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Solar geoengineering could start soon if it starts small [1]

['David W. Keith', 'Wake Smith']

Date: 2024-02-05

Such complacency is ill-advised. The barrier between research and deployment may be less distinct than is often assumed. Our analysis suggests a country or group of countries could conceivably start a subscale solar geoengineering deployment in as little as five years, one that would produce unmistakable changes in the composition of the stratosphere. A well-managed subscale deployment would benefit research by reducing important uncertainties about SAI, but it could not be justified as research alone—similar research could be carried out with a much smaller amount of aerosol particles. And it would have a non-negligible impact on the climate, providing as much cooling as sulfur pollution from international shipping did before the recent cleanup of shipping fuels. At the same time, the magnitude of the cooling would be small enough that its effects on climate, on a national or regional scale, would be very difficult to detect in the face of normal variability.

While the climate impact of such a subscale deployment would be small (and most likely beneficial), the political impact could be profound. It could trigger a backlash that would upend climate geopolicy and threaten international stability. It could be an on-ramp to large-scale deployment. And it could be exploited by fossil fuel interests seeking to slow the essential task of cutting emissions.

We oppose near-term deployment of solar geoengineering. In accord with the Climate Overshoot Commission, the most senior group of political leaders to examine the topic, we support a moratorium on deployment until the science is internationalized and critically assessed, and until some governance architecture is widely agreed upon. But if we are correct that such subscale deployments are plausible, then policymakers may need to confront solar geoengineering—its promise and disruptive potential, and its profound challenges to global governance—earlier than is now widely assumed.

Obstacles to early deployment

Humans already emit a huge quantity of aerosols into the troposphere (the turbulent lowest layer of the atmosphere) from sources such as shipping and heavy industry, but these aerosols fall to Earth or are removed by rainfall and other processes within about a week. Volcanic eruptions can have a more lasting effect. When eruptions are powerful enough to punch through the troposphere into the stratosphere, the aerosols deposited there can endure for roughly a year. SAI would, like the largest volcanic eruptions, inject aerosols or their precursors into the stratosphere. Given their vastly longer atmospheric endurance, aerosols placed there can have a cooling impact 100 times larger than they would if emitted at the surface.

Getting aerosols to the stratosphere is another matter. Passenger jets routinely reach the lower stratosphere on transpolar flights. But to get efficient global coverage, aerosols are best deployed at low latitudes, where the stratosphere’s natural overturning circulation will carry them poleward and thus distribute them worldwide. The average height of the top of the troposphere is about 17 kilometers in the tropics, and models suggest injection needs to be a few kilometers higher than that to be captured in the upwelling stratospheric circulation. The altitude for efficient deployment is commonly assumed to be at least 20 kilometers, nearly twice the height at which commercial jets or large military aircraft cruise.

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[1] Url: https://www.technologyreview.com/2024/02/05/1087587/solar-geoengineering-could-start-soon-if-it-starts-small/

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