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Global warming, global fever
29 June 2002
From New Scientist Print Edition.
Mark Schrope

As global warming heats the Earth, diseases will become more severe and widespread, affecting everything from trees and marine life to birds to people. That's the dark vision spelled out by a respected team of researchers this week.

While they accept that their findings are controversial, they say there is already enough evidence to justify fears that diseases will sweep north into higher latitudes, infecting people, disrupting fragile ecosytems and possibly wiping out some species. "We don't want to be alarmist, but we are alarmed," says team member Richard Ostfeld from the Institute of Ecosystem Studies in Millbrook, New York.

But some experts dismiss their conclusions as a "leap of faith" that could distract governments and health services from dealing with the epidemics already facing us.

The effect of climate change on the severity and spread of disease has been vigorously debated for years, but the link has rarely been studied in detail. Because disease outbreaks happen anyway, it's hard to judge whether they are caused by global warming or other factors such as the appearance of a new host or a change in the local ecology. Economic changes and failed prevention measures can also encourage the spread of disease.

The team has now published a major review of the literature in Science (vol 296, p 2158), including both epidemiological studies and projections using climate models. Put together, they reveal that climate change can have an impact in a number of ways. For instance, warmer winters will increasingly let pathogens and their carriers survive all year round, or allow them to move into previously cold areas. Pathogens could also spread via warmer waters, and infect hosts that used to live in different climates.

The researchers examined studies which looked at the recent spread of human diseases, including malaria, Lyme disease and yellow fever. Most show that the flies or ticks carrying these diseases move into higher latitudes as they warm up. And many lab and field experiments have shown that as the temperature rises these bugs reproduce and bite more.

The link is even stronger for diseases that affect plants and animals, says lead author Drew Harvell, a marine ecologist at Cornell University in Ithaca, New York. Viruses, bacteria, fungi and parasites have all been implicated in disease outbreaks in hosts that include corals, oysters, plants and birds.

For instance, there is compelling evidence that coral bleaching has been exacerbated by coral diseases that thrive in warmer water. Such events are typically tied to El Niņo, whose connection to climate change is uncertain. But Harvell insists that coral's problems are unprecented: "I don't know how to say it strongly enough. This just is not normal."

The researchers also found that oak trees, wheat, butterflies and lions have all been struck by diseases linked to global warming. "These other diseases are expanding, and clearly they're not subject to the collapse of public infrastructure and political unrest," says Ostfeld.

Climate change may slow the spread of some diseases, such as the cool-loving chytrid pathogen thought to be wiping out frogs in Central America. But this is outweighed by the diseases that are likely to get worse, say the scientists. The team accepts that the evidence isn't unequivocal for any one example, and acknowledges that it can be difficult to predict how hosts will adapt to new pressures. "Scientific certainty is a very elusive phenomenon," says Ostfeld. But he sees this as one more reason to try to halt global warming.

James McCarthy, co-chair of the Intergovernmental Panel on Climate Change working group on impacts, adaptation and vulnerability, says the team's work strengthens the case for a link between warming and an increased threat of disease. But so many other factors are involved that it is extremely difficult to prove.

The evidence is not enough to convince Simon Hay, an epidemiologist at Oxford University. He accepts that climate may have an impact on pathogens and carriers such as mosquitoes. But he says the evidence isn't strong enough to predict what will happen in future, particularly for human diseases. Hay also believes it is dangerous to focus too much on future impacts rather than present threats. "Most of the epidemiologists I deal with wouldn't rank climate change as one of the major risks facing our populations," he says. "That's not because we say it won't happen - we really don't know - but because our priorities are elsewhere."

From issue 2349 of New Scientist magazine, 29 June 2002, page 22

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