A hotter world will mean more people at risk of malaria, right? That's certainly the impression the U.N. gave in its 2007-2008 Human Development Report, which noted that increases in rainfall, temperature, and humidity will help malaria-carrying mosquitoes expand their geographic range. That report estimated that as many as 400 million people could be at risk—and this is a disease that already kills one million people per year.
But the science on this subject has never been so clear cut. If you look at what the IPCC concluded in the "human health" chapter of its 2007 report, there was a lot of hedging:
Despite the known causal links between climate and malaria transmission dynamics, there is still much uncertainty about the potential impact of climate change on malaria at local and global scales, because of the paucity of concurrent detailed historical observations of climate and malaria, the complexity of malaria disease dynamics, and the importance of non-climatic factors, including socio-economic development, immunity and drug resistance, in determining infection and infection outcomes.
In other words, yes, changes in humidity and temperature can affect the travel patterns of mosquitoes, but there are so many other social factors at work—from access to health care to prevention strategies—that it's never so simple. And, in fact, there's a new study out in Nature by Oxford's Peter Gething that concludes that, over the past century, we've been able to fight malaria much, much faster than a warming climate has been able to spread it around, and there's plenty of reason to think that will continue to be the case:
First, widespread claims that rising mean temperatures have already led to increases in worldwide malaria morbidity and mortality are largely at odds with observed decreasing global trends in both its endemicity and geographic extent.
Second, the proposed future effects of rising temperatures on endemicity are at least one order of magnitude smaller than changes observed since about 1900 and up to two orders of magnitude smaller than those that can be achieved by the effective scale-up of key control measures.
That's certainly good news. Though two caveats: One, as Kevin Lafferty of the U.S. Geological Survey points out, climate change will still help malaria spread into new areas, even as the overall global picture improves. So it will still create plenty of headaches for public health authorities.
Second, as the IPCC details in its chapter on climate and human health, malaria isn't the only disease-based problem we're likely to face in a warming world—there's malnutrition, heat-wave deaths, shifting vectors for infectious and diarrheoal diseases, and so forth. Interestingly, the effects on malaria seemed to be the one thing this IPCC chapter was "mixed" about, even though malaria turned out to get all the headlines whenever the subject of global warming and disease came up. Let's see if this new study changes that.