Over the next 100 years, many scientists predict, 20 percent to 30 percent of species could be lost if the temperature rises 3.6 degrees to 5.4 degrees Fahrenheit. If the most extreme warming predictions are realized, the loss could be over 50 percent, according to the United Nations climate change panel.

Polar bears have become the icons of this climate threat. But scientists say that tens of thousands of smaller species that live in the tropics or on or near mountaintops are equally, if not more, vulnerable. These species, in habitats from the high plateaus of Africa to the jungles of Australia to the Sierra Nevada in the United States, are already experiencing climate pressures, and will be the bulk of the animals that disappear.

In response to warming, animals classically move to cooler ground, relocating either higher up in altitude or farther toward the poles. But in the tropics, animals have to move hundreds of miles north or south to find a different niche. Mountain species face even starker limitations: As they climb upward they find themselves competing for less and less space on the conical peaks, where they run into uninhabitable rocks or a lack of their usual foods  or have nowhere farther to go.

“It’s a really simple story that at some point you can’t go further north or higher up, so there’s no doubt that species will go extinct,” said Walter Jetz, professor of ecology and evolutionary biology at Yale, whose research last year predicted that a third of the 1,000 mountain birds he studied, or 300 species, would be threatened because warming temperatures would decimate their habitats.