A heat surge like that is enough to cause today's worrisome animal behavior to turn tragic, according to two studies published in the January 8, 2004, issue of Nature. Even slight increases in temperature can push species toward the poles or to higher elevations, but for many animals, migration to a cooler habitat is impossible. Scientists calculate that up to one third of the 1,103 plants and animals they studied could plunge into extinction by 2050. If extrapolated globally, more than one million species could disappear because of climate change by midcentury. Meanwhile, warming could flood cities and cause human health problems such as respiratory disease and increased rates of malaria.