2. WEATHER: Climate extremes will increase as Earth warms, study warns (ClimateWire, 06/20/2008)

Lauren Morello, ClimateWire reporter

Climate change is reshaping weather across the United States and its territories, bringing more intense rains, severe droughts and hotter days and nights, according to a new report by government scientists.

It's a trend that's not likely to abate anytime soon, says the report released yesterday by the federal Climate Change Science Program, which examines weather patterns in North America, Hawaii and U.S. territories in the Caribbean Sea and the Pacific Ocean.

The analysis calls the rise in "extreme weather," expected to intensify in coming decades, one of the "most serious challenges to society in coping with a changing climate."

Many of the changes are already apparent, scientists said.

The heavy downpours that lead to recent flooding in the Midwest may be a harbinger of things to come, said Tom Karl of the National Oceanic and Atmospheric Administration, a lead author of the report.

If greenhouse gas emissions continue to rise, extremely intense rainfall that now occurs every 20 years could happen as often as every five years by the end of the century, Karl said, with the change most pronounced in the northern United States.

Another scientist involved with the report, Gerald Meehl of the National Center for Atmospheric Research, said there is already "convincing scientific evidence" that human-produced greenhouse gas emissions are behind recent increases in the number of very heavy rainstorms.

At the same time, the new report also warns that climate change will likely intensify droughts, especially in the American Southwest, which is now in the midst of a drought that started in 1999.

The idea that climate change will lead to both heavier downpours and more drought seems counterintuitive, Karl said, but there's a common-sense explanation.

"When it rains, it rains harder," he said, describing the future conditions predicted in the report. "When it's not raining, the temperatures are warmer, so more water evaporates -- and that means droughts become more intense and last a bit longer."

That increased dryness, combined with earlier spring snowmelt driven by increasing temperatures, will likely mean less available freshwater in the western United States, an area already grappling with intense conflicts over water distribution rights.

A 90% chance of warmer days and nights

Other apparent changes include the rise in average air temperatures, the report notes. Six of the last 10 years are among the hottest 10 ever recorded, with 1998 at the top of the list.

Looking at it another way, there have been fewer cold snaps in the last decade than in any other 10-year period in the historical record of temperature data, which stretches back to 1895.

"We have fewer nights when temperatures go below freezing, and we've seen a decrease in frost days," Meehl said. The end result, he said, is higher average temperatures and a greater risk of very hot days, much like the heat wave that gripped Europe in summer 2003.

According to the report, there is a 90 percent or greater chance that the increase in hot days and nights will continue through the end of the century.

By the middle of the century, if greenhouse gas emissions continue to increase at their current rate, a day so hot it now occurs every 20 years could occur as often as every three years in the continental United States and every five years in Canada.

By the end of the century, a day that hot could occur every other year -- or more.

The report also concludes that:

• Human-produced greenhouse gas emissions are the main factor driving the increase in global average temperatuers observed over the last half-century.

• The strength and frequency of Atlantic hurricanes increased dramatically in recent decades, likely driven by warmer sea surface temperatures. Future climate change is likely to increase hurricane wind speeds, the amount of rainfall the storms produce and storm surge levels.

• Cold-season storms in the Atlantic and Pacific oceans will likely see increasingly strong winds and higher wave heights by the end of the century as a result of climate change.

• Arctic Ocean sea ice will continue to decline and may disappear entirely during summers in coming decades, exposing Alaskan and Canadian coasts to ocean waters and increasing the risk of severe coastal erosion.

Click here for the CCSP report.

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