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Global warming poses greatest problems for organisms in the tropics, UW scientists say

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Challenging common assumptions, research by UW scientists estimate future impacts due to climate change may be greatest on organisms in the tropics. Their findings are described in a paper published in the Proceedings of the National Academy of Science.

"There is an implicit assumption that impacts will be greatest at high latitudes because the rate of warming is greater there," said Curtis Deutsch, co-lead author of the paper and UCLA professor of atmospheric and oceanic sciences. "Our results counteract that expectation using a conceptual framework that quantifies the effect of temperature change on population growth and fitness."

The group's results not only contribute an estimate of biological impacts using the best empirical data available, but also provide a check to some of the more provocative images of climate change impacts.

The study used biological data for ectotherms (cold-blooded animals) including insects, frogs, lizards and turtles living in a variety of locations worldwide to construct performance curves relating fitness to temperature. Ectotherms are dependent on their environment for sources of body heat and are known to function best at optimal temperatures.

Reductions in performance are seen as the temperature rises or falls to a maximum or minimum critical temperature.

Combining this data with projections of future climate change allowed the scientists to estimate changes in population growth rates 100 years into the future. This was achieved by comparing observations of climate to projections from the most recent assessment report of the Intergovernmental Panel on Climate Change (IPCC) and integrating the change over the performance curves.

"The strongest decreases in fitness should be found where the sensitivity is greatest," Deutsch said. "Sensitivity is greater where the range of temperature tolerance is smallest, not necessarily where the rate of temperature change is highest."

Joshua Tewksbury, a UW professor of biology, the group's other leader, explained that organisms living in the tropics are already living near their optimal temperature.

"When you change the temperature in the tropics, the entire year gets too warm," Tewksbury said. "Organisms don't have the option of retreating to a better season and performance suffers."

The tropics receive the most solar radiation of anywhere on Earth and are at the warmest latitudes. Because the amount of incoming energy varies less throughout the year, there is no pronounced seasonal cycle of temperatures customary to the mid-latitudes.

The scientists emphasized the importance of biological sensitivity over the magnitude or rate of warming, but both said that their study is limited in its scope.

"Scientists like to take one piece of a complex puzzle, hold everything else constant and see how it impacts some other property," Tewksbury said. "Here we are looking at the direct effects of temperature on organisms. Now we can put that puzzle piece back into the big picture and ask whether it still holds."

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