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November 2006

NCAR scientists predict a warmer, wetter Earth

New study based on computer models offers strong evidence for more extreme weather events in the future.

It’s rare to turn on the news these days without hearing about a heat wave in one place, a catastrophic flood in another, or perhaps an unusually strong snowstorm.

Claudia Tebaldi

Claudia Tebaldi.

Indeed, a new study by scientists in SERE and ESSL suggests that wild weather is in fact on the rise, and that some regions of the world will get hit harder than others. The study, slated for publication in the December issue of the journal Climatic Change, has already drawn ample media coverage in the United States and abroad. It foretells a warmer, wetter world in which the build-up of greenhouse gases in the atmosphere causes a greater number of extreme weather events, particularly heat waves and intense bursts of precipitation.

The study is one of the first of its kind to draw on the extensive computer modeling recently carried out for the Intergovernmental Panel on Climate Change’s upcoming report, due in early 2007.

“We now have the first model-based consensus on how the risk of dangerous heat waves, intense rains, and other kinds of extreme weather will change in the next century,” says Claudia Tebaldi (ISSE/CGD/IMAGe), who led the study. The team includes Julie Arblaster and Jerry Meehl, both in CGD. A fourth collaborator, Katherine Hayhoe, is based at Texas Tech University.

The power of multiple models

Previous studies have analyzed how average temperature and rainfall might change over the next century as greenhouse gases increase; the new study focuses specifically on how weather extremes would change. It is unique in that it incorporates simulations from nine computer models, giving the scientists greater confidence in its results.

“Past studies have looked at maybe one or two models, but by looking at these nine models and getting similar answers, it makes the results more ­credible,” Jerry says.

To generate data for the study, ­modeling centers around the world used supercomputers to run simulations for both the 20th and 21st centuries. Within this 200-year stretch, the researchers singled out the periods 1980–1999 and 2080–2099 to compute future changes in extremes. (Simulating past years and comparing the results to real weather observations lets scientists verify the accuracy of their models.) Each model simulated the future years three times, varying the amount of greenhouse gases that could accumulate in the atmosphere over the next hundred years to account for how society may or may not act to reduce emissions.


Vegetation struggles to grow in dry soil in eastern Colorado. Dry spells and droughts could lengthen significantly across the western United States, a hot spot for future weather extremes. (Photo by Carlye Calvin)

For all three scenarios, the models agree that by 2080–2099:

  • The number of extremely warm nights and the length of heat waves will increase significantly over nearly all Earth’s land areas.

  • Dry spells, which can produce and intensify droughts, could lengthen significantly across the western United States, the Mediterranean, eastern Brazil, and other mid- to low-latitude areas.

  • Most areas above about 40°N will experience a significant increase in the number of days with heavy precipitation (more than 0.4 inches). This includes the northern tier of the United States, Canada, and most of Europe.

  • The average growing season could increase significantly across most of North America and Eurasia.

The study pinpoints the western United States as a hot spot for some of the worst extremes. Some places, such as the Pacific Northwest, are expected to have longer dry spells punctuated by heavier rainfall.

Claudia emphasizes that weather extremes, not averages, are what damage society and ecosystems. The most ominous results are the predictions for heat waves and warm nights, she says. During heat waves, warm nights are often associated with fatalities because people and buildings have less chance to cool down.

She also notes that the scenario with the lowest increase in greenhouse gas emissions produced the least increase in extreme events. “We see this finding as the only hopeful note from this study,” she says. “Even if not positive in itself, we hope it will have a motivating effect for initiating long overdue measures to curb greenhouse gases, since it shows a very straightforward relation between the severity of the most extreme events to come and future concentrations of human-produced gases in our atmosphere.”

• by Nicole Gordon

On the Web

“Going to the Extremes: An Intercomparison of Model-Simulated Historical and Future Changes in Extreme Events”

In this issue...

NCAR scientists predict a warmer, wetter Earth

The end of the world as we know it?

“A New Light on Science”

Keeping science in the news

Short Takes

Random Profile: Chrystina Tasset

Delphi Question: Webhire formatting issues

Just One Look


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