Some studies say yes. One found that temperature increases from 0.5 degrees to 3 degrees can double the population of Anopheles mosquitoes, which carry malaria, and double the speed at which dengue incubates in the Aedes aegypti mosquito, suggesting that its infectious life would increase. And temperature rises can extend a mosquito’s range and lengthen the biting season. Climate models have the risk of catching dengue rising to 74 percent by 2050 and the number of malaria infections by 50 million to 80 million cases per year by 2100.
Many scientists are skeptical, however, of the link between climate and disease. Historically, neither dengue nor malaria has ever been an exclusively tropical, or even a lowland, disease, says the Institut Pasteur’s Paul Reiter, a specialist in mosquito-borne diseases. Malaria epidemics were once common in Northern Europe, and one almost reached the Arctic Circle in the 1920s. Both diseases’ disappearing from the north have more to do with economic conditions than climate, he says. Outbreaks also aren’t always closely linked to mosquito populations. Between 1980 and 1999, for instance, northern Mexico had 62,000 cases of dengue, but Texas had only 64, despite having twice as many mosquitoes. “It’s very simplistic to say that higher temperatures mean more malaria or dengue,” says Reiter. “There are a host of more important factors.”
Chief among these factors is human activity—deforestation, irrigation, the migration of people into malaria-prone areas, war and civil strife, drug and insecticide resistance and, above all, poverty. Whereas the draining of marshes for dryland cultivation helped nearly eliminate malaria in Europe, turning deserted swamps into populous rice fields has caused dramatic increases in infection in Asia. Likewise, Mexicans who live without mosquito screens or closed sewers are far more vulnerable to mosquito-borne diseases than Texans who have air conditioning. Climate-disease models tend to gloss over these details—making picking future winners and losers an uncertain business.
