"Global mean rainfall changes are more sensitive to solar forcing than to carbon dioxide forcing," Govindasamy Bala of Lawrence Livermore National Laboratory, US, told environmentalresearchweb. "Therefore, if you turn down the sun to counter carbon dioxide effects, you will end up with less rainfall and droughts."
Schemes such as arrays of mirrors in space or the injection of sulphate particles into the atmosphere are all – often controversially – on the drawing board to reduce the amount of sunlight reaching Earth. A 2% reduction in sunlight would probably be sufficient to counteract temperature increases caused by a doubling of carbon dioxide levels.
Bala and colleagues used equilibrium climate simulations to show that the hydrological sensitivity – the change in global mean precipitation per degree of warming – is 2.4% per Kelvin for solar forcing, but only 1.5% per Kelvin for carbon dioxide forcing. That meant that a doubling in carbon dioxide levels would increase global mean rainfall by around 4%, but a 1.8% reduction in sunlight to counteract the resulting changes in surface temperature would decrease rainfall by 6%.
Rainfall is more sensitive to changes in sunlight levels than to carbon dioxide as sunlight has a greater effect on evaporation. Radiative forcing by carbon dioxide mainly heats the troposphere while solar forcing mainly heats the surface of the Earth.
"For the same surface temperature change, insolation changes result in relatively larger changes in net radiative fluxes at the surface; these are compensated by larger changes in the sum of latent and sensible heat fluxes," wrote the researchers in a paper in PNAS.
The researchers stress that, as well as the reduction in the rate of global mean water cycling, there are many other reasons not to engage in geoengineering schemes for climate stabilization. The techniques would not mitigate ocean acidification caused by high carbon dioxide levels, some could damage the ozone layer, systems would need to remain in place for centuries, technical failure could be catastrophic and lead to large temperature spikes, and it would be hard to bring about an international consensus.
The team focused on hydrological changes on a global scale because at a regional level the changes in precipitation are quite small relative to interannual variability.
Bala is now trying to quantify the changes to hydrology of different mechanism, such as carbon dioxide forcing, solar forcing, carbon dioxide fertilization and land use change.