Mitigating these grim projections, reductions of greenhouse gas emissions may lessen the intensification of dangerously hot days by as much as 50 percent, the study shows.
"Rare events today, like the 2003 heat wave in Europe, become much more common as greenhouse gas concentrations increase," says Noah S. Diffenbaugh, of Purdue University in West Lafayette, Ind., who led the study. Indeed, they "become the norm and the extreme events of the future are unprecedented in their severity," he says.
The analysis indicates that daily temperatures currently found in the hottest two weeks of the summer instead will be found in the coldest two weeks of the summer. In Paris, for example, temperatures that occurred there during the heat wave in 2003 are exceeded a couple dozen times every year in the simulated future.
Fueling the new projections is an intensified impact of global warming at the high end of the Mediterranean's summer temperature range. The researchers find that warming and reduced precipitation in the region contribute to preferential warming of the hottest days.
The most scorching summer days "warm more than the typical summer days warm," Diffenbaugh explains. "One might expect that an average warming of four degrees would equate to each day warming by four degrees, but in fact the hottest days warm quite a bit more," he says.
This is due, in large part, to a surface moisture feedback, the scientists propose. The surface gets dryer as it gets hotter and the dry soil leads to less moisture in the area and less evaporative cooling. The locations of intensified warming on hottest days of the year match the locations where surface drying occurs, Diffenbaugh says.
In addition to threatening people's lives, soaring temperatures could harm the Mediterranean region's economy, notes study co-author Jeremy S. Pal of Loyola Marymount University in Los Angeles, Calif.
The region extends into 21 European, African, and Asian countries that border the Mediterranean Sea. Its metropolitan areas include Rome, Paris, Barcelona, Algiers, Cairo, Istanbul, and Tel Aviv. Negative consequences in the area could affect human health, water resources, agriculture, and energy demand, Pal adds.
The ultimate severity of the damage depends on what steps are taken today. "Technological and behavioral changes that are made now will have a big influence on what actually happens in the future," says Diffenbaugh. "Decreases in greenhouse gas emissions greatly reduce the impact."
Still, "we see negative effects even with reduced emissions," he notes.
In the new work, Diffenbaugh, Pal, and colleagues in Italy and China, analyze climate simulations covering two time periods: 1961 to 1989 and 2071 to 2099. They report their results in the June 15 Geophysical Research Letters, a publication of the American Geophysical Union.
The simulations assume emissions scenarios as proposed in 2000 by the United Nations Intergovernmental Panel on Climate Change (IPCC), a leading scientific organization that evaluates climate- change-related science. One scenario anticipates that greenhouse gas emissions will continue to increase exponentially. The other, reduced-emissions scenario incorporates diminished population growth and greater environmental concern.
Although newer emissions scenarios have been generated since 2000, a recent assessment by IPCC found that those scenarios differ little in their emissions ranges from the older ones, Diffenbaugh says.
The Mediterranean region study also uses the National Weather Service Heat Index in the analysis of the heat stress response to increasing greenhouse gas concentrations.
The areas most likely to face substantial increases in dangerous heat index are concentrated largely in coastal areas, the researchers find. The team can discern such localized effects because the climate model used in the study has a resolution of 20 kilometers (12 miles) -- perhaps the highest spatial resolution available for the Mediterranean region. Much as increased resolution in a photograph makes a clearer picture and allows one to zoom in without blurring the image, the powerful resolution of the climate model allows researchers to gather detailed information about particular areas. The researchers used a supercomputer in the National Climate Center in Beijing to run the climate model.
"This is the first time this amplification signal over coastal areas could be seen and quantified," says co-author Filippo Giorgi of the International Centre for Theoretical Physics in Trieste, Italy. "Coastal regions are particularly vulnerable because they will likely be affected by other important climate change related stresses, such as a rising sea level." Many large cities in the Mediterranean region are on the coast, Giorgi notes.
The study was funded by the Italy-USA collaborative agreement on climate change research and the National Science Foundation.
Peter Weiss | American Geophysical Union
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