Distinguished Professor, Associate Editor of Reviews of Geophysics
Department of Environmental Sciences
Geoengineering, also called climate engineering or climate intervention, has been proposed as a “solution” to 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. 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 may be 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 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 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. This would have devastating impacts on natural ecosystems. As climate velocities (how fast animals would have to move to keep the same climate) would more than double in speed, rapid climate fragmentation would occur in biomes where temperature and precipitation velocity vectors diverge by > 90° from each other. Rapid geoengineering termination would increase significantly the threats to biodiversity from climate change.
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.