John M. Last
Department of Epidemiology and Community Medicine, University of Ottawa, Ottawa, Canada KIH 8M5
Mass med1a (1-3), the Worldwatch Institute (4), technical reports (5, 6), international expert groups (8), the World Health Organization (9, 10), international conferences (11, 12), and science journals (13, 14) have all depicted damage to the biosphere caused by human activity. The topic has also received some attention in the medical press (15-21). This paper is partly speculative, but it is based on the observations and judgment of scientists, in many fields, who agree that the global environment is changing in ways that could have serious consequences for human health. Public health workers face new challenges and responsibilities in coming decades.
Global change is a complex process; many interconnected factors can be regarded as either causes or consequences. There is controversy about several aspects, particularly those that have economic and political implications. Since the late 1970s, attention has been focused on stratospheric ozone depletion, which is leading to increased ultraviolet (UV) radiation of the biosphere (22), and on accumulation of atmospheric greenhouse gases, which many authorities believe to be inducing global warming (23-25).
Ozone depletion and global warming and their health effects are discussed in some detail in this paper. Related features of global change include environmental contamination with pesticides (26) and toxic chemicals (27) and damage to aquatic ecosystems, agriculture, certain kinds of vegetation, and building materials due to acidic deposition (28). Like ozone depletion and global warming, these features are associated with industrial development and energy use, especially fossil fuel combustion and dispersal into the air, water and soil of industrial products, and toxic wastes. Other features of global change are depleted supplies of arable land, fresh water, and renewable and nonrenewable resources (29), and species extinction that reduces biodiversity (30, 31). Underlying all of these phenomena are rising population pressure and increased migration, which lie at the root of many armed conflicts (32) (Table 1).
Ionizing and UV radiation act on oxygen to produce ozone. In the troposphere, ozone is a toxic constituent of photochemical smog, created by interaction of UV radiation with automobile and other exhaust fumes; ozone also occurs in some industrial emissions. In the stratospheric, ozone shields the biosphere from what otherwise would be lethal amounts of UV radiation. The stratospheric ozone layer is situated at an altitude of 12-45 km; the ozone moiety would be only a few millimeters thick at surface pressure and temperature, but of course expands greatly at high altitudes.
Ultraviolet radiation comprises UV-A, with a wavelength of 320-400 nm; UV-B, which is 290-320 nm; and UV-C, which is 200-290 nm. The most dangerous, UV-C, is prevented from reaching the earth's surface by stratospheric ozone. The least harmful UV-A passes through the stratosphere; it contributes to tanning of fair-skinned persons. Biologically harmful UV-B reaches the earth's surface in amounts inversely proportional to the concentration of atmospheric ozone, UV-B is impeded by urban air pollution, suspended particulates, aerosols, and ozone in the troposphere, as well as by stratospheric ozone.
The stratospheric ozone layer is a fragile shield. Among the chemicals that destroy ozone are chlorine monoxide, which began to accumulate in the atmosphere following the development and widespread use of refrigerants, and volatile solvents containing chlorofluorocarbons (CFCs). Chlorofluorocarbons are stable, lighter-than-air chemicals, but break down to release chlorine monoxide when exposed to UV radiation. In 1979, an expert panel of the National Academy of Sciences concluded that CFCs would damage the ozone layer, with serious consequences for human health and other adverse biological effects (33). The panel recommended restricting the use of CFCs.
The ozone layer has been under observation from satellites, high-altitude balloons, and surface stations for many years. Some thinning was observed in the 1970s (34). In 1985, observations in Antarctica showed severe attenuation, often described as a "hole" (35, 36); since then, seasonal fluctuations have been observed, with maximum attenuation in the Antarctic spring. These fluctuations have become more pronounced and widespread each year; in 1990 and 1991, they extended into southern regions of Chile, Argentina, New Zealand, and Australia (37). In the northern hemisphere, ozone depletion was first observed more recently (38): there was significant seasonal thinning over parts of Asiatic Russia in 1990-1991 and the northeastern region of North America in 1992. In early 1992, NASA air quality monitoring revealed high concentrations of CFCs and chlorine monoxide in the atmosphere above New England and eastern Canada (39). These high concentrations are expected to persist for decades and cause further deterioration of the ozone layer. The upper limit of the ozone layer, at an altitude of 45 km, declined by about 7% in 1980-1989, which suggests that chlorine monoxide or other ozone-destroying chemicals have already diffused to very high altitudes (40).
