I know that global warming causes extreme weather and melts glaciers

I know that global warming causes extreme weather and melts glaciers and causes sea level rises. But how does it increase the spread of disease?

—Curran Clark, Seattle, WA

Climate change accelerates the spread of disease primarily because warmer global temperatures enlarge the geographic range in which disease-carrying animals, insects and microorganisms—as well as the germs and viruses they carry—can survive. Analysts believe that, as a result of global temperature rises, diseases that were previously limited only to tropical areas may show up increasingly in other, previously cooler areas.

For example, mosquitoes carrying dengue fever used to dwell at elevations no higher than 3,300 feet, but because of warmer temperatures they have recently been detected at 7,200 feet in Colombia’s Andes Mountains. And biologists have found malaria-carrying mosquitoes at higher-than-usual elevations in Indonesia in just the last few years. These changes happen not because of the kinds of extreme heat we’ve experienced in recent months, but occur even with minuscule increases in average temperature.

But extreme heat can also be a factor, and the nexus of global warming and disease really hit home for North Americans in the summer of 1999, when 62 cases of West Nile virus were reported in and around New York City. Dr. Dickson Despommier, a Columbia University public health professor, reports that West Nile Virus is spread by one species of mosquito that prefers to prey on birds, but which will resort to biting humans when its normal avian targets have fled urban areas during heat waves.

“By reproductive imperative, the mosquitoes are forced to feed on humans, and that’s what triggered the 1999 epidemic,” Despommier says. “Higher temperatures also trigger increased mosquito biting frequency. The first big rains after the drought created new breeding sites.” He adds that a similar pattern has been recognized in other recent West Nile outbreaks in Israel, South Africa and Romania.

Bird flu is another example of a disease that is likely to spread more quickly as the Earth warms up, but for a different reason: A United Nations study found that global warming—in concert with excessive development—is contributing to an increased loss of wetlands around the world. This trend is already forcing disease-carrying migrating birds, who ordinarily seek out wetlands as stopping points, to instead land on animal farms where they mingle with domestic poultry, risking the spread of the disease via animal-to-human and human-to-human contact.

A Congressionally-mandated assessment of climate change and health conducted in 2001 predicted that global warming will cause or increased incidences of malaria, dengue fever, yellow fever, encephalitis and respiratory diseases throughout the world in coming decades. The assessment also concluded that insect- and rodent-borne diseases would become more prevalent throughout the U.S. and Europe.

The news isn’t good for less developed parts of the world either. Researchers have found that more than two-thirds of waterborne disease outbreaks (such as cholera) follow major precipitation events, which are already increasing due to global warming.

CONTACTS: Natural Resources Defense Council Consequences of Global Warming, www.nrdc.org/globalWarming/fcons.asp.