Tropical diseases in a changing climate.

Global warming poses dangers more subtle than melting ice caps and flooded cities. According to epidemiologists at the World Health Organization, climate change could increase the range of many diseases that are now predominantly tropical. Most pathogens are geographically constrained by the distribution of their host species, or by environmental restrictions on their life-cycles. For example, incubation periods (the time between infection and when the host is capable of spreading the disease) are often temperature-determined. And the length of time a pathogen can survive without a host is usually governed by temperature and the amount of moisture available.

WHO scientists have been looking at the effects of global warming on such diseases as malaria, dengue, and yellow fever--and on the insects and mammals that spread them. Their modeling indicates that with a warming of just a few degrees Celsius, and with regional precipitation increases of 7 to 15 percent, these diseases could spread to more temperate latitudes, or into areas that are currently too dry for them.

In the 1960s, widespread immunization and treatment programs brought many tropical diseases under control. Interest in--and funding for--tropical epidemiology has since declined. But impending climate change, combined with some pathogens' growing resistance to the drugs used to treat them, has some researchers worried that the diseases may be poised to expand their ranges at a time when we are least prepared to counter them.

Researchers aren't sure whether to blame climate change or ineffective control measures for recent outbreaks of tropical diseases in areas formerly marginal to them. Careful monitoring might clarify the causes and help determine the best methods for combating infection. Of course, another sensible step would be further reductions in greenhouse gas emissions.