by Jonathan A. Patz, MD, MPH
Department of Occupational and Environmental Medicine
Johns Hopkins
School of Public Health
According to the United Nations' Intergovernmental Panel on Climate Change (IPCC), anthropogenic greenhouse gas emissions are significantly altering the earth's climate. By the year 2100, average global temperatures are projected to rise by 2.0-2.5°C (range 1.5-4.0°C). This projected rise in temperature represents a five-fold faster rate of warming than that observed over the past century. These IPCC figures even assume a reduction in global economic growth rate, slowed population growth over the next half century and improved conservation measures. These projections are consistent with climate sensitivity to atmospheric CO2 concentrations observed from ice core data extending over 158,000 years. Sea level also is expected to rise by about 34-52 cm. by the year 2100 as a result of ocean thermoexpansion and by melting of glaciers.
While uncertainties always will accompany predictive climate modeling, there is increasing agreement between climate projections arising from varying methodologies in multiple climate centers internationally. The medical community is beginning to examine the consequences that these projections may portend for public health, and the World Health Organization considers global warming as a serious public health challenge for the future.
Under the conditions of global warming, direct hazards to human health (e.g., urban heat-island effect and harmful air pollution) may become significant public health problems given current trends in urbanization. Warmer temperatures combined with increased ambient UV radiation could worsen photochemical smog, especially over urban areas. Elevated night-time temperature readings are the most significant meteorological variable contributing to heat-related mortality; the greenhouse effect is predicted to especially affect these minimum temperatures, and studies estimate a 3 to 4-fold increase in heat-mortality in large temperate US cities under a doubled atmospheric CO2 scenario (which could occur by the year 2040 if current fossil fuel emission trends continue).
Infectious agents which cycle through cold-blooded insect vectors to complete their development are quite susceptible to subtle climate variations. In temperate regions, climate change would affect vector-borne diseases by altering the vector's range, reproductive and biting rates, as well as pathogen development rate within the vector host.
Malaria and dengue fever serve as prime examples of climate sensitive diseases. The geographic range of malaria is generally limited to the tropics and subtropics because the Plasmodium parasite requires an average temperature above 16°C to develop. Malaria has been observed in non-endemic high elevations in Africa during unseasonably warm conditions.
Freezing temperatures kill overwintering eggs of Aedes aegypti, the mosquito carrier of dengue and yellow fever. Warming trends, therefore, can shift vector and disease distribution to higher latitudes or altitudes, as was observed in Mexico when dengue reached an altitude of 1,700 meters during an unseasonably warm summer in 1988. In an earlier study in Mexico, the most important predictor of dengue prevalence in communities was found to be the median temperature during the rainy season.
Temperature also drives epidemic dynamics of dengue transmission. Warmer water temperatures in breeding vessels reduces the size of emerging adults that subsequently must feed more frequently to develop an egg batch. Viral development time inside the mosquito also shortens with higher temperatures, increasing the proportion of mosquitoes that become infectious at a given time. Thus, mosquitoes bite more frequently and are potentially more infectious at warmer temperatures.
Climate-related increases in sea surface temperature can lead to higher incidence of water-borne cholera and shellfish poisoning. Marine phytoplankton blooms include red tides that cause diarrheal and paralytic diseases. Vibrio cholera has been found to be associated with marine zooplankton, and blooms from warmer sea surface temperatures could expand this important reservoir from which cholera epidemics may arise.
Human migration and damage to health infrastructures from the projected increase in climate variability and severity of storms could threaten human shelters and public health infrastructures and indirectly contribute to disease transmission. Human susceptibility to disease might be further compounded by malnutrition due to climate impacts on agriculture.
As experts in the field of Occupational and Environmental Medicine, we need to better understand the linkages between climatological and ecological change as determinants of public health. Our field of medicine must begin to grapple with some of these "ecologically-based" environmental health hazards. Addressing this newly recognized threat will require interdisciplinary cooperation among health professionals, climatologists, biologists and social scientists, and will necessitate research beyond conventional dose-response linear relationships to address complex systems-based ecological processes.
The long-term insideous nature of the "exposure" of climate change also requires shifting attention to the health of future generations. Sustainable public health means promoting health for people today, without compromising on resources needed by future generations to achieve the same level of health. No less priority should be given to current health crises, however, the scope of Occupational and Environmental Medicine needs a broader focus to begin to anticipate intergenerational health challenges, already requiring preventive measures. In planning adaptive measures (e.g., air conditioning and vector control strategies) long-term impacts must therefore be considered. For example, increased energy demand for air conditioning will exacerbate greenhouse gas emissions and widespread pesticide applications can promote insect resistance, as well as impact human health directly through toxic exposures.
New understanding of linkages between public health and "global life-support systems" is emerging in the literature. Prime examples are those of stratospheric ozone depletion, climate change, and threatened fisheries which have enormous implications for public health, but may not be immediately perceptible. Through new collaborative efforts we can begin to confront these tough challenges and advance that much further in the practice of true preventive medicine.
Recommended Reading:
1. WHO. Potential health effects of climatic change. World Health Organization, 1990.
2. Wigley, TML, Raper SCB. Implications for climate and sea level of revised IPCC emissions scenarios. Nature 1992;357:293-300.
3. McMichael, AJ., Global environmental change and human population health: conceptual and scientific challenge for epidemiology. Intnl J Epidem 1993;22(1):1-8. 4. Patz JA, Epstein PR, Burke TA, Balbus-Kornfeld JM. Global climate change and emerging infectious diseases. JAMA 1996;275(Jan.):(in press).