What is solar geoengineering … and why is it controversial?

A new report from the National Academies of Science, Engineering and Medicine addresses a controversial question: Is solar geoengineering – an approach aimed at cooling the Earth by reflecting sunlight back into space or modifying clouds – a possible tool to counter climate change?

The report, produced by a committee of 16 experts from various fields, does not stand, but concludes that the concept should be studied. It calls for the creation of a multidisciplinary research program, coordinated with other countries and administered by the U.S. Global Change Research Program, which aims to fill the vast knowledge on this issue.

The study emphasizes that such research does not replace greenhouse gas reduction and should be a minor part of the U.S. response to climate change. It notes that “realizing 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 uses physical sciences, social sciences and ethics and includes public contributions.

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

Three options, many questions

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

Solar geoengineering strategies are highly controversial inside and outside the climate science community. It is an important step forward for 16 experts from various disciplines to agree that now is the time to establish a research program on this topic. Our committee has come a long way in achieving this recommendation, working on many complex and contentious issues to reach consensus, but we have done it collegially and productively. Each of us has learned a lot.

The three options we considered raise many questions:

– Stratosphere aerosol injection would increase the number of small reflective particles (aerosols) in the upper atmosphere to increase reflection of sunlight back into space. While there is strong evidence that this approach can cause cooling globally, there is limited understanding of how cooling potential relates to the amounts of injected aerosols, their location and type, and the subsequent regional climate responses and effects.

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

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

– Cirrus thinning would seek to lessen the formation of thin, thin clouds that retain heat radiating upward from the earth’s surface. The effectiveness of this approach is unknown due to a very limited understanding of cirrhosis properties and the microphysical processes determining how cirrhosis can be altered. Existing climate model simulations have yielded contradictory results.

Given the risks of rapid warming and its effects, 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 fairly safe and effective option. A cross-disciplinary, coordinated and well-managed research program could prove that more investment is justified. Or it could indicate that solar geoengineering should no longer be considered. The key point is that both results will be guided by healthy science.