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Disease Rise Global Warming*

June 21, 2002 Talk about it E-mail story Print

Study Links Warming to Epidemics

Science: The survey lists species hit by outbreaks and suggests that humans are also in peril.


A wide-ranging survey of world ecosystems shows that warmer temperatures have sparked a host of epidemics in plants and animals, suggesting that global warming could ravage the planet's ecology and accelerate disease in a number of species--including our own.

The study is a litany of species--oysters, oaks, monarch butterflies and colorful Hawaiian forest birds called honeycreepers--that have succumbed in recent years to new or more virulent disease outbreaks due to even small increases in temperature.

Although many of the deaths were related to short-term temperature spikes from phenomena such as El Nino, scientists said the findings bolster theories that long-term global warming could usher in similar devastation--and on a much broader scale. "It's not only going to be a warmer world, it's going to be a sicker world," said Andrew Dobson, an epidemiologist at Princeton University and co-author of the report published in today's issue of Science.

Almost everywhere the authors looked--on land and sea, in tropical climes and temperate ones--they found examples of plants and animals falling prey to disease as temperatures rose.

"When you see the same pattern across so many organisms, you need to start taking the climate signal seriously," said Richard S. Ostfeld, an ecologist at the Institute of Ecosystem Studies in Millbrook, N.Y. "We don't want to be alarmist, but we are alarmed."

The link between global warming and disease has been controversial since it was first proposed in 1991. Many critics argue that such concerns are overblown since a connection has yet to be firmly established. The problem has proved to be maddeningly difficult to analyze because of the multitude of factors, such as pollution and habitat destruction, that play major roles in the spread of disease.

The most contentious aspect of the debate has been the effect of warming on human epidemics, which are far more difficult to dissect because they involve a range of factors such as sanitation and vaccination.

The new paper sidesteps that debate somewhat by focusing largely on wild plants and animals that aren't subject to a public health infrastructure. But its implication is clear that humans are at risk too. "The mechanisms that cause malaria to be transmitted to honeycreepers are exactly the same mechanisms that transmit malaria to children," Dobson said.

The authors chronicle a variety of ways that warmer temperatures could fuel the severity and proliferation of disease. Warm nights and shorter winters allow the fungus that causes Dutch elm disease to thrive. Higher temperatures have allowed malaria-carrying mosquitoes to invade higher elevations, leading to more malaria deaths in mountain-dwelling Hawaiian songbirds. Ligurian Sea sponges that suffer heat stress in warmer waters are more vulnerable to fatal infection.

The authors also included examples in which warm temperatures could lead to less disease. These include a number of diseases in fish, insects and amphibians that spread only in cool conditions.

"It seems clear the importance of temperature is pretty overwhelming," said Drew Harvell, a Cornell University marine ecologist and lead author of the study.

Harvell said her team could not yet make specific forecasts on how warming might alter ecosystems, but she said there was concern because "diseases are capable of reshaping whole communities very rapidly." She pointed to Caribbean corals, where many species that existed for 4,000 years have been wiped out by disease in just 15 years.

The changes have occurred during a century in which the planet's temperature has risen by 1 degree Fahrenheit. Most scientists agree that human-produced greenhouse gases have been the major factor and predict the Earth will warm several more degrees by 2100.

The new study does a good job of compiling temperature-related changes in plant and animal diseases, and of not oversimplifying such relationships, said Mark L. Wilson, an ecologist and epidemiologist at the University of Michigan who has long been critical of previous studies. Early work, he said, ignored factors such as evolution or human intervention that could mitigate the effects of climate change.

He lauded the authors for illuminating the complexity of the issue by including examples of how warmer temperatures could decrease some disease rates. "That's the kind of rigorous thinking I'd like to see more of."

The field has a long way to go, Wilson added, before it can predict how climate change will play out. "The bottom line is, with climate change things are very likely to change an awful lot, but in what direction and at what rate?"

Scientists still disagree about how much effect temperature change has on the spread of human diseases.

"Most of the epidemiologists I deal with wouldn't rank climate change as one of the major risks facing our populations, not because we say it won't happen but because our priorities are elsewhere," said Simon Iain Hay, a zoologist at Oxford University who in February published a study in Nature suggesting that increased malaria rates in East African highlands were not due to rising temperatures.

Paul Reiter, a dengue expert with the Centers for Disease Control and Prevention's Puerto Rico office, argues against the relative importance of climate in human disease by pointing to periods in the past when malaria and other tropical diseases pervaded cooler regions. He argues that the spread of malaria is more closely linked to deforestation, agricultural practices, human migration, poor public health services, civil war, strife and natural disasters.

"Claims that malaria resurgence is due to climate change ignore these realities and disregard history," he wrote in an article about malaria spreading through England during the Little Ice Age, which began about 1450 and lasted for several hundred years.

Other epidemiologists argue that humans may be able to counter new temperature-sensitive plagues of disease with technology and better public health services.

A 1995 dengue pandemic that spread through Mexico, for example, practically stopped at the Rio Grande. "You don't have such epidemics [in the U.S.] today because you live in screened, air-conditioned houses and have first-world health care," said Hay.

Arguments that strong public health will shield us from emerging plagues only frustrate Jonathan Patz and Paul Epstein, two physicians who have spent more than a decade working on--and warning the world about--what they see as a coming epidemic of climate-induced disease.

"This is a truly disturbing panorama," said Epstein, of Harvard Medical School's Center for Health and the Global Environment. "I'm afraid we've underestimated the true costs of what climate change is doing to our environment and our society."

Patz, who directs the Program on Health Effects of Global Environmental Change at the Johns Hopkins Bloomberg School of Public Health, agreed with those who said public health officials need to address acute issues of poverty, sanitation and vaccination.

But he said it would be dangerous to ignore climate-induced problems that could wreak havoc.

"That's like saying we'll only treat heart attacks and ignore hypertension," he said. "This is not a black-and-white issue."

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