23 August 2010, by Adele Rackley

As the Earth heats up through global warming, rainfall increases. But researchers have shown that this 'hydrological sensitivity' varies over time, and in extreme cases can cause a short-term reduction in rainfall.

Clouds and sky

This has important implications for our ability to predict the effects of climate change on rainfall, floods and drought across the world.

Scientists have shown that rainfall will increase globally by 2-3 per cent for every degree Celsius that the Earth's surface heats up.

It sounds straightforward, but not every study into this relationship was producing the same results. So a team of researchers from the University of Leeds and the Met Office Hadley Centre looked at a range of climate change scenarios to find out why. Their results

are published in Geophysical Research Letters.

'Clearly there's something else going on,' says Dr Timothy Andrews of Leeds University, lead author of the report.

'We found that precipitation isn't just affected by temperature change at the surface. It also responds directly to increased heating in the atmosphere.'

And because the atmosphere responds to heating much more quickly than the surface – over days rather than decades – the effect on rainfall is also much faster.

The various mechanisms that cause global warming work differently on the atmosphere and the surface. Increased solar radiation, greenhouse gases or dark aerosols – particles in the air – all eventually warm the Earth's surface. But they act differently on atmospheric temperature, and it's these effects that can produce an immediate change in rainfall.

And crucially, this fast atmospheric response seems to suppress rather than increase rainfall.

Black carbon aerosols – soot to you and me – can absorb enough sunlight in the atmosphere to suppress the rainfall response to global warming, and the immediate result will be less rainfall.

'Simply projecting the 2-3 per cent change into the future might not work if the balance of factors causing global warming changes.'

Dr Timothy Andrews, University of Leeds

So there are two effects going on in tandem: a fast atmospheric response that is closely linked with the type of climate change mechanism, and a slower response to surface temperature change that happens regardless of the climate change mechanism.

'The slow effects do produce the predicted 2-3 per cent increase in rainfall,' says Andrews, 'but on top of this are the fast changes, which kick in much sooner.'

'You need to understand the balance of the different mechanisms affecting the climate in order to properly understand what the combined effects are going to be,' he continues.

Attempts to stabilise different climate change mechanisms could actually lead to an increase in rainfall if these fast atmospheric responses aren't taken into account.

'Simply projecting the 2-3 per cent change into the future might not work if the balance of factors causing global warming changes,' Andrews explains. 'But until now no one has studied these fast atmospheric responses – everyone's looked at the overall effects.'

It's not just about how hard it's going to rain. The accuracy of our predictions of hydrological sensitivity will affect our ability to predict associated changes in the frequency and strength of droughts and flooding too.

The researchers will now follow up their global study by looking at the regional implications of these fast, atmospheric influences on rainfall.