Geoengineering projects are controversial, even though they are largely theoretical at this point. They range from mimicking the effects of volcanic eruptions by releasing sulphur dioxide into the atmosphere, to deploying giant mirrors in space to deflect the sun's rays.

In this new study scientists from Germany, Norway, France and the UK used four different computer models that mimic the earth's climate to see how they responded to increased levels of CO2 coupled with reduced radiation from the sun. Their scenario assumed a world with four times the CO2 concentration of the preindustrial world, which is in the range of what is considered possible at the end of this century.

They found that global rainfall was reduced by about 5% on average using all four models. Under the scenario studied, rainfall diminished by about 15%, or about 100 millimetres per year, compared to pre-industrial levels, in large areas of North America and northern Eurasia. Over central South America, all the models showed a decrease in rainfall that reached more than 20% in parts of the Amazon region.