Global warming now threatens millions of species.

For more than a decade, World Watch has reported on the growing evidence that a rapidly changing climate could drive many species into extinction by shrinking or altering their habitats. Now, the most comprehensive study yet published on that prospect warns that as many as a quarter of the planet's species could disappear within the next half-century. Published in January by the journal Nature, the report is coauthored by conservation biologist Chris Thomas of the University of Leeds and Lee Hannah of Conservation International. The authors headed a team of researchers from the United Kingdom, United States, Mexico, South Africa, and Brazil.

The researchers projected future populations of 1,103 types of native plants and animals, representing about 20 percent of the Earth's surface. Extrapolating to the planet as a whole, they calculated that if global temperature rises between 2 and 6 degrees F during the next century, as predicted by the UN's Intergovernmental Panel on Climate Change (IPCC), between 18 and 35 percent of the planet's species could be extinguished by the year 2050. They did not attempt to project what might happen after mid-century.

Thomas and Hannah stress that while many of our planet's species have successfully adapted to changing climate in the past, a key difference now could be the speed of the current warming. Human-caused releases of carbon dioxide and other climate-warming gases appear to have triggered a more sudden change in the planet's temperature than many species will be able to adapt to. If habitat is destroyed faster than a particular species can migrate or evolve (other studies have calculated limits to the speeds with which various trees can migrate to higher latitudes or altitudes, for example), the species will be unable to survive.

The authors also stress that a mass extinction of this magnitude--possibly the largest since the dinosaurs were killed off 65 million years ago--would have profound impacts on human life, as well. "Where we can produce food will change," Thomas said. "The spread of diseases will change."