It's nearly hurricane season again — and government forecasters say they expect it to be a busy one. They predict seven to 10 hurricanes between June and November. Scientists still don't agree on precisely how climate change affects the storms, but they agree that several new trends increase the risk of severe damage.
Kevin Trenberth, a climate scientist at the federally funded National Center for Atmospheric Research in Boulder, Colo., says one area of scientific agreement that doesn't get much attention involves the amount of rain coming from hurricanes and other storms.
It is increasing because warmer water puts more water vapor into the air, and more water vapor means more rain.
"Since about 1970, we've been able to determine that there's about a 4 percent increase in water vapor over the global oceans," Trenberth said, "and so we're seeing that, indeed, when it rains, it pours — much more so now than it did a few years ago."
The factor that worries scientists most, though, doesn't have anything to do with global warming. It has to do with the dramatic increase in the number of people living along coasts.
Roger Pielke Jr. of the University of Colorado at Boulder says Miami offers a good example of why that is a problem.
"In 1926, there was a really major Category 4 or stronger hurricane that hit what is now downtown Miami," said Pielke, an environmental studies professor. Back then, only about 100,000 people lived on the coast there, he said.
Now, millions of people live there, and millions more have spread along the rest of the East Coast and the Gulf Coast. Pielke says that means there are fewer and fewer places a hurricane can land without doing a lot of damage.
And he says it means there's likely to be more damage from any one storm.
"The potential for losses is doubling every 10 years," Pielke said. "So a storm like Hurricane Andrew in 1992 was about a $30 billion storm. Today it might be $60 or $70 billion. Ten years from now, it might be $120, $140 billion."
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