It seems self-evident that global warming should result in malaria spreading to new places, especially to people who live in parts of the world where mosquitoes only appear during the warmer months. Like so many things, however, the answer isn’t that simple. Accurate predictions are challenging because of the inherent difficulty of taking all possible variables into account. Studies of changing malaria patterns, meanwhile, yield debatable results.
Climate Doesn’t Limit Malaria
In 2000, Paul Reiter pointed out in Emerging Infectious Diseases (Vol 6) that malaria is already absent from parts of the world where the climate would support it and appropriate vector mosquito species are present (“From Shakespeare to Defoe: Malaria in England in the Little Ice Age”). Malaria, introduced to North America by explorers and immigrants, was still present in much of the United States and even southern Canada in the early 20th century.
Reiter argues that the disease known as ague in Medieval England is consistent with malaria, and reports of it there continued throughout the colder period following the mid-16th century and lasting over a hundred years. The last case of malaria acquired in England was recorded in 1950. History also records severe epidemics of malaria in the former USSR.
Reiter lists a number of factors that cause malaria to return and/or surge: “population increase, forest clearance, irrigation and other agricultural activities, ecologic change, movement of people, urbanization, deterioration of public health services, resistance to insecticides and antimalarial drugs, deterioration of vector control operations, and disruptions from war, civil strife, and natural disasters.”
Evidence That Global Warming Spreads Malaria
A 2002 report in Nature (Vol 415) calmed some fears after Simon Hay and others compared increases in malaria in East Africa caused by Plasmodium falciparum with climate data from 1901 to 1995 (“Climate Change and the Resurgence of Malaria in the East African Highlands”). They found no significant climate change during that period.
In March 2010, however, a report of reanalysis of the data used by Hay and co-authors, and of other published studies, suggested that climate change did indeed occur and play a significant role. Luis Fernando Chaves and others found that statistical errors in data analysis explain the difference in results (“Climate Change and Highland Malaria: Fresh Air for a Hot Debate.” The Quarterly Review of Biology 85:1).
Chaves and co-authors, however, acknowledge that lots of things influence the spread of malaria, writing that “more biological data will improve our overall understanding of malaria and will allow scientists to propose more general and accurate models on the impacts of climate change on malaria transmission.” Indeed, the absence of climate change with increasing malaria does not rule out a relationship, and the presence of warming with increasing malaria does not prove cause and effect.
Predictions That Malaria Will Not Spread as the Climate Warms
Some researchers have attempted to predict future trends in malaria spread. In 2000 David Rogers and Sarah Randolph identified multiple climate variables, established using the current P. falciparum distribution (“The Global Spread of Malaria in a Future, Warmer, World,” Science; 289). The two then applied the climate variables to worst case scenario climate change predictions to generate a world map of possible future P. falciparum distribution. The maps for the year 2000 and for 2050 differed very little.
In May 2010, David Gething and others published a report in Nature (Vol 465) in which they compared changes in malaria patterns over the last century with predictions for future warming and spread of the parasite. The results of this new way of looking at the malaria picture did not suggest that malaria is likely to spread significantly in a warmer world (“Climate Change and the Global Malaria Recession”).
Gething and co-authors write that “predictions of an intensification of malaria in a warmer world… 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.” In other words, warmer climate may have the potential to spread malaria, but other factors, especially human actions that either favor malaria or seek to eradicate it tend to overwhelm any climatic effects.
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Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
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