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Beyond actual inundation, a rising sea level would put millions of people at greater risk of occasional storm-related flooding. Under current conditions, about 46 million coastal residents experience some flooding from storms each year. The number at risk from flooding would double if sea level rises 0.5 meters and nearly triple with a 1-meter rise, according to one study. In an area like Bangladesh, where storm surges can reach as far as 200 kilometers inland during some intense cyclones, the increase in flood risk could greatly magnify the already high toll associated with such storms. Coastal erosion would also increase substantially, endangering natural protective features such as sand dunes, mangroves, and barrier islands, and exacerbating the flood risk [284].
Climate change could influence air pollution profiles and the health effects that come from exposure to polluted air by altering the rate of chemical reactions in the atmosphere that form or destroy pollutants, or by influencing the factors such as wind and precipitation that regulate how pollutants accumulate or disperse. For example, higher temperatures favor the formation of pollutants like ground-level ozone the main constituent of smog. Preliminary calculations by U.S. EPA show that a 4° C increase in ambient air temperatures in the San Francisco Bay area would likely increase ozone levels by 20 percent and double the size of the area that does not meet national air quality standards [285]. Higher temperatures would also increase the evaporation of volatile liquids such as gasoline or organic solvents, again adding to the urban smog problem.
Changes in regional wind and rainfall patterns accompanying climate change could also affect air pollution levels. If winds increase in a given area, they would tend to disperse and dilute air pollutants, thereby lowering human exposures. By contrast, a decrease in winds with an increased tendency to form local inversion layers where warm, still air aloft traps pollutants close to the surface would increase pollution exposures. Likewise, in areas where rainfall increases, pollutant loads may decline, since precipitation scours many pollutants from the air. A decrease in rainfall, on the other hand, may increase pollution levels since fewer pollutants are washed out of the atmosphere [286] [287]
Indirect impacts
Climate change will likely raise the already considerable toll of infectious diseases worldwide. This impact is likely to occur because factors such as temperature and rainfall can affect the abundance and distribution of disease vectors or disease-causing microbes, as well as the vulnerability of populations to these diseases. It is impossible to predict exactly how disease rates will change in response to climate change because the interactions between environment and disease are so complex, and the effects of climate change will vary so much from location to location. But considerable evidence shows that many diseases are quite sensitive to variations in climate and are likely to increase their range and incidence as temperatures rise and precipitation patterns change.
Mosquitoes are quite sensitive to changes in temperature and rainfall and are among the first organisms to extend their range when environmental conditions become favorable [288]. Thus, higher temperatures could influence the incidence of diseases such as malaria, dengue fever, yellow fever, and several types of encephalitis. Cold temperatures are often the limiting factor in mosquito survival, so any increase in minimum winter temperatures would likely extend mosquito ranges into temperate regions or higher altitudes where they do not survive now.
Higher temperatures also speed the life cycles of both the mosquito and the disease organisms they harbor and make adult mosquitoes bite more often. At 30° C, the dengue virus takes 12 days to incubate in the Aedes aegypti mosquito, but only 7 days at 32° C. The shorter incubation period translates to a potential threefold higher transmission rate of the disease. Higher temperatures also produce smaller adult mosquitoes that must feed more often to develop an egg batch, which in turn increases the chances for disease transmission [289].
Although temperature most determines the potential range of the mosquito and the disease organism, precipitation principally governs the availability of breeding sites and the overall population of mosquitoes. Thus, the combination of temperature and rainfall changes modified by many other factors such as land use changes, human population densities, and whether exposed populations have any built-in disease immunity will determine how the patterns of mosquito-borne diseases change [290]. In some areas, the interplay of these factors will increase disease incidence; in other areas, incidence may decline.
Argentina provides an example of the complex changes in malaria distribution that climate change could bring. Currently, most of Argentina lies just south of the zone in which malaria occurs. But if global warming increases rainfall in central Argentina and makes it semitropical, as models project, the malaria-carrying mosquitoes might be able to expand south into the pampas and savanna regions, introducing malaria to these areas. On the other hand, northwestern Argentina, where malaria mosquitoes can now be found, might well become drier with global warming, making it less conducive to mosquito survival and reducing malaria outbreaks there [291].
References and notes
284. A. McMichael et al., eds., Climate Change and Human Health (World Health Organization, Geneva, 1996), pp. 146, 149-150, 154.
285. U.S. Environmental Protection Agency (U.S. EPA), The Potential Effects of Global Climate Change on the United States, EPA Report No. EPA-230-05-89-050 (U.S. EPA, Washington, D.C., 1989), p. 199.
286. A. McMichael et al., eds., Climate Change and Human Health (World Health Organization, Geneva, 1996), p. 65.
287. John Balbus, Assistant Professor of Medicine, The George Washington University, “Air Pollution and Climate Change,” presentation at the Annual Meeting of the Society for Occupational and Environmental Health (SOEH), National Institutes of Health, March 6, 1997.
288. S. Lindsay and M. Birley, “Climate Change and Malaria Transmission,” Annals of Tropical Medicine and Parasitology, Vol. 90, No. 6 (1996), p. 580.
289. J. Patz, et al., “Global Climate Change and Emerging Infectious Diseases,” Journal of the American Medical Association, Vol. 275, No. 3 (1996), p. 218.
290. A. McMichael et al., eds., Climate Change and Human Health (World Health Organization, Geneva, 1996), p. 81.
291. A. McMichael et al., eds., Climate Change and Human Health (World Health Organization, Geneva, 1996), pp. 84-86.
