by SRM. To make these tradeoffs explicit across a range of policy portfolios, in our report we consider three specific climate risk management scenarios with different relative contributions of mitigation and SRM. The specific response patterns of climate and countervailing risks to varying levels of SRM are not currently well known and are likely to depend on the particular technology, deployment strategy, and governance mechanisms employed. Determining these patterns of risk response to SRM levels should be a research topic of high priority. 


Tyler Felgenhauer is Director of Climate Research at the Duke Center on Risk, Duke University.

Govindasamy Bala is Professor at the Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science.

Mark Borsuk is Professor of Civil and Environmental Engineering, Pratt School of Engineering, and Co-Director of the Duke Center on Risk, Duke University.

Matthew Brune is a Research Fellow at the Duke Center on Risk, at Duke University.

Inés Camilloni is Profesor at Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Departamento de Ciencias de la Atmósfera y los Océanos and CONICET – Universidad de Buenos Aires. Centro de Investigaciones del Mar y la Atmósfera.

Jonathan B. Wiener is the Perkins Professor of Law and Professor of Environmental Policy and Public Policy, at the Law School, Nicholas School, and Sanford School, and Co-Director of the Duke Center on Risk, at Duke University.

XU Jianhua is Associate Professor in the Department of Environmental Management, College of Environmental Sciences and Engineering and Institute for Global Health and Development at Peking University.

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