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Warming most evident at high latitudes, but greatest impact will be in tropics

August 15, 2005

The impact of global warming has become obvious in high latitude regions, including Alaska, Siberia and the Arctic, where melting ice and softening tundra are causing profound changes. But, contrary to popular belief, the most serious impact in the next century likely will be in the tropics, says a group of researchers headed by a University of Washington ecologist.

Scientists have noted warming at higher latitudes that already appears to be causing some flowers to bloom earlier than usual and seems to be altering some wildlife migration and hibernation patterns.

"You see this and you think the higher latitudes are really being hammered by climate change. We are arguing that this might not be true," said Joshua Tewksbury, a UW assistant professor of biology. "To predict the impact of climate change, we need to know the amount of change and how organisms are able to tolerate that change. Previous research has focused on change alone and ignored tolerance."

The more dramatic impact could actually be in the moist tropics, despite modeling that indicates temperatures there will warm just 2 or 3 degrees by 2100 compared with 6 degrees or more at higher latitudes, Tewksbury said. That is because organisms in the tropics normally do not experience much temperature variation because there is very little seasonality, so even small temperature shifts can have a much larger impact than similar shifts in regions with more seasonal climates.

"Temperatures in the tropics don't fluctuate that much, so the relatively small temperature shifts predicted by climate change models will be very large in relation to what organisms are adapted to tolerate," he said. "It's only going to be perhaps a 2-degree change, but in many tropical areas organisms have never experienced a 2-degree change."

By contrast, higher latitudes can have vast temperature fluctuations from hot summers to cold winters, and so plants and animals already are adapted to a wide range of temperatures. For example, it is not unusual for temperatures in Alaska's interior to reach far below zero in the winter and into the mid-70s during the summer. Flowers that bloom earlier and animals that change their hibernation or migration patterns in response to warming temperatures could be an illustration of the adaptability of life at higher latitudes.

Tewksbury's group collected studies that examined several closely related populations of plants, animals and insects to determine how the populations tolerated changing temperatures. Then they developed models based on the average between a region's highest temperature in the warmest month of the year and lowest temperature in the coldest month and programmed the region's expected temperature changes because of climate warming.

"What we find is that organisms in the tropics have very low tolerance," he said. "The evidence suggests that the range of temperatures an organism experiences dictates its tolerance to changing climate, or defines the temperature envelope in which it can live."

Tewksbury will present the findings Friday at the Ecological Society of America's annual meeting in Montreal. Collaborators include Raymond Huey, a UW biology professor, and UW biology doctoral students David Haak, Paul Martin and Kimberly Sheldon, who is lead author for the work.

Tewksbury noted that while direct estimates of tolerance are not available for all organisms or for all regions of the globe, there is good information on the degree of seasonal temperature fluctuation worldwide. That seasonality, he said, can be used to predict how well organisms will tolerate climate change.

The findings also imply that warming could forever alter life on Earth because the vast majority of species live in the tropics and many could be driven to extinction because of their inability to adapt.

"Evolution only happens if you don't go extinct. From an evolutionary standpoint, a model of the climate change impact reflects a race between adaptation and extinction," Tewksbury said. "Climate models project 6 degree temperature shifts in temperate zones during the next 90 years and 2 to 3 degrees in equatorial climates. That is such a rapid change that longer-lived organisms such as trees will have very little opportunity for adaptation - survival might be a matter of tolerance alone. In contrast, for an insect that has three generations a year the evolution of tolerance might play a larger role.

"It's kind of a bleak outlook for tropical organisms, and it shows how the lack of seasonal temperature variation can magnify the impact of climate warming.\\\

University of Washington