Wednesday, April 13, 2016
4:00 pm
South Mudd 365

Environmental Science and Engineering Seminar

Stratospheric Sulfur Geoengineering - Benefits and Risks
Alan Robock, Distinguished Professor, Department of Environmental Sciences, Rutgers University

Geoengineering, also called climate engineering, 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 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 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.  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.  SRM combined with CO2 fertilization would have small impacts on rice production in China, but would increase maize production.  SRM using stratospheric aerosols would reduce stratospheric ozone and enhance surface UV-B radiation.  The enhanced downward diffuse radiation would increase the surface CO2 sink.  We are currently investigating the various mechanisms involved in impacts on the land biosphere with climate models.

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.

Contact Kathy Young katyoung@gps.caltech.edu at 626-395-8732
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