Stratospheric Sulfur Geoengineering—Benefits and Risks

Geoengineering, also called climate engineering, has been proposed to address global warming, involving “solar radiation management (SRM)” by injecting particles into the stratosphere, brightening clouds, or blocking sunlight with satellites between the Sun and Earth. (“Geoengineering” also refers to carbon dioxide reduction, a completely different proposed technology, with different costs and governance. It is not addressed here.) While volcanic eruptions have been suggested as innocuous examples of stratospheric aerosols cooling the planet, the volcano analog actually argues against stratospheric geoengineering because of ozone depletion and regional hydrologic responses. No such systems to conduct stratospheric geoengineering now exist, but a comparison of different proposed stratospheric injection schemes, using airplanes, balloons, and artillery, shows that using airplanes to put sulfur gases into the stratosphere would not be expensive. Nevertheless, it would be very difficult to create stratospheric sulfate particles with a desirable size distribution.

Our Geoengineering Model Intercomparison Project (GeoMIP), conducting climate model experiments with standard stratospheric aerosol injection scenarios, is ongoing. We have found that if we could counteract increasing greenhouse gases with global insolation reduction we could keep the global average temperature constant, but global average precipitation would reduce, particularly in summer monsoon regions around the world. Temperature changes would also not be uniform. The tropics would cool, but high latitudes would warm, with continuing, but reduced sea ice and ice sheet melting. New experiments with time- and space-varying sulfate injections, or that combine stratospheric SRM with surface brightening, show that it may be possible to control to some extent these regional differences. Temperature extremes would still increase, but not as much as without SRM.

If SRM were halted all at once, there would be rapid temperature and precipitation increases at 5-10 times the rates from gradual global warming. Sudden geoengineering termination would more than double temperature velocities for the land and ocean, and would more than triple temperature velocities in multiple global biodiversity hotspots. These geoengineering-associated velocities exceed even the most optimistic dispersal rate estimates for many species, increasing local extinction risks. Rapid geoengineering implementation and termination would significantly increase the threats to global biodiversity and ecosystems from climate change.

SRM combined with CO₂ fertilization would have small impacts on rice production in China, but would increase maize production. New experiments with the Community Earth System Model from the National Center for Atmospheric Research, which includes comprehensive tropospheric and stratospheric chemistry, show that SRM using stratospheric aerosols would reduce stratospheric ozone and enhance surface UV-B radiation. The enhanced downward diffuse radiation would increase the surface CO₂sink. Surface ozone and tropospheric chemistry would likely be affected by SRM, but the overall effect is strongly dependent on the SRM scheme.

If there were a way to continuously inject SO₂ into the lower stratosphere, it would produce global cooling, stopping melting of the ice caps, and increasing the uptake of CO₂ by plants. But there are at least 27 reasons why stratospheric geoengineering may be a bad idea. These include disruption of the Asian and African summer monsoons, reducing precipitation to the food supply for billions of people; ozone depletion; no more blue skies; reduction of solar power; and rapid global warming if it stops. Furthermore, there are concerns about commercial or military control, and it may seriously degrade terrestrial astronomy and satellite remote sensing. Global efforts to reduce anthropogenic emissions (mitigation) and to adapt to climate change are a much better way to channel our resources to address anthropogenic global warming.