To the list of predicted outcomes from global warming, researchers have added a particularly troubling possibility in recent years: increased incidence of disease. The logic behind that projection is easy enough to understand. Disease-transmitting insectsmosquitoes, for exampleoften flourish in warmer climates. The disease perhaps most worrisome in that regard is malaria, which already kills thousands of people every day. A recent surge of malaria cases in four high-altitude parts of East Africa seemed to fit neatly with the idea. Researchers writing today in the journal Nature, however, assert that in fact meteorological data do not support a link between the resurgence of malaria in East Africa and global warming.

Working from a 95-year data set of global terrestrial climate, Simon Hay of the University of Oxford and his colleagues scrutinized long-term climate trends for the four highland sites. The temperature, rainfall, vapor pressure and number of months when conditions were suitable for malaria transmission, the researchers report, "have not changed significantly during the past century or during the period of reported malaria resurgence."

But if rising temperatures didn't cause the observed increases in malaria, what did? According to the team, economic, social and political factors can explain the trend. "The more certain climatologists become that humans are affecting global climates," they conclude, "the more critical epidemiologists should be of the evidence indicating that these changes affect malaria."