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.O°C, and sea level is anticipated to rise by nearly 50 cm. (1) This represents a five-fold faster rate of warming and a three-fold higher rate of sea level rise than that observed over the past century. While uncertainties accompany predictive climate modeling, the increasing agreement between climate projections arising from varying methodologies in multiple climate centers is cause for concern among the medical community. The World Health Organization already considers global warming as a serious threat to sustainable public health. (2,3)
Paralleling the concept of sustainability in the context of development, sustainable health is the promotion of health for people today without compromising resources needed by future generations to achieve the same level of health. The long-term nature of the health impacts from climate change, therefore, will necessitate increased attention by economic strategists as they plot a course for sustainable development. After all, sustainable economic development will be undermined if the health of the population is threatened.
Expected Health Effects
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 3-4-fold increase in heat-mortality in large temperate US cites under a doubled atmospheric CO2 scenario. (4)
Infectious agents which cycle through cold-blooded insect vectors to complete their development are quite susceptible to subtle climate variations. (5) 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. (6)
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. (7) Malaria has been observed in non endemic high elevations in Africa during unseasonably warm conditions. (8)
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, (9) as was observed in Mexico when dengue reached an altitude of 1,700 meters during an unseasonably warm summer in 1988. (10) 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. (11)
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. (12) Viral development time inside the mosquito also shortens with higher temperatures, increasing the proportion of mosquitoes that become infectious at a given time. (13) 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 zooplankton, and blooms from warmer sea surface temperatures could expand this important reservoir from which cholera epidemics may arise. (14,15)
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. (16,17)
Sustainable Health and Development
The World Commission on Environment and Development clearly illustrated that national governments can no longer separate economic development from environmental issues. (18) Now it is also clear that both development and environment can not be separated from human health. In the short-term, development generally improves human health which is usually related to per capita income. The health implications from climate change, however, present a unique "exposure pathway" never before addressed by the health community. Adverse outcomes that arise from disturbance of global environmental systems, such as the climate system will require more integrated preventive measures which take into account both long-term environmental and economic forces.
Development is central to policy directed towards climate change. Development accounts for the creation of anthropogenic greenhouse gases (causing the root exposure hazard), and defines the extent to which nations remain vulnerable to the consequences of climate change. (19) With heightened awareness of the linkages between global climate/ecological systems and sustainable public health, we can no longer make health assessments based solely on local environmental conditions. Local and regional development must be internationally coordinated with consideration of these long-term population health consequences.
Developing countries will suffer disproportionately more from the consequences of climate change, though industrialized nations are presently more responsible for GHGs in the atmosphere. In the future, poorer countries will influence the course of global warming significantly more, (20) both through the technologies they choose to adopt as their development accelerates, and by land use practices. Much of the responsibility for insuring prudent policy shifts lies with those nations already having established health infrastructures; their development occurred prior to the realization of the adverse impacts of short-sighted economic goals on global systems that serve as foundations for sustaining human health.
The insidious nature of the "exposure" of climate change also requires shifting attention to the health of future generations. No less priority should be given to current health crises, however, the scope of public health 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.
Precautionary measures aimed at reducing greenhouse warming will also have inherently beneficial side-effects, or "ancillary benefits" for public health. Reducing air pollution and population growth, for example, have obvious short-term positive impacts on health. In a sense these short-term gains weigh favorably in the policy debate which must consider potentially irrevocable and widespread health consequences together with vast uncertainties.
Finally, we must be cautious in our interpretation of the concept of sustainable development. One interpretation dangerous to sustainable health is that of an "economically stable but ecologically impoverished world." (21) The ecologically-based nature of the health effects of climate change necessitate full- cost-accounting of both environmental and health concerns stemming from economic development.
New understanding of the linkages between public health and "global life-support systems" is emerging in the literature. (22) 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 challenges and advance that much further in the practice of "sustainable" preventive medicine.
What is uniquely challenging about global environmental change is that the issues transcend local communities. The causes are collective and multifactorial, and the impacts will be global and unconfined by national borders. The public health community is now confronted by indirect and delayed effects on life-sustaining systems on a global scale. (23) The attributable risks of such population-wide exposures can lead to tremendous disease burdens, and therefore, demands increased attention by the public health and economic development communities.
In the history of public health, our reductionist approach to research and prevention has to this point been quite successful in identifying single etiologic agents, including microorganisms, direct chemical toxins, and specific harmful human behaviors. The health effects expected from global climate change, however, will involve exposure factors (or processes) that will likely be ecologically-based and difficult to quantify over extended time scales. (24) This relationship between ecosystem stability and long-term human health underscores the importance of far-sighted sustainable development and reinforces the need for integration of sustainable health and development policies.
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