Rainfall Changes Tied to Global Warming

New satellite data shows that tiny airborne particles are changing rainfall patterns around the world. These man-made particles, mostly from burning fossil fuels, make it more difficult for clouds to form and less likely to rain if they do form.

Daniel Rosenfeld of the Hebrew University of Jerusalem says that because they block sunlight, these tiny particles slow down the evaporation from lakes and oceans ?so they suppress clouds in the first place.? The particles are too small to form the seeds of raindrops, the way dust usually does, ?so the clouds that do form?have a hard time [producing] rain.?

This analysis is based on new data from a joint American-Japanese satellite that uses radar to examine particles in shallow clouds near the Earth?s surface and the large, high clouds that contribute most of the rainfall. Rainfall from even the biggest thunderclouds can be cut in half by this kind of pollution, Rosenfeld says, and can completely stop rain from shallow clouds.

The evaporated water eventually does come down as rain, but not where it normally does. For instance, he said, the satellite data show that rain which used to fall in the tropics is being transported to higher latitudes. The areas most affected are those with the worst air pollution, and those tend to be the most populated areas, where water is essential.

Man-made pollution also is affecting the strength of the monsoons of Asia, according to Veerabhadran Ramanathan, of the Scripps Institution of Oceanography, in La Jolla, California. A low-level haze over most of the Indian Ocean is blocking sunlight and changing the pattern of evaporation. As a result, the monsoons of southeast Asia have more rain, while the rest of the region gets less precipitation.

Brazilian researcher Paulo Artaxo, of the University of Sao Paulo, reports a similar discovery in the forests of Amazonia. Between August and October, when farmers burn off woodland, the haze of particles has a dramatic cooling effect on the ground below.This cooling effect may be as much as 47 degrees Fahrenheit, which is enough to interfere with evaporation and cloud formation. He says, ?To have a cloud form, you have to have water vapor from evaporation.?

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Other scientist predict more extreme rainfall and greater flooding for the rest of this century. It will be particularly extreme at northern latitudes?across Canada, Alaska, northern Europe, and northern Asia, the regions that already receive the most precipitation. But the equatorial tropics and Southeast Asia are also likely to have increased rainfall and flooding. Two scientific teams, one from the United States and the other from Europe, blame the rainfall pattern on global warming.

Christopher Milly, of the U.S. Geological Survey, and his colleagues reviewed data on 100-year floods that occurred in the last century for 29 major river basins around the world. ?By definition, a 100-year flood is really extreme and rare,? says Milly. ?What we can observe when we look at those records is that the number of these extreme flooding events occurred disproportionately in the last decades of the 20th century. The difference is large enough to make you raise your eyebrows. It?s hard to believe it could happen by chance, enough that it?s worth looking for other reasons why there were so many floods in the last few decades.?

Scientists use computer models to predict possible changes in climate. The models uses a variety of data on various conditions, such as atmospheric concentrations of greenhouse gases and ocean circulation patterns. By changing these variable conditions, scientists can simulate what might happen in the real world when climate conditions change.

Milly and his team combined a climate model with a river model, and examined different scenarios to identify trends. The trends for the 20th century, particularly at high altitudes and in equatorial regions, closely mirrored what had actually happened, giving the scientists confidence that the model works.

Based on the model?s projections, it?s reasonable to assume that increased flooding over the past century was related to global warming, and the increase is likely to continue. ?The models suggest that instead of the chances of a 100-year flood occurring once every 100 years, which is what you would expect, the risk will increase in the 21st century to somewhere between 3 to 6 chances in 100, which is a manifold increase,? he says.

In the another study, Tim Palmer of the European Center for Medium Range Weather Forecasts in the U.K. and Jouni R

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