NewsSeptember 18, 2012

Warming May Increase Tropical Rainfall, Study Shows

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Michael D. Lemonick

By Michael D. Lemonick

It’s one of the classic predictions of global-warming science: as the planet heats up, extreme precipitation should become more common. That’s because warm air can hold more moisture than cool air, and when that moisture finally comes down as rain or snow, there’s more of it to fall.

Exactly how much more, however, is something scientists are still working out — and a paper just published in Nature Geoscience has taken a step in that direction. According to author Paul O’Gorman, an atmospheric scientist at MIT, heavy downpours in the tropics are likely to increase by ten percent for every 1°C (1.8°F) increase in global average surface temperature.

This NASA satellite image shows the flooded city of Ayuthaya, Thailand, during major flooding in October 2011. Click on the image for a larger version.
Credit: NASA.

That’s a greater increase than scientists have already come up with for more temperate latitudes, a figure they’ve pegged at about five percent for every degree of warming. The disparity, O’Gorman said in an interview, is because the kind of rain patterns that affect the mid-latitudes and the tropics are different. Although you can get both widespread and localized rain events (such as thunderstorms) in both sorts of places, he said, local events are more common in the tropics.

“Because these are smaller in extent,” O’Gorman said, “climate models have more trouble representing them.” The result: projections for rainfall are all over the map, ranging from no increase at all to a leap of 25 percent for every degree of warming, depending on which model you choose.

So O’Gorman came at the problem in a different way. He looked at how climate models deal with rainfall changes caused, not by global warming, but by the naturally occurring El Niño—Southern Oscillation (ENSO) phenomenon, in which the eastern equatorial Pacific Ocean warms up for a year or two, then cools back down. “ENSO isn’t generally a good proxy for global warming,” O’Gorman said, “because it’s a regional event. It causes an increase in rainfall in some areas and a decrease in others.”

Nevertheless, when he looked at the climate models that did best at simulating El Niño-related rainfall changes, they also provided the most accurate projections of actual rainfall trends in the tropics, based on rainfall observations from satellites. 

It’s good news, of course, that climatologists are getting better at projecting the likely changes coming as the planet warms. But this particular result won’t make anyone happy. “Unfortunately, the results of the study suggest a relatively high sensitivity of tropical extreme rainfall to global warming,” O’Gorman said in a press release, which could lead to more floods in countries like Bangladesh, Thailand and Honduras that are already prone to inundations that kill thousands and destroy property.

The only silver lining, O'Gorman said, is that the study's results provide an estimate of what that sensitivity [to warming] is, which should be of practical value for planning.”

In addition to flooding concerns, changes in tropical rainfall patterns could also affect the health of tropical forests, which are a major “carbon sink,” taking in planet-warming greenhouse gases and helping to put a brake on manmade global warming. Any decrease in the effectiveness of such sinks could cause warming to accelerate. 

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