From the more serious climate sceptics we often hear calls for mitigation strategies to cope with anthropogenic climate change. That is, technical fixes designed to deal with the consequences of a warming world. One such mitigation strategy, which had been advocated on television chat shows by so-called “Sceptical Environmentalist” Bjørn Lomborg, among others, would involve pumping sulphate aerosols into the high atmosphere. This would dim the Sun’s light, thereby reducing the global average temperature.
Geoengineering with sulphate aerosols is a blue sky concept beset by a number of uncertainties. Colorado climate scientist Tom Wigley, who tends to favour geoengineering, cautions that, even if sulphate injections into the atmosphere were found to be technically and economically viable, they would provide a grace period of up to 20 years before major cutbacks in greenhouse gas emissions are required. Popular non-scientists such as Lomborg, on the other hand, argue for geoengineering solutions that remove the need for a radical reduction in greenhouse gas emissions.
When it comes to the details of sulphate aerosol geoengineering, much work is being done by scientists around the world. A recent contribution comes from Rutgers University atmospheric modeller Ben Kravitz and others, who in a recent paper discuss the effects of geoengineering on the middle atmosphere, and recommend a number of experiments that could be used to explore the extent to which geoengineering may offset the climate change forecast in various models.
Scientific questions of interest to Kravitz and his colleagues include the relative effects of continuous aerosol injection versus transient ones, such as those from erupting volcanos, and the extent to which regional changes in rainfall would be compensated by variations in the uptake by the atmosphere of moisture from the Earth’s surface and vegetation.
This importance of this regional aspect of the water cycle cannot be understated, yet it is conveniently ignored by those who in the public arena advocate geoengineering as a political solution to climate change. For example, there is a significant probability that geoengineering could reduce the summer monsoon rainfall in Africa and Asia, threatening the food supply for billions of people. How do we factor such a tangible risk into political decision making that is reliant upon necessarily uncertain science?
Rasch et al., “An overview of geoengineering of climate using stratospheric sulphate aerosols”, Phil. Trans. R. Soc. A 366, 4007 (2008)
TML Wigley, “A Combined Mitigation/Geoengineering Approach to Climate Stabilization”, Science 314, 452 (2006)
Kravitz et al., “The Geoengineering Model Intercomparison Project (GeoMIP)”, Atmos. Sci. Lett. (2011)