Researchers at Caltech and MIT have used climate simulations from many modeling centers to show that the heavier rainstorms expected as a result of global warming will not be uniformly spread around the world, and that their intensity—the amount of rain produced in them—will go up by 5 to 6 percent for every one degree Celsius increase in temperature.

Paul O'Gorman, a former postdoc at Caltech and now an assistant professor at MIT, and Tapio Schneider, professor of environmental science and engineering at Caltech, explain that because warmer air can hold more water vapor, the warming of the atmosphere is expected to result in more vapor in the atmosphere. Thus more water vapor can condense and rain out in heavy storms.

But the intensity of the heaviest storms does not increase by as much as the water vapor in the atmosphere increases. This is because while water condenses out as rising air cools, the rate of cooling for the rising air will be less in a warmer climate. "This moderates the increase in precipitation extremes," says Schneider.

This pattern of changes in extreme precipitation as the climate heats up is seen clearly and consistently in climate models outside of the tropics, the team says, but the models give conflicting results within the tropics. More research will be needed to determine the likely outcomes in tropical regions.

The report was published online in the Proceedings of the National Academy of Sciences.