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NEW YORK, 10 September 2006
Dear EarthTalk: 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.
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