Droughts will get drier, storms will get stormier and floods will get deeper with a warming climate across North America, U.S. government experts said in a report billed as the first continental assessment of extreme events.
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.
"Heat waves and heavy downpours are very likely to further increase in frequency and intensity," the report stated. "Substantial areas of North America are likely to have more frequent droughts of greater severity. Hurricane wind speeds, rainfall intensity and storm surge levels are likely to increase. The strongest cold season storms are likely to become more frequent, with stronger winds and more extreme wave heights."
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 Karl, director of the National Climatic Data Center, said in a briefing Thursday.
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 flooding to climate change.
The report itself noted that "intense precipitation (the heaviest 1 percent of daily precipitation totals) in the continental U.S. increased by 20 percent over the past century while total precipitation increased by 7 percent."
But the report cautioned that preparing for weather that 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 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.
Hotter days more often
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.
"It is well established through formal attribution studies that the global warming of the past 50 years is due primarily to human-induced increases in heat-trapping gases," the report itself states.
Other future projections cited in the report include:
- Sea ice extent in the Arctic Ocean is expected to continue to decrease and may even disappear in summer in coming decades;
- Precipitation, on average, is likely to be less frequent but more intense;
- Droughts will likely be more frequent and severe in some areas;
- Hurricanes will likely spawn increased precipitation and wind;
- The strongest cold-season Atlantic and Pacific storms are likely to create stronger winds and higher extreme wave heights.
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.
The full report is online at www.usgcrp.gov