Conference on Human Health and Global Climate Change: Summary of the Proceedings (1996)

greenhouse gas; the protective ozone layer resides some 10 to 40 kilometers, or 6 to 25 miles, above the Earth's surface. Solar energy recombines diatomic oxygen (O₂) into triatomic ozone (O₃); these molecules are broken down to O₂ by naturally occurring compounds containing nitrogen, hydrogen, and chlorine; and the cycle begins again. In the past 50 years human activities have added millions of tons of ozone-depleting chemicals to the atmosphere, primarily through the widespread use of chlorofluorocarbons (CFCs) in refrigerators, spray cans, foam insulation, and cleaning compounds.

In theory, these ozone-depleting chemicals rise up in the atmosphere and destroy the ozone layer faster than it is naturally restored. Indeed, in 1985, researchers reported dramatic declines in ozone concentrations over Antarctica during the southern spring. This seasonal “hole” in the ozone shield has grown larger and appeared earlier in subsequent years. Many other factors might contribute to these findings, including sunspot cycles and the isolation and extreme cold of the Antarctic weather system, but CFCs and other ozone-depleting compounds were clearly implicated2.

More recent measurements have confirmed that ozone depletion is in fact a global phenomenon, although it is less acute in the tropics and more pronounced toward the poles, particularly in the Southern Hemisphere. At present there is a 5 to 7 percent ozone depletion over the United States during the summer, when people are most likely to be outdoors; about 11 percent over southern temperate areas; and more than 50 percent over Antarctica. Every 1 percent decrease in ozone can lead to a 2 percent increase in nonmelanoma skin cancer. This phenomenon is expected to continue for the next decades, despite international efforts to ban CFCs and to phase out other ozone-destroying compounds. Peak ozone depletion will occur around the turn of the century; recovery is expected to occur over the following 50-year period.

Conference participants noted that the anticipated human health risks caused by global climate change will not be localized; instead, they will occur on a large scale, impinging on entire populations. In addition to increasing the familiar, direct effects of climate (i.e., extreme weather events such as heatwaves and floods), global change will also involve a variety of indirect risks arising from the disturbance of natural systems (e.g., the ecology of infectious diseases, food production, and fresh water supplies). Forecasting these risks is a complex, uncertain task, and encompasses a long time horizon. (Box 2 summarizes a pair of presentations on El Niño as an analogue for long-term global climate change.)

The health effects of global climate change span a continuum from direct to indirect, as shown in Figure 3. In the long run, the indirect effects of disturbing natural systems may have greater cumulative impacts on human health, and most of those impacts will be adverse. As summarized in the most recent assessment of the United Nations Intergovernmental Panel on Climate Change (IPCC, 1995) and by various speakers during the first day of the conference, the most likely and most serious health risks and health effects of global climate change and ozone depletion would be adverse changes in the following:

• heat stress;

• skin cancer, cataracts, and immune suppression;

• vector-borne infectious diseases;

• non-vector-borne infectious diseases;

• food production and nutritional health;

• water quality and quantity;

• air pollution and allergens;

• weather disasters and rising sea level; and

• social and demographic dislocations.

Climate influences the distribution, frequency, types, and severity of infectious diseases in humans. The interaction between climate and infectious diseases derives from the impact of climate on infectious organisms (such as bacteria and viruses), on the human host, and on vectors and reservoir hosts and their ecosystems. Climate change can increase the probability of contact between humans and infectious organisms.

Temperature and rainfall influence the abundance and distribution of insect vectors and animals—one source of infectious diseases in humans. Global climate change is likely to affect the geographic distribution of animals and insects and could expand transmission of infectious diseases carried by mosquitoes (e.g., malaria, dengue, and yellow fever) and other vectors, such as ticks, sandflies, and fleas. Altered distributions of vectors are likely to involve expansion of vector-borne diseases into new geographic areas and populations and disappearance from other areas. For a vector-borne disease to persist in an area, climatic conditions must support a complex interaction that may involve plants, animals, insects and human activities.

Extreme events, such as flooding and hurricanes, that lead to displacement of populations into crowded, temporary shelters, or movement into previously uncleared lands, could also contribute to an increase in vector-borne infections.

