The Hard Questions of Climate Change

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Pictured: Two vectors at work. Suwon City Mosquito Monument, Suwon South Korea. Image courtesy Flickr user wmjas via creative commons.

A wise person once said, "It's hard to make predictions, especially about the future."* When you start talking about the future of complex systems, it only gets more difficult. Case in point: The effects of climate change on vector-borne diseases.

Climate change: More pestilence-carrying pests in more places. At least, that's the fear. In 2008 alone, there were some 4000-odd peer-reviewed papers published on the topic, according to the journal Ecology. That should give you an idea of how twitchy this possibility makes actual scientists. Top it off with a glass full of TV news Kool-aid, and you've got yourself a regular panic. But there's currently a scientific debate raging over what, exactly, this means for people. Some researchers are now saying that the issue is more complicated than it appears on the surface. We can't simply assume that rising temperatures automatically equal higher rates of human disease, they say. At least, not all the time.

Why such a twisty answer? Because predicting the spread of disease involves more than just sticking out a thermometer. You have to account for a lot of other things, including where and how people live, the other ways they're changing the environment and how heat affects the disease, itself.

Case in point: Lyme disease. Rates in North America have skyrocketed since the 1970s, and the habitat of the Lyme-carrying deer tick has spread to cover large swaths of the U.S. At the same time, those same regions have also been getting warmer...yada, yada, yada, we're all gonna die.

Not necessarily. Deer Ticks and Lyme disease are moving North, says Durland Fish, professor of epidemiology at the Yale School of Public Health. But they're also moving East and South. In fact, he says, what looks like a climate-triggered expansion actually has more to do with reforestation, re-population of wild deer herds and suburban lifestyles that put more people in contact with both. So, then, hooray! Climate change isn't a problem and the hippies can suck it.

Well, again, not necessarily. By 2080, the global mean temperature is expected to increase by more than 5.4° F. Based on this, Fish and his colleagues mapped deer ticks' future habitat and found it'll likely grow by more than 20% in the U.S.--mostly in areas of Kentucky, Missouri, Kansas and Nebraska that aren't particularly Lyme-ridden today. Thankfully, those regions aren't terribly popular with humans, either, but Fish also says trends toward earlier Springs and warmer, longer-lasting Falls seem to favor a type of bacteria that causes more severe cases of Lyme. The result could be a future where Lyme doesn't infect significantly more Americans, but causes worse illness when it does.

The story for other vector-borne illnesses is equally complicated. For instance, the mosquitoes that spread malaria do favor tropical temperatures. So you'd think climate change would put more people at risk as regions bordering the tropics heat up. But Kevin Lafferty, a research ecologist with the U.S. Geological Survey, pointed out in the April issue of Ecology that rising temperatures also mean some currently malaria-prone areas will become too hot for mosquitoes. Equally important, according to Lafferty, is the fact that many of those border regions are far wealthier than current malaria hotbeds, so it's really unlikely malaria will make a comeback in places like the United States. Sure, the South is getting warmer, but Americans still live air-conditioned, indoor lifestyles, and still have relatively high levels of access to mosquito repellants and malaria treatments. In his Ecology paper, Lafferty reported data from computer models suggesting that, while the worst strain of malaria could expand beyond its current habitat to gain 23 million new human hosts by 2050, it's also going to lose access to some 25 million people.

*Also tough: Attributing pithy quotes. According to the Internets, this saying comes from Yogi Berra, Mark Twain, Woody Allen, Groucho Marx, Confucius, Will Rogers, and Niels Bohr. Presumably first uttered during a great conversation aboard the TARDIS.