Is a Warmer World a Sicker World?

As scientists piece together how climate impacts disease, strange patterns are emerging: mosquito outbreaks can follow drought, shorter migrations can make butterflies sick, and more birds (not fewer) can ward off West Nile virus.

By Roberta Kwok

In the late 1990s, a set of alarming maps created a stir in the scientific community. Based on predictions by a team of Dutch and Australian researchers and initially published in the journal Environmental Health Perspectives, the maps charted how global warming could increase the risk of malaria in seemingly unlikely locales: northern countries such as Poland, the Netherlands, and Russia.

Over the next several years, versions of the maps continued to appear in journals and at scientific meetings as researchers raised the disquieting possibility that climate change could trigger an expansion of disease. An article in Scientific American reprinted one iteration of the maps and declared that “by the end of the 21st century, ongoing warming will have enlarged the zone of potential malaria transmission from an area containing 45 percent of the world’s population to an area containing about 60 percent.” Statements like these added to the popular perception that a warmer world will automatically be a sicker one.

But what if this isn’t true, or is only partially true? Kevin Lafferty, an ecologist at the U.S. Geological Survey, is among a handful of scientists now raising these questions and rethinking conventional wisdom. Lafferty recently published a controversial article in the journal Ecology suggesting that, while climate change may shift the ranges of certain diseases, it won’t necessarily increase the total amount of territory they affect. (1) And Sarah Randolph, a parasite ecologist at the University of Oxford, has reviewed recent disease outbreaks—some of which have been attributed to global warming—and concluded that human actions and other factors may have played a larger role than climate.

Lafferty’s and Randolph’s opinions have stirred intense debate. While there are credible arguments on both sides, the overriding point is that some scientists are beginning to see the ecology of disease as far too complicated to support simple declarations about the impact of global warming. It turns out that disease ecology is made up of a multitude of moving parts, ranging from precipitation patterns to animal migrations, that constantly shift and adjust in relation to each other. And when climate changes, the end result may be an increase in disease—or not.

Nature vs. Nurture
When tick-borne encephalitis spread throughout the Baltics, was the culprit climate change or the fall of the Soviet Union?

Sarah Randolph has spent more than 30 years studying vector-borne diseases (diseases transmitted to hosts by insects and other animals). She’s also a maverick who has devoted the latest chapter in her career to digging beneath what she calls “seductive mindsets.” As she wrote in a response to Lafferty’s Ecology article, one of these mindsets is that recent disease outbreaks are caused by climate change, “adding fuel to the fire of predicted impending doom.” (2)

Take tick-borne encephalitis (TBE), a nasty viral disease that can cause inflammation of the brain. Beginning in the 1980s and 1990s, a rise in temperature appeared to correspond with a TBE surge in several European countries, where thousands of people were stricken with headaches, fever, and other unpleasant symptoms. In Sweden, some scientists suggested that warming had triggered the rise in TBE cases and that future climate change would exacerbate the scourge.

Randolph decided to conduct her own investigation. In a 2007 study, her team examined county-level TBE trends in the Baltic countries of Estonia, Latvia, and Lithuania; they found patterns that couldn’t be explained solely by climate. While temperatures rose in 1989 across the Baltics, TBE cases in individual counties began spiking anytime between 1990 and 1998.

To investigate further, Randolph’s team studied the region’s social and economic history. After the Baltics broke from Soviet rule in the early 1990s, unemployment rates—and poverty—surged. Poorer people were less likely to be vaccinated, the researchers found, and more likely to forage for food in tick-filled forests. This suggested to Randolph that, contrary to popular assumptions, the disease surge probably had far more to do with human actions than planetary changes.

The TBE case isn’t unique. “In the last two decades,” Randolph argues, “there’s practically no examples where a vector-borne disease can be pinned on climate change.”

Of course, Randolph is only one player in a contentious debate, and other scientists say they have found links between climate and diseases such as malaria, dengue, and cholera. Just because disease is influenced by a myriad of factors doesn’t mean we should ignore climate, warns Richard Ostfeld of the Cary Institute of Ecosystem Studies. “To me, that’s kind of like saying because we know that obesity is also a risk factor for heart disease, we don’t need to worry about smoking,” he says.

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