When it rains, it pours

Extreme precipitation in the tropics comes in many forms: thunderstorm complexes, flood-inducing monsoons and wide-sweeping cyclones like the recent Hurricane Isaac.

Global warming is expected to intensify extreme precipitation, but the rate at which it does so in the tropics has remained unclear. Now an MIT study has given an estimate based on model simulations and observations: With every 1 degree Celsius rise in temperature, the study finds, tropical regions will see 10 percent heavier rainfall extremes, with possible impacts for flooding in populous regions.

''The study includes some populous countries that are vulnerable to climate change,'' says Paul O'Gorman, the Victor P. Starr Career Development Assistant Professor of Atmospheric Science at MIT, ''and impacts of changes in rainfall could be important there.''

O'Gorman found that, compared to other regions of the world, extreme rainfall in the tropics responds differently to climate change. ''It seems rainfall extremes in tropical regions are more sensitive to global warming,'' O'Gorman says. ''We have yet to understand the mechanism for this higher sensitivity.''

Results from the study are published online this week in the journal Nature Geoscience.

A warm rain will fall
Global warming's effect on rainfall in general is relatively well-understood: As carbon dioxide and other greenhouse gases enter the atmosphere, they increase the temperature, which in turn leads to increases in the amount of water vapor in the atmosphere. When storm systems develop, the increased humidity prompts heavier rain events that become more extreme as the climate warms.