Vector-borne diseases are already a major cause of illness and death in tropical countries, where 2.4 billion people are at risk from malaria and 1.8 billion from dengue fever (see Table 1). The numbers of people at risk from these and other diseases will increase with warmer temperatures and humidity, particularly because these changes are occurring simultaneously with changes in human behavior that increase the dangers of infectious disease—most notably international travel, population growth, rapid urbanization, poor sanitation, and changes in land-use patterns that increase habitat or bring humans in contact with insect or rodent vectors. Climate-related migrations could also contribute to the dissemination of previously localized diseases. Several vector-borne diseases have been increasing rapidly in recent years, including some that were previously considered to be under control, such as dengue fever and malaria. Strong support for public health programs both domestically and internationally would help to reduce this risk.

Changes in water temperature and the resulting proliferation of aquatic microorganisms would tend to increase the range and severity of cholera and other food- and water-related diseases that can cause epidemics of diarrhea and dysentery. Cholera epidemics are typically associated with seacoasts and rivers, for instance, where the cholera organism, Vibrio cholerae, survives by sheltering under the mucous coating of tiny invertebrates called copepods. These hosts, in turn, respond both to water temperature and to nutrients (fertilizer, wastewater) in stream runoff. Researchers are currently evaluating the connection between water temperature, coastal currents, algal blooms, and subsequent outbreaks of cholera like those in Peru in 1991 and Bangladesh in 1992.

Higher temperatures contribute to faster reproduction by disease organisms. Rates of genetic mutation also increase in times of stress. Furthermore, disease-causing organisms are remarkably resilient and can respond rapidly to changes in the physicochemical environment. Climatic and other environmental changes are contributing to the selection and emergence of genetic strains that are resistant to drugs and other controls.

An increase in average temperature would probably be accompanied by an increase in the number and severity of extreme heatwaves in some areas. This would cause an increase in illness and death, particularly among the young, the elderly, the frail, and the ill, especially in large urban areas. Climate change would exacerbate an already large urban heat island effect that exists in many large cities. In fact, heat-related mortality may prove to be the largest direct health threat from global climate change. The deaths of 726 people that were attributed to a heatwave in Chicago in the summer of 1995 may be an extreme example, but it serves as a possible indicator of what might occur if climate change scenarios are correct.

Mid-latitude cities that experience irregular, but intense, heatwaves appear to be most susceptible—cities like St. Louis, Washington, D.C., and New York. Tropical and subtropical cities seem to be less susceptible, in part because populations have acclimatized to the regularity of hot weather (although a 1995 incident in New Dehli indicates the susceptibility of tropical populations as well). People in mid-latitude cities might also acclimatize, and air conditioning can mitigate perhaps 25 percent of heat-related mortality (while also requiring increased energy and refrigerant use, thereby increasing greenhouse gas emissions). In addition, summer mortality increases might be partially offset by declines in winter mortality. However, much of the research points to a substantial increase in weather-related mortality under climate change conditions. Despite these uncertainties, there is a clear need to develop an adequate warning system to alert the public and government agencies when oppressive air masses are expected—extended periods of extreme high temperature, light winds, high humidity, and intense solar radiation.

*Excerpts from a special briefing at the Conference on Human Health and Global Climate Change, September 11, 1995.

FIGURE 2. Pictoral representation of global climate impact anamolies due to ENSO. (Provided by NOAA, based on work of C. Ropelewski and collaborators)

Ozone depletion can have both direct and indirect effects on ecological systems and human health. Increased exposure to ultraviolet radiation (especially UV-B) can have harmful effects on photosynthesis (on land and sea), with potentially disruptive impacts on food production and the stability of ecosystems. The most important direct human health effect would be an increase in nonmelanoma skin cancers, especially in fair-skinned populations. Such cancers are already a major problem in the United States, with about 1 million new cases per year. Furthermore, current models suggest a two percent increase in incidence for every one percent decrease in stratospheric ozone.

