Long-Range Climate Forecast: Pack Your Umbrella

Global warming, which helps supercharge rain clouds by pumping more water vapor into the atmosphere, will lead to more intense rain and snowfall than some climate models predict, according to a new study.

Scientists who analyzed two decades of real-world satellite observations say that global precipitation could increase about 13 percent above 2000 levels by the end of this century if current trends continue. That, they say, is dramatically higher than the 1 percent to 3 percent increase that some computer models have projected for the future.

"There's going to be considerably more precipitation in our future than one might have expected from current model results," said Frank Wentz, a physicist with Remote Sensing Systems in Santa Rosa, Calif. Wentz co-authored the NASA-funded study, which appears today in the online journal Science Express.

Climate scientists already know that as Earth's atmosphere has warmed, it has increased capacity to hold water that later falls in more extreme rain and snow events. For every 1-degree Fahrenheit temperature increase, the total amount of water circulating in the atmosphere goes up by 4 percent.

The vast majority of the world's climate scientists agree global warming is caused in part by people releasing heat-trapping greenhouse gases into the atmosphere largely by burning fossil fuels. The resulting temperature increase, say experts, is already significantly increasing the intensity of precipitation events in some parts of the world.

In February, the Intergovernmental Panel on Climate Change -- made up of thousands of climate scientists from around the globe -- reported that precipitation had already increased as much as 8 percent in parts of North America, northern Europe and parts of Asia between 1900 and 2005.

Results Need More Scrutiny

Some climate modelers praised the new research, but said more work needed to be done to reconcile why the satellite observations offered such different predictions than the climate models.

"That's how this is supposed to work," said Gerry Meehl, a research scientist and climate modeler at the National Center for Atmospheric Research. "When we get new observations, or a new way of looking at observations, I think that really is helpful to us to try and improve the model."

Meehl, who was not involved in the new study, said the climate models accurately recreated the general patterns found in the satellite observations. One explanation for the discrepancy in the two predictions, Meehl said, is that 20 years of satellite observations may be too short to establish long-term trends.

Wentz said there might also be some limitations to the models. All agree, though, the question of exactly how much additional precipitation global warming will bring deserves more scrutiny.

More Rain? Not So Fast

Not every part of the globe will see increased amounts of rain or snow, however, and experts warn increased precipitation will not necessarily mean an end to severe droughts.

Many parts of the world, including the American Southwest, are expected to see worsening drought conditions. The Intergovernmental Panel on Climate Change also projects that portions of southern Africa, the Mediterranean and southern Asia will likely be much drier as a result of a warming world.

Climate models generally predict that dry places will get drier and wet places will get wetter, said Noah Diffenbaugh, a climate scientist at Purdue University.

The downpours will likely be bigger and more unpredictable.

"In general, you may see larger precipitation events that are farther apart," Diffenbaugh said.

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