Global warming may prove worse for insects—and other cold-blooded critters—living in the steamy tropics than for their counterparts living closer to the frigid polar regions, according to a new study in Proceedings of the National Academy of Sciences USA. Even though climate change is likely to affect areas near the poles, tropical insects are already living in conditions that verge on being too hot for them, which means they could be teetering on the edge of extinction.

Take the shield bug—also known as the stinkbug for the nasty smelling liquid it spews when attacked. There are varieties of the insect in both the U.K. and Kenya. But although the shield bugs in the former may prosper as a result of a warmer climate in their region, their counterparts in Kenya (and other parts of Africa) may find themselves unable to cope with the heat, according to the research—and, if they cannot adapt or move, they may perish. "The current climate is at its optimum temperature," says study co-author and biogeochemist Curtis Deutsch of the University of California, Los Angeles. "Any warming was going to push them towards reduced fitness."

Deutsch, Joshua Tewksbury of the University of Washington in Seattle, and other colleagues compared anticipated temperatures in both the tropics and higher latitudes with the optimum temperatures preferred by a variety of cold-blooded creatures—specifically, insects, frogs, lizards and turtles.

Although they found that temperature increases are likely to be greater near the poles—more than two degrees Celsius (3.6 degrees Fahrenheit)—northern insects, for one, might benefit from less cold winters and even warmer summers. But their tropical cousins live in an area without extreme seasonal swings in temperature and are therefore very specifically adapted to live within a very narrow thermal range. Predicted warming of a fraction of a degree may be too much for them. "They are more highly sensitive to a given amount of warming," Deutsch notes. "In the tropics, organisms will be more rapidly approaching temperatures that are too warm for them to reproduce."

The same is likely to be true for tropical frogs, lizards and turtles, according to the study. That's consistent with what is already happening. For example, the frogs of La Selva Biological Station in Braulio Carrillo National Park in Costa Rica's Caribbean lowlands have endured a 75 percent drop in population since 1970, perhaps due to climate change, according to a study by biologist Steven Whitfield of Florida International University in Miami, who was not affiliated with this study.

The fact that the tropics have such a large share of Earth's species means the effects could have a serious impact on biodiversity. "Organisms do have temperature ranges in which they can thrive and wider ones in which they can exist at all," says entomologist John Losey of Cornell University, who was not involved with the research. "There is indeed much more diversity in the tropics and it may in fact be in peril if temperatures rise quickly enough."

"Tropical species in Brazil or Kenya will indeed be much more sensitive to small temperature shifts than their temperate counterparts in places like England and Saskatchewan," adds biologist Robert Pringle of Stanford University's Center for Conservation Biology. This worrying finding "applies to all organisms that can't regulate their own body temperatures metabolically, which is the majority of species on Earth."

Still, cold-blooded tropical animals have survived climate changes in the past and may be able to once again adapt, acclimate, migrate or even change their behaviors. For example, beetles might burrow into the ground to stay cool during the hottest parts of the tropical day. And insects benefit from reproducing quickly, which could allow them to adapt more quickly than their slower-breeding lizard counterparts, who are also cold-blooded.

"We simply can't quantify right now how useful those strategies are going to be, especially in the context of this climate change," Deutsch says. "This change is extremely rapid compared to changes that have occurred in the geological past." Nor is it clear how other, less well understood climate changes, such as more or less rainfall, could affect insects.

And in more temperate regions, bugs could even benefit: As the pine beetles eating their way through the vast boreal forests of western North America prove, changes in climatic patterns can help insect pests by allowing them to survive through the winter. Pine beetles aside, it is in humankind's interest that certain beneficial insect species—what Deutsch calls the "unsung heroes in ecosystems"—survive, although some may not.

There are just too many factors to predict all the effects with any certainty, although it is clear that there will be some winners and some losers. "Direct impact of warming on physiology is a fundamental and direct consequence of warming, but it's really only one foundation stone for the response of the ecosystem," Deutsch says. "That is going to be vastly more complex."