The current scenario for phaseout of CFCs predicts a 25 percent increase in skin cancer by 2050 at 50°N latitude, relative to the 1980 incidence. Melanoma is a less frequent but far more deadly skin cancer, whose relationship to UV-B exposure remains uncertain. Both types of skin cancer have a long lag time between exposure and disease; the effects of increased UV-B may not be seen until after 2050. Increased UV-B can also be expected to increase the frequency of cataracts, which can lead to blindness in all populations. Current estimates indicate a 0.3 to 0.6 percent increase in new cataract cases for every 1 percent decrease in stratospheric ozone. Ozone depletion may also contribute to the frequency, severity, and duration of some infectious diseases due to ultraviolet's ability to suppress the immune system. There are many uncertainties about the effect of UV-B on immune responses, although it appears that neither pigmentation nor sunscreens offer effective protection.

Global climate change would have mixed effects on the productivity of agriculture, livestock, and fisheries. In tropical and subtropical areas, global climate change may lead to droughts, flooding, and the emergence of new plant diseases, decreasing food production in many areas where food supplies are already insecure. Meanwhile, crop productivity may increase in other regions, mostly in the higher temperate latitudes such as Canada, Siberia, and Patagonia. However, agricultural projections are strongly dependent on assumptions about technological advances and patterns of consumption.

Over 800 million people are chronically undernourished today, particularly in the developing world, and malnutrition is an underlying cause of childhood mortality. With further population growth, malnutrition may increase the vulnerability of these populations to endemic diseases and epidemics. Some areas may need to change crops, planting practices, and diet, further increasing vulnerability during the period of transition. Such regions might be helped by advance warning of conditions that might cause crop failures.

Overall, models project the world may be able to produce enough food to feed future populations. However, changes in regional patterns of production could be significant, and in the long term, nutritional security can only be ensured through education and training, higher incomes, favorable market mechanisms, political stability, and population controls.

Great spatial and temporal variability characterize water availablility. Climate change may exacerbate such variations. Today 1 billion people lack access to clean and abundant drinking water, and even more are without adequate sanitation. Adjustments to water shortages can be managed where physical infrastructure (reservoirs, pipelines, and canals) and water management institutions exist. Increasing populations dependent on limited sources served by isolated systems are at more risk. Landscapes may erode or stabilize as precipitation alters vegetative cover, thus affecting runoff and transport of sediment and pollutants.

The same industrial processes that produce greenhouse gases will also produce increased urban air pollutants, and they too can pose major health risks. Levels of fine particulates (from fossil fuels and wood smoke) and ozone (from photochemical reactions) are known to be associated with higher levels of hospital admissions for respiratory diseases. Fine particulates also appear to be associated with admissions for heart disease and with general mortality. In the United States, where air pollution is relatively low compared with Mexico City and some Asian cities, it nevertheless contributes to 70,000 excess deaths and 1 million additional hospitalizations annually. In the future, as global increases in energy production lead to higher levels of particulates, and increases in

temperature and ultraviolet radiation accelerate the reactions that produce ozone and other secondary pollutants, the health effects of air pollution on a global scale could be staggering. Higher temperatures and humidity may also lead to higher concentrations of plant pollen and fungal spores that cause allergic disorders such as asthma and hay fever.

El Niño is associated with increased rainfall and floods in some regions. Long-term climate change over the entire planet may result in an increase in extreme weather events, such as droughts, floods, and cyclones. These events could increase the number of deaths and injuries and the incidence of infectious diseases and psychological disorders, as well as causing indirect effects through food shortages and the proliferation of disease vectors.

A 40-centimeter rise in sea level would approximately double the number of people who are currently exposed to flooding each year in areas like Bangladesh. It could also contribute to the loss of coastal and delta farmland, as in Egypt, and to the destruction of food supplies. Rising sea level also increases the vulnerability of costal cities, low-lying areas, and small islands to damage during storms.

Global climate change would alter patterns of employment, wealth distribution, and population settlement throughout the world. Physical conflicts might also arise over depleted environmental resources such as farmland, surface water, and coastal fisheries. Biodiversity would also be affected (see Box 3). The greatest destabilizing effects would likely be experienced in areas of Africa which are already highly vulnerable. At the same time, populations may be moving out of

FIGURE 3. Ways in which global climate change may affect human health. (Adapted from IPCC, 1995)