If global warming continues unabated, many of the world's climate zones may disappear by 2100, leaving new ones in their place unlike any that exist today, according to a new study. Researchers compared existing patterns of temperature and precipitation with those that may exist at the turn of the century, based on scenarios put forth in the recent report of the Intergovernmental Panel on Climate Change (IPCC). They found that if greenhouse gas emissions continue rising at the same rate, up to 39 percent of Earth's continental surface may experience totally new climates, primarily in the tropics and adjacent latitudes as warmer temperatures spread toward the poles.

Researchers say the analysis was intended to more precisely gauge the ecological consequences of climate change. Studies have already estimated that species such as butterflies are creeping toward the poles at a rate of six kilometers per decade as temperatures rise. Some species, however, may not be able to keep pace with future changes potentially leading to new regional ecosystems as novel climate patterns emerge, possibly leading to extinctions if some climates disappear entirely.

To evaluate the range of possible outcomes, ecologists John Williams of the University of Wisconsin–Madison, and Stephen Jackson of the University of Wyoming, along with U.W. Madison climatologist John Kutzbach compared global climate projections published last month by the fourth IPCC with current regional climates, looking specifically at average summer and winter temperatures and precipitation. They considered scenarios of either unchecked greenhouse gas emissions or a global reduction in the rate of emissions growth.

They found that the business-as-usual scenario comes with large climate changes the world over and would create entirely new patterns of temperature and precipitation for 12 to 39 percent of Earth's land area. An additional 10 to 48 percent of land would see its climate zones disappear, replaced by patterns of temperature and precipitation now occurring elsewhere, such as rain forest becoming savanna or evergreen forest becoming deciduous. In the reduced-emissions scenario, the group reports that the two kinds of change would each take hold over 4 to 20 percent of land.

In the case of unchecked emissions, "we are going to be seeing climates that certainly are completely outside the range of modern human experience," Jackson says. According to the analysis, new climates would be most dramatic in the rain forests of the Amazon and Indonesia, but would extend as far toward the poles as the American southeast.

Climate disappearance would occur in tropical mountains and near the poles, including regions such as the Andes, the African highlands, Indonesia and the Philippines, parts of the Himalayas and near the Arctic. With nowhere to go, species in these regions might become extinct, the group notes in this week's Proceedings of the National Academy of Sciences USA.

Jackson says that prior studies have concentrated on ecological changes closer to the poles, but the tropical changes might be more dramatic. "If [the climate of] Memphis moves to Chicago, we have a Memphis there to say what Chicago will look like," he says. "For an area where we don't have a modern analogue, there's really nothing to look at to say, this is what the environment will look like."