Wednesday, May 19, 2010

Climate Change and Malaria

One of the purported harms of global warming is an increase in the prevalence of malaria. Given that malaria is a tropical disease, it makes sense to assume that warmer temperatures would bring an increase in malaria.

As I noted before, the actual data on this point doesn't match onto our intuitions. And the latest issue of Nature has a study that suggests that temperature rises do not impact malaria risk. Here is a sample from the News summary of the study:

Of the many climate-change catastrophes facing humankind, the anticipated spread of infectious tropical diseases is one of the most frequently cited — and most alarming. But a paper in this week's Nature adds to the growing voice of dissent from epidemiologists who challenge the prediction that global warming will fuel a worldwide increase in malaria.

On the surface, the connection between malaria and climate change is intuitive: higher temperatures are known to boost mosquito populations and the frequency with which they bite. And more mosquito bites mean more malaria.

Yet when epidemiologists Peter Gething and Simon Hay of the Malaria Atlas Project at the University of Oxford, UK, and their colleagues compiled data on the incidence of malaria in 1900 and 2007 (see page 342), they found the opposite: despite rising temperatures during the twentieth century, malaria has lost ground. According to the models the researchers used to tease out the factors affecting the incidence of malaria, the impact of public-health measures such as improved medications, widespread insecticide use and bed nets have overwhelmed the influence of climate change. "Malaria is still a huge problem," says Gething. "But climate change per se is not something that should be central to the discussion. The risks have been overstated."

And here is the abstract of the paper:

The current and potential future impact of climate change on malaria is of major public health interest1, 2. The proposed effects of rising global temperatures on the future spread and intensification of the disease3, 4, 5, and on existing malaria morbidity and mortality rates3, substantively influence global health policy6, 7. The contemporary spatial limits of Plasmodium falciparum malaria and its endemicity within this range8, when compared with comparable historical maps, offer unique insights into the changing global epidemiology of malaria over the last century. It has long been known that the range of malaria has contracted through a century of economic development and disease control9. Here, for the first time, we quantify this contraction and the global decreases in malaria endemicity since approximately 1900. We compare the magnitude of these changes to the size of effects on malaria endemicity proposed under future climate scenarios and associated with widely used public health interventions. Our findings have two key and often ignored implications with respect to climate change and malaria. 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. Predictions of an intensification of malaria in a warmer world, based on extrapolated empirical relationships or biological mechanisms, must be set against a context of a century of warming that has seen marked global declines in the disease and a substantial weakening of the global correlation between malaria endemicity and climate.