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What’s solar geoengineering… and why is it controversial?

A new report from the National Academies of Sciences, Engineering and Medicine tackles a controversial question: Is solar geoengineering – an approach designed to cool the Earth by reflecting sunlight back into space or modifying clouds – a potential tool for countering climate change?

The report, produced by a committee of 16 experts from diverse fields, does not take a position but concludes that the concept should be studied. It calls for creating a multidisciplinary research program, in coordination with other countries and managed by the U.S. Global Change Research Program, that seeks to fill in the many knowledge gaps on this issue.

The study emphasizes that such research is not a substitute for cutting greenhouse gas emissions and should be a minor part of the U.S. response to climate change. It notes that “engineering the climate” would not address the root cause of climate change – greenhouse gas emissions from human activities. And it calls for a research program that draws on physical science, social science and ethics and includes public input.

These perspectives from three members of the study committee underline the complexity of this issue.

Three options, many questions

James W. Hurrell, Professor and Scott Presidential Chair of Environmental Science and Engineering, Colorado State University

Solar geoengineering strategies are very controversial within and beyond the climate science community. It is a major step forward to have 16 experts from different disciplines agree that now is the time to establish a research program on this topic. Our committee traveled a long road to reach this recommendation, working through many complex and contentious issues to reach consensus, but we did it collegially and productively. Each of us learned a great deal.

The three options we considered raise many questions:

– Stratospheric aerosol injection would increase the number of small reflective particles (aerosols) in the upper atmosphere to increase reflection of sunlight back into space. While strong evidence exists that this approach can induce cooling at a global scale, there is limited understanding of how cooling potential relates to the amounts of injected aerosols, their location and type, and the ensuing regional climate responses and impacts.

Marine cloud brightening would add materials to low clouds over the ocean to make them more reflective. Water vapor in clouds condenses into droplets when it comes into contact with particles, such as salt; adding particles produces more droplets, making the clouds more reflective.

Where and by how much the brightness of clouds can be modified, and whether feedback processes will mask or amplify some of the effects, are important research questions. Key processes occur at scales too small to include directly into the current generation of global climate models, and these process uncertainties will need to be reduced in order to develop reliable projections of climate impacts.

Cirrus cloud thinning would seek to reduce the formation of wispy, thin clouds that retain heat radiating upward from Earth’s surface. The efficacy of this approach is unknown because of very limited understanding of cirrus cloud properties and the microphysical processes determining how cirrus clouds may be altered. Existing climate model simulations have yielded contradictory results.

Given the risks of rapid warming and its impacts, it is important to consider a portfolio of response options, and to understand as quickly and efficiently as possible whether solar geoengineering could be a reasonably safe and effective option. A transdisciplinary, coordinated and well-governed research program might prove that more investment is warranted. Or it could indicate that solar geoengineering should not be considered further. The key point is that either outcome will be guided by sound science.