If you can't stand global warming, get out of the tropics.

That's the advice from scientists studying conditions in Costa Rica.

The most significant harm from climate change so far has been in the polar regions.

But these scientists say tropical plants and animals may face an even greater threat.

"Many lowland tropical species could be in trouble," says the team of researchers, led by Robert K. Colwell of the University of Connecticut.

Their study appears in Friday's edition of the journal Science.

"The tropics, in the popular view, are already hot, so how could global warming harm tropical species? We hope to put this concern on the conservation agenda," Colwell said.

That's because some tropical species, such as insects, are living near their maximum temperatures already and warmer conditions could cause them to decline, Colwell explained.

"We chose the word 'attrition' to emphasize slow deterioration," he said. "How soon that will be evident enough for a consensus is difficult to say."

But the researchers estimated that a temperature increase of 3.2 C over a century would make 53 per cent of the 1,902 lowland tropical species they studied subject to attrition.

That doesn't mean today's jungles will one day be barren, however.

"'Tis an ill wind that blows nobody any good. Some species will thrive," Colwell said. "But they are likely to be those already adapted to stressful conditions," such as weeds.

Some species may shift to higher elevations

What of the others?

There are few nearby cooler locations for tropical plants and animals fleeing rising temperatures.

In the tropics in particular, going up rather than out may be an answer.

That's because tropical species with small ranges would have to shift thousands of kilometres north or south to maintain their current climatic conditions. "Instead," Colwell said, "the most likely escape route in the tropics is to follow temperature zone shifts upward in elevation on tropical mountainsides."

For example, moving uphill, the researchers said, temperature declines between 5.2 and 6.5 degrees for every 1,000 metres. To get a similar reduction moving north or south, species would have to travel more than 1,000 kilometres.

Of course moving won't work for everyone. Species already living on mountaintops will have no place to climb.

The study provides an important illustration of the potential risk to tropical species from global warming, Jens-Christian Svenning of the University of Aarhus, Denmark, and Richard Condit of the Smithsonian Tropical Research Center note in a commentary on the findings.

"These numbers suggest large risks," but are likely to be controversial because there remain large gaps in the knowledge of species' sensitivity to climate change, added Condit and Svenning, who were not part of the research team.

Meanwhile, a separate paper in Science reports that warming climate has already scrambled the ranges of small mammals in Yosemite National Park.

Ranges for some high-elevation mammals such as the alpine chipmunk have shrunk, while animals living at low elevations, such as the harvest mouse, have expanded their ranges into higher reaches, Craig Moritz of the University of California, Berkeley, and colleagues report.

Earlier in 2008, a study of 171 forest species in Western Europe showed most of them are shifting their favoured locations to higher, cooler spots. For the first time, research can show the "fingerprints of climate change" in the distribution of plants by altitude, and not only in sensitive ecosystems, said Jonathan Lenoir of AgroParisTech in Nancy, France.

His team found "a significant upward shift of species optimum elevation, the altitude where species are the most likely to be found over their whole elevation range."