Extreme Weather Events Predicted for Warming North America

Droughts, heavy downpours, excessive heat, and intense hurricanes are likely to become more common as humans continue to increase concentrations of heat-trapping greenhouse gases in the atmosphere, according to first comprehensive analysis of observed and projected changes in weather and climate extremes in North America.

The federal government's U.S. Climate Change Science Program and its Subcommittee on Global Change Research released the scientific assessment late last month.

"This report addresses one of the most frequently asked questions about global warming - what will happen to weather and climate extremes? This synthesis and assessment product examines this question across North America and concludes that we are now witnessing and will increasingly experience more extreme weather and climate events," said report co-chair Tom Karl, PhD, who directs the National Climatic Data Center in Asheville, North Carolina.

A Des Moines city worker checks debris caught in the arches of the Grand Avenue bridge over the swollen Des Moines River. June 11, 2008. (Photo by Greg Henshall courtesy FEMA)

The UN's Intergovernmental Panel on Climate Change previously evaluated extreme weather and climate events on a global basis in this same context but this is the first specific assessment across North America.

"We will continue to see some of the biggest impacts of global warming coming from changes in weather and climate extremes," said report co-chair Gerry Meehl, PhD, of the National Center for Atmospheric Research in Boulder, Colorado. "This report focuses for the first time on changes of extremes specifically over North America."

Global warming of the past 50 years is due primarily to human-induced increases in heat-trapping gases, according to the Climate Change Science Program report.

For the future, the report forecasts more abnormally hot days and nights, along with heat waves, while cold nights are very likely to become less common.

Sea ice extent is expected to continue to decrease and may disappear in the Arctic Ocean in summer in coming decades.

Precipitation, on average, is likely to be less frequent but more intense, the report finds, and at the same time droughts are likely to become more frequent and severe in some regions.

Hurricanes will likely have increased precipitation and wind.

The strongest cold season storms in the Atlantic and Pacific are likely to produce stronger winds and higher extreme wave heights, according to the report.

Hot, dry weather means more wildfires. Here, Lanny Thomise of the Juniper Valley Fire Department cuts into a burning tree in Colorado's Nash Ranch Fire as it reaches 1,100 acres in size. June 27, 2008. (Photo by Bryan Dahlberg courtesy FEMA)

The National Oceanic and Atmospheric Administration, an agency of the U.S. Commerce Department, plays a key role in the Climate Change Science Program, which is responsible for coordinating and integrating climate research, observations, decision support, and communications of 13 federal departments and agencies.

The report warns that these more extreme weather patterns will be tough on the U.S. economy.

"During the period 1980-2006, the U.S. experienced 70 weather-related disasters in which overall damages exceeded $1 billion at the time of the event," the report states. "Clearly, the direct impact of extreme weather and climate events on the U.S. economy is substantial."

Many changes in the North American climate have already occurred, the report explains.

Most of North America is experiencing more unusually hot days and nights and fewer unusually cold days. The last 10 years have seen fewer severe cold waves than any other 10 year period in the historical record, which dates back to 1895. The number of heat waves has been increasing since 1950.

There has been a decrease in frost days and a lengthening of the frost-free season over the past century.

Heavy downpours have become more frequent and intense and now account for a larger percentage of total precipitation.

Droughts are becoming more severe in some regions.

Atlantic tropical storm and hurricane destructive potential has increased substantially since about 1970.

Storm tracks have shifted northward in both the North Atlantic and North Pacific over the past 50 years. The strongest cold season storms are becoming even stronger in the North Pacific.

To read the report, "Weather and Climate Extremes in a Changing Climate," click here.

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