The World
Health Organization estimates that warming and precipitation trends over the
past 30 years killed 150,000 people annually and caused health problems in an
additional 5 million people. And with the Intergovernmental Panel on Climate
Change predicting average global temperatures to rise between 1.4 and 5.8 degrees
Celsius by 2100, climate scientists, physicians and others are warning of a
possible increase in heat-related deaths and disease outbreaks.
Ponds, such as this one near Alem Kitmama,
Ethiopia, are breeding grounds for mosquitoes, which can carry malaria. New
research indicates that climate change may bring on more incidences of malaria,
as well as other diseases. Image copyright of WHO/P. Virot.
The climate events that researchers are now looking at with an eye toward human
health include not only heat waves but also extreme events such as hurricanes.
For example, in 1998, Hurricane Mitch dropped almost 2 meters of rain on Central
America in three days, with related incidences of malaria, dengue fever and
cholera soaring in the following weeks. In 2000, rain and three cyclones inundated
Mozambique for six weeks, after which malaria rose fivefold.
More recently, amid the many lasting repercussions of Hurricane Katrina’s
fatal strike on the U.S. Gulf Coast, is something known as “Katrina’s
cough,” says Paul Epstein, a doctor and researcher at Harvard Medical School.
The cough may be caused by the prevalence of mold, toxins in the air and water,
or the oil spills left over from the blow.
“We have long known about the immediate problems associated with extreme
events, such as flooding, but it is the aftermath and the biological impacts
that we have been underestimating,” says Epstein, who discussed these links
in the Oct. 6 New England Journal of Medicine and in a report released
Nov. 1 called Climate Change Futures. Taken individually,
he says, extreme events in recent years might be anomalous, but together, they
indicate a trend: “Climate is changing, and such events are becoming more
probable.”
Certainly some links between climate change and human health are stronger than
others, and understandably there is some skepticism, says Jonathan Patz, a researcher
at the University of Wisconsin in Madison, who published an article in the Nov.
17 Nature about such connections. For example, although models show a link between
climate change and increased intensity of hurricanes, the link between hurricane
frequency and climate change is more tenuous (see Geotimes,
December 2005). Models, however, indicate that the risks of extreme events,
such as the European heat wave of 2003, have more than doubled because of climate
change, he says.
The strongest climate and health connection is heat-related mortality, says
Jeffrey Shaman, a climate scientist at Oregon State University in Corvallis.
“Here, we have a direct mechanism [for the deaths], and temperatures are
the culprit,” he says. Unlike connections between diseases and climate
change, there is no chain of events with temperature-related deaths: It’s
a simple cause-effect relationship. More tenuously connected, however, are climate
and diseases, such as malaria or West Nile Virus, Shaman says.
Although water, temperature and other environmental factors can affect the survival
and distribution of infectious agents (for example, bacteria and viruses) and
their disease-carrying vectors (for example, mosquitoes and ticks), “I
haven’t seen the follow-through” to indicate what the direct causes
of disease outbreaks may be, Shaman says. The “causal mechanism” of
the spread of disease is not as clear-cut.
Still, Patz says, “we can definitely see correlations,” pointing to
malaria outbreaks that have been associated with anomalously high temperatures
in Kenya and India, and dengue fever outbreaks in Central America, which have
been linked to higher temperatures and moisture. “It is clear that the
intense rains and increased flooding associated with a changing climate can
precipitate clusters of infectious diseases carried by mosquitoes, rodents and
by water,” Epstein says.
Other connections, such as that between plague outbreaks in the American Southwest
and climate change, may not be quite as obvious, Patz says. Researchers, however,
have observed that following anomalously heavy rainfall, rodent populations
that carry the plague expand, thus leading to more human cases. And, Patz says,
his team’s model simulations have correctly “predicted” the past
observed trends.
“Nailing down the exact effects of climate change on health is difficult,”
partially because so many factors are involved in determining how bad a disease
outbreak will be, says Cynthia Rosenzweig, a climate researcher with NASA’s
Goddard Institute for Space Studies in New York City. “It is important
that we learn about all the factors that are involved” in the spread of
disease, she says, especially with the World Health Organization estimating
that climate-change-induced health risks will more than double in the next 25
years.
Megan Sever
Links:
"Global climate
affects storms?" Geotimes, December 2005.
"Malaria
Mapping and Prevention," Geotimes, May 2005.
Climate
Change Futures
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