Geoengineering

Solar Radiation Management Methods

SRM methods focus on reducing the amount of solar radiation absorbed by the Earth by an amount sufficient to offset the increased trapping of infrared radiation by rising levels of GHGs. These schemes essentially put a “dimmer switch” on the sun. Researchers have concluded that the warming effects associated with a doubling of atmospheric CO2 concentrations could be offset by reducing solar radiation inflows by 1.8 percent. The primary SRM geoengineering schemes that have been discussed to date are: stratospheric sulfur aerosol injection, cloud albedo enhancement, and space-based systems.

Perhaps the most widely discussed climate geoengineering option is enhancement of planetary albedo (the surface reflectivity of the sun's radiation) using stratospheric aerosols. While most proposals have focused on the use of sulfur, other potential options include aluminum, hydrogen sulfide (H2S), carbonyl sulfide, ammonium sulfide, soot, and engineered nanoscale particles. A recent study by Victor Brovkin and colleagues concluded in 2009 that the amount of sulfur emissions required to compensate for projected warming by 2050 would be between 5 and 16 teragrams (Tg) per year, increasing to 10–30 Tg/year by the end of the century. Potential delivery vehicles for stratospheric sulfur dioxide injection include aircraft, artillery shells, stratospheric balloons, and hoses suspended from towers. Supporters of stratospheric aerosol injection tout the fact that it could be an extremely cheap option, perhaps costing only a few billion dollars annually.

However, sulfur aerosol injection schemes could have negative ramifications. Reductions in evaporation associated with deployment could substantially weaken Asian and African monsoons, threatening the food and water supplies of billions of people. Moreover, there is substantial concern that injection of sulfur particles into the stratosphere could imperil recovery of the ozone layer by catalyzing chemical reactions that deplete ozone. This could delay recovery of the [Page 613]ozone layer in the Antarctic by between 30 and 70 years, substantially increasing the incidence of skin cancer and other maladies associated with UV-B radiation. Finally, unless GHG emissions were substantially reduced after deployment of a sulfur-aerosol-injection scheme, or any other SRM approach, termination of such a scheme could result in warming of more than 20 times the current rate. This would be a consequence of the buildup of CO2 that had accrued in the atmosphere in the interim, with its suppressed warming effect, as well as the temporary suppression of climate-carbon feedbacks.

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