 Global warming could re-make the world's climate zones by 2100, with some polar and mountain climates disappearing altogether and formerly unknown ones emerging in the tropics, scientists said on Monday. And when climate zones vanish, the animals and plants that live in them will be at greater risk of extinction, said Jack Williams, lead author of a study published in the journal Proceedings of the National Academy of Sciences. "What we've shown is these climates disappear, not just regionally, but they're disappearing from the global set of climates, and the species that live in these climates really have nowhere to go as the system changes," said Williams, a geographer at the University of Wisconsin-Madison. Previous studies have raised the concern about species extinctions in specific areas -- such as the cloud forest of Costa Rica or the Cape region in South Africa -- but this is the first to predict this global change, Williams said in a telephone interview. As Earth warms, predicted to happen by up to 15 degrees F (8 degrees C) at some latitudes by the end of this century, climate zones are likely to shift away from the equator and toward the poles, the study said. "It's those climates near the poles or at the tops of mountains that are being pushed out...," Williams said. "It's getting too hot." Polar bears and ring seals, which depend on Arctic ice, could be among those species threatened by the shifting of climate zones, Williams said, but the study did not specifically address the fate of these animals. As polar climate zones disappear, new zones will be created in the parts of the world that are already the hottest, the study predicted, using models of climate change. The change in temperature is likely to be greater in the Arctic and Antarctic because when snow and ice melt, their ability to reflect sunlight goes away too, accelerating the warming effect. However, because normal fluctuations in temperature and rainfall are smaller in the tropics, even small changes in temperature can make a big difference in this warm region, co-author John Kutzbach, also of the University of Wisconsin-Madison, said in a statement. Williams attributed the warming to the building of carbon dioxide and other greenhouse gases in the atmosphere. A report in February by the Intergovernmental Panel on Climate Change said that with 90 percent probability, human activities are responsible for the warming of the planet.