Malaria and Global Warming in Perspective?

To the Editor: The two reports from the International Panel on Climate Change (IPCC) (1,2) cited in the letter by Pim Martens (3) are widely regarded as "the standard scientific reference for all concerned with climate change and its consequences," yet the contents of these reports are often misleading. The quoted passage does not acknowledge the devastation caused by malaria in temperate regions. The reassurance that "existing public health resources" would "make reemergent malaria unlikely" ignores the nonclimatic factors that led to its disappearance and continued absence. Moreover, although malaria/climate models are not meant to predict future worlds, the IPCC chapter (1) on human health--one-third of which is devoted to vector-borne disease--makes extensive use of such models to warn of substantial "actual climate-related increases in malaria incidence" and "highly likely" extensions of its distribution. The chapter does include statements that the "predictions" of such models should be viewed cautiously "until they have been validated against historical data sets," and "malaria is most likely to extend its spread ... in tropical countries." The past presence of malaria in "southern Europe" is also mentioned, but such qualifiers are applied to predictions of 10- to 100-fold increases in epidemic potential in temperate climates. These predictions are frequently cited as evidence of a major threat to humanity (4,5).

The IPCC reports state "... anopheline mosquito species that transmit malaria do not usually survive where the mean winter temperature drops below 16 [degrees] C -18 [degrees] C." Similarly, two oft-quoted publications (6,7) define the vector's limit of survival as the 15 [degrees] C winter isotherm, i.e., in the northern Sahara. However, in the past the limit was the 15 [degrees] C summer isotherm. In fact, much of Europe and all of the United States are within the 20 [degrees] C or 25 degrees] C summer isotherms, and malaria was once prevalent in parts of southern Canada and up to 64 [degrees] N in Russia and Siberia. The same publications state that Aedes aegypti, the principal urban vector of dengue and yellow fever, cannot survive mean temperatures below 10 [degrees] C, but with global warming "... dengue could extend into the southern United States." This statement has been repeatedly quoted (5), although Ae. aegypti is common where winter temperatures of -15 [degrees] C are not unusual and epidemics of dengue and yellow fever have occurred as far north as Boston and Dublin. Repeated claims that global warming may have already led to increases in these diseases in the tropics are equally indefensible (8,9).

References

(1.) Intergovernmental Panel on Climate Change (IPCC). Impacts, adaptations and mitigation of climate change: scientific-technical analyses. Watson RT, Zinyowera MC, Moss RH, editors. New York: Cambridge University Press;1996. Chapter 18.

(2.) Intergovernmental Panel on Climate Change. The regional impacts of climate change: an assessment of vulnerability. Working Group II. Intergovernmental Panel on Climate Change. New York: Cambridge University Press; 1998. Chapters 5,8.

(3.) Martens P. Malaria and global warming in perspective? [letter] Emerg Infect Dis 2000;6:313-4.

(4.) Ross A. Strange weather: culture, science and technology in the age of limits. London: Verso; 1996.

(5.) Gelbspan R. The heat is on: the high stakes battle over Earth's threatened climate. New York: Addison-Wesley; 1997.

(6.) Patz JA, Epstein PR, Burke TA, Balbus JM. Global climate change and emerging infectious diseases. JAMA 1996;275:217-23.

(7.) Epstein PR, Diaz HF, Elias S, Grabherr G, Graham NE, Martens WJM, et al. Biological and physical signs of climate change: focus on mosquito-borne diseases. Bulletin of the American Meteorological Society 1998;79:409-17.

(8.) Reiter P. Global warming and vector-borne disease in temperate regions and at high altitude. Lancet 1998;351:839-40.

(9.) Mouchet J, Manguin S, Sircoulon J, Laventure S, Faye O, Onapa AW, et al. Evolution of malaria in Africa for the past 40 years: impact of climatic and human factors. J Am Mosq Control Assoc 1998;14:121-30.

Paul Reiter Centers for Disease Control and Prevention, San Juan, Puerto Rico, USA