Polar bears may have it relatively easy. It's the tropical creatures that could really struggle if the climate warms even a few degrees in places that are already hot, scientists reported.
That doesn't mean polar bears and other wildlife in the polar regions won't feel the impact of climate change. They probably will, because that is where the warming is expected to be the most extreme, as much as 10 degrees Celsius by the end of this century. But there are far fewer species living in the Arctic and Antarctic and in the temperate zones than in the tropics, said Curtis Deutsch of the University of California at Los Angeles.
Many of these tropical creatures are living at the edge of their temperature tolerance already. Even the slight tropical warming predicted by 2100 - three degrees Celsius - could push them to the brink, Dr Deutsch said in a telephone interview.
In research published in Proceedings of the National Academy of Sciences, Dr Deutsch and his co-authors investigated what could happen to cold-blooded animals in the tropics over the next 100 years if the predictions of greenhouse warming hold true.
They chose cold-blooded creatures - mostly insects but also frogs, lizards and turtles - because warm-blooded animals have other ways of regulating their body temperatures, such as growing a thick coat of fur to guard against cold and shedding when it gets warm.
Cold-blooded organisms can either seek shade when it's hot or sun themselves when it's cool, but otherwise they are limited, Dr Deutsch said.
"If nothing else happens, if they were just subjected to warming temperatures and everything else in their environment stayed the same, we would predict that their populations would crash more quickly," he said, meaning that many would die and their reproductive rates would plummet.
These animals do have other options besides a species crash, he said: They can migrate uphill or toward the poles to seek cooler climates, or they can evolve, and those with the best tolerance for heat would survive.
If they migrate or mutate, this could have an important impact on humans living outside the tropics, Dr Deutsch said, since insects particularly play key roles in pollinating agricultural crops and breaking down organic matter into essential nutrients for other creatures.
"The direct effects of climate change on the organisms we studied appear to depend a lot more on the organisms' flexibility than on the amount of warming predicted for where they live," said co-author Joshua Tewksbury of the University of Washington.
"The tropical species in our data were mostly thermal specialists, meaning that their current climate is nearly ideal and any temperature increases will spell trouble for them," Dr Tewksbury said in a statement.