Droughts will get dryer, storms will get stormier and floods will get deeper with changing climate, a government research report said Thursday.
Events that have seemed relatively rare will become commonplace, said the latest report from the U.S. Climate Change Science Program, a joint effort of more than a dozen government agencies.
There has been an increase in the frequency of heavy downpours, especially over northern states, and these are likely to continue in the future, Thomas R. Karl, director of the National Climatic Data Center, said in a briefing.
For example, Karl said, by the end of this century rainfall amounts expected to occur every 20 years could be taking place every five years.
Such an increase "can lead to the type of events that we are seeing in the Midwest," said Karl, though he did not directly link the current inundations to climate change.
But the report cautioned that preparing for weather than has been relatively common can leave people vulnerable as extreme events occur more and more.
"Moderate flood control measures on a river can stimulate development in a now 'safe' floodplain, only to see those new structures damaged when a very large flood occurs," the report said.
At the same time heavy rains increase, there'll be more droughts, especially in the Southwest, Karl said.
"When it rains, it rains harder and when it's not raining, it's warmer - there is more evaporation, and droughts can last longer," he explained.
The Southwestern drought that began in 1999 is beginning to rival some of the greatest droughts on record including those of the 1930s and 1950s, he added.
Gerald A. Meehl, a senior scientist at the National Center for Atmospheric Research, said there has been a trend toward increasing power in hurricanes since the 1970s in the Atlantic and western Pacific, a change that can be linked to rising sea surface temperatures.
There is a statistical connection between rising sea surface temperatures and hurricane activity, Meehl said, but linking changes in hurricanes to human actions will require more study.
More easily attributed to human impact, through release of greenhouse gases, is an overall increase in temperatures, he said.
It's not getting as cold at night as it did in earlier decades and there are fewer nights with frosts, a trend expected to continue into the future, Meehl said.
"A day so hot that it is experienced only once every 20 years would occur every three years by the middle of the century," under the mid-range projections of climate models, the report said.
Researchers can use computer models of climate to separate out cause and effect of this warming, he explained - looking at the effect of things like changes in solar radiation or volcanic eruptions - and the result is to attribute climate warming to the burning of fossil fuels.
Participating in the Climate Change Science Program are the Agency for International Development, Department of Agriculture, National Oceanic and Atmospheric Administration, National Institute of Standards and Technology, Department of Defense, Department of Energy, National Institutes of Health, Department of State, Department of Transportation, U.S. Geological Survey, Environmental Protection Agency, National Aeronautics and Space Administration, National Science Foundation and the Smithsonian Institution.