If there were a way to continuously inject SO2 into the lower stratosphere, it would create a permanent cloud there, producing global cooling, stopping melting of the ice caps, and increasing the uptake of CO2 by plants. A comparison of different proposed injection schemes, using airplanes, balloons, and artillery (Figure 1), shows that putting sulphur gases into the stratosphere would be comparatively inexpensive. But there are at least 27 reasons why stratospheric geoengineering may be a bad idea (Table 1). 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 geoengineering stops. Furthermore, there are concerns about commercial or military control, and serious degradation of terrestrial astronomy and satellite remote sensing.
Clearly, the solution to the global warming problem is mitigation (reduction of emissions of gases and particles that cause global warming, primarily CO2). Society will also need to adapt to impacts that are already occurring. Whether geoengineering should ever be used will require an analysis of its benefits and risks, as compared to the risks of not implementing it. Research so far has pointed out both benefits and risks from geoengineering and has shown that it is not a solution to the global warming problem, but at some time in the future, despite mitigation and adaptation measures, society may be tempted to try to control the climate to avoid dangerous impacts. Much more research on geoengineering is needed for society to be able to make informed decisions about the fate of Earth, the only planet in the universe known to sustain life.