June 04, 2007
Monday

In a report that challenges conventional wisdom, Earth might become much rainier if planetary warming continues unabated, a team of experts on climate change announced.

Over the next 100 years, global rainfall could increase by about 20 percent -- three times as fast as the rate projected previously by global-warming scientists -- if greenhouse gases in the atmosphere continue unabated, said physicist Frank Wentz and colleagues at Remote Sensing Systems in Santa Rosa, Calif. Their report appears in the latest issue of Science Express, an online publication associated with Science magazine.

Their study is not precise enough to forecast how increasing global warming will affect rainfall in specific regions such as California, Wentz said. Still, his team's analysis of 19 years of planetary rainfall and humidity data hints that global warming might portend "a general tendency to make the wetter areas wetter and the drier areas drier - which, when it comes to climate change, is a pretty gloomy scenario," he told The San Francisco Chronicle.

Kelly T. Redmond, deputy director of the Western Regional Climate Center at the Desert Research Institute in Reno, Nev. called the report a very interesting paper.

"It's the kind of subject we need to be investigating," said Redmond, who is not connected with the Santa Rosa team. "It's a very fundamental issue: What is rainfall on Earth going to do (during) climate change?"

As everyone knows from childhood, the sun evaporates water, causing it to rise into the sky, where it eventually cools and falls back to Earth. The humidity scale reveals how much evaporated water is in the air at any given time.

In the past, climate modelers have generally assumed that as global warming evaporates water and makes the planet more humid, the rainfall rate will rise more slowly. In other words, precipitation won't intensify as fast as the humidity. Initially, the reason seems obvious: Warmer air can hold more water vapor, delaying its eventual cooling and falling back to Earth as raindrops, snow, sleet or hail.

However, when members of the Santa Rosa team analyzed satellite measurements of planetary changes in humidity and rainfall from 1987 to 2006, they were surprised by what they found: Over that period, the global rainfall rate rose at almost exactly the same rate as humidity, like two race NASCAR drivers racing neck and neck. The difference between the rise in rainfall and the rise in humidity was about 1 percent, Wentz said.

"We were very startled by the fact that our absolute numbers are as close to each other," Wentz said. "It casts some doubt on the (earlier computer) models" for how global warming affects precipitation.

The implication, he said, is that as global warming continues, planetary rainfall -- far from lagging behind the humidity rise -- will increase at about the same rate and, thus, much faster than projected by earlier computer models. The scientists are not sure why, but Wentz said, "we're opening a new door" for researchers studying how climate change will affect the future Earth.

One of Wentz's co-authors, atmospheric physicist Lucrezia Ricciardulli, said climate experts previously generally agreed that the average planetary temperature will rise by about 5 degrees Fahrenheit over the next century if cars, factories and other sources continue to emit greenhouse gases at projected rates -- in effect, without being cut back. Older computer models projected this would mean a 6 percent increase in rainfall over the next century, but the Santa Rosa team thinks 20 percent is closer to the mark, she said.

(She cautioned that this doesn't mean the actual total amount of rainfall will multiply three-fold. Rather, the increase in rainfall is expected to be three times greater than the previously projected increase.)

The other co-authors are meteorologist Kyle Hilburn and physicist Carl Mears. For years, the private research firm in Santa Rosa has been a major player in analyzing satellite measurements of meteorological data related to climate change.

The 19 years of satellite data came from six weather satellites that are part of the U.S. Defense Department's Meteorological Satellite Program.

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