Judith Burns, BBC News 7 Aug 09;
The use of geo-engineering to slow global warming may increase the risk of drought, according to a paper in Science journal.
Methods put forward include reflecting solar radiation back into space using giant mirrors or aerosol particles.
But the authors warn that such attempts to control the climate could also cause major changes in precipitation.
They want the effect on rainfall to be assessed before any action is taken.
Gabriele Hegerl of the Grant Institute at University of Edinburgh and Susan Solomon of National Oceanic and Atmospheric Administration (NOAA) at Boulder, Colorado, write that "if geo-engineering studies focus too heavily on warming, critical risks associated with such possible "cures" will not be evaluated appropriately".
They argue that climate change is about much more than changes in temperature. So using temperature alone to monitor the effects of geo-engineering could be dangerous.
Underestimating effects
They cite the powerful effects on rainfall of volcanic eruptions which also prevent solar radiation reaching the Earth's surface, albeit by throwing up dust rather than reflecting the radiation back into space.
For example in 1991, the eruption of Mount Pinatubo not only reduced global temperatures but also led to increases in drought.
The pair correlated 20th Century weather records with data for the increase in greenhouse gases and dates for major volcanic eruptions.
This revealed that greenhouse emissions tend to slightly increase rainfall in the short term but also showed that reduction in rainfall in the months following a major volcanic eruption is far more dramatic.
The authors note that current climate models tend to underestimate the effects on precipitation of both greenhouse gases and of volcanic eruptions.
The article warns that geo-engineering of this type, combined with the effects of global warming could produce reductions in regional rainfall that could rival those of past major droughts, leading to winners and losers among the human population and possible conflicts over water.
They conclude: "optimism about a geo-engineered 'easy way out' should be tempered by examination of currently observed climate changes."