Quality and Coherence of Evidence
Measurements include atmospheric concentrations of CFCs and chlorine monoxide; stratospheric ozone density; UV radiation flux; quantitative and qualitative changes in small organisms, such as phytoplankton; and data from epidemiological and other studies of adverse human health effects that may be attributable to UV radiation. Atmospheric physics and chemistry are arcane sciences: only minuscule proportions of the huge atmospheric volume can be examined. Instruments to measure UV radiation (expensive UV spectrophotometers and relatively cheap Robertson-Berger meters) are sparsely distributed; only 25 US recording stations existed in 1990 (41). The readings have uncertain validity in relation to human exposure. No increase in surface-level UV-B was observed in the US during 1974-1985 (42), but the measuring stations are in urban areas where air pollution confounds the readings. Some increase has been observed at a high-latitude measuring station in Switzerland (43) and Antarctica (44). Use of personal dose-meters would resolve uncertainty about human exposure in urban areas.
The 1991 Update of the United Nations Environment Programme (UNEP) Report on the Environmental Effects of Ozone Depletion (44) summarized the situation. Seasonal fluctuations continue, but there is a long-term trend toward further attenuation of stratospheric ozone; regions that will be vulnerable as attenuation progresses include populated parts of the northern hemisphere in the mid-latitudes. a reduction in total ozone of about 3% has occurred over the last ten years (45), associated at least in some places in the southern hemisphere with increased UV-B radiation readings at ground level (44). Anthropogenic tropospheric ozone and aerosols have masked the effect of ozone depletion on UV-B radiation readings in urban areas, but little or no such compensation occurs in places that are remote from industrial emissions.
Biological Effects of UV Radiation
UV-B has harmful effects on a wide range of biological systems (44, 46, 47). It causes DNA damage proportional to the intensity and duration of exposure; small, delicate organisms suffer more damage than large robust species, such as humans. UV-B impairs the growth and photosynthesis of certain plants, e.g. seedlings of maize, rye, and sunflowers. UV-B impairs the motility and reproductive capacity of phytoplankton. Change is evident in the composition of phytoplankton in aquatic ecosystems; these ecosystems are already under UV-B stress, and further increases in UV radiation are expected to cause detrimental effects, including disruption of some food chains. Moreover, marine phytoplankton metabolize a great deal of atmospheric CO2 (48); reduction of phytoplankton would decrease the uptake of CO2 and thus aggravate the greenhouse effect, as I discuss later. In short, the ecological effects of increased surface level UV-B radiation, although not fully predictable, are likely to be widespread and harmful. We need more studies of the effects of UV-B radiation on forestry, marine and fresh-water ecosystems, and food production.
Effects of UV Radiation on Human Health
The effects of UV radiation on human health are summarized in Table 2.
IMMUNOSUPPRESSION Induction of immunosuppression by UV radiation has been demonstrated in animals and humans (49-51). This is independent of skin pigmentation, so all people everywhere are at risk from potential adverse effects on the immune system, including increased incidence and severity of infectious disease and enhanced risk of malignant changes.
DERMATOLOGICAL EFFECTS Acute exposure to UV-B causes sunburn; chronic exposure leads to loss of elasticity and accelerated aging of the skin (52). Some fair-complexioned persons experience photo-allergy, which can be severe.
CANCER The most serious effect of UV-B is the enhanced risk of malignant, melanoma and nonmelanoma skin cancer (53). Increased risk of other malignancies, e g. cancer of the lip and salivary glands and intra-ocular melanoma, is uncertain. The relationship of UV radiation to cancer is discussed in detail in a current International Agency for Research on Cancer monograph (54), which concludes that there is convincing evidence for a causal relationship of malignant melanoma and nonmelanoma skin cancer to UV radiation. Case-control studies (55-57) suggest a higher risk of malignant melanoma related to a few episodes of acute sunburn, especially in childhood, than to prolonged low-level exposure with tanning. Cancer registry statistics demonstrate that the incidence rates of malignant melanoma have been rising and the age at onset has been declining for some years. Many, if not all, of these trends are almost certainly attributable to the popularity of sunbathing, rather than to exposure to rising concentrations of UV radiation; incidence rates can be expected to rise even more, perhaps rapidly, under the influence of higher concentrations of UV radiation. Mortality rates, however, have remained unchanged or have declined (58, 59); this probably reflects early detection and efficacious treatment.
OCULAR DAMAGE Ultraviolet radiation increases the risk of postcapsular and nuclear cataract and probably macular degeneration (60-62). Pterygium also occurs more frequently. Whether these effects are directly caused by UV radiation or by the combined effects of sunlight, heat, and dust is uncertain. Many other factors contribute to the risk of cataract formation; the role of UV radiation requires more careful risk assessment.
A sustained 10% decrease in atmospheric ozone has been estimated to increase the risk of cataract by 5% per annum (1.6-1.75 million additional cases worldwide). The risk of malignant melanoma has increased by 10%, and the risk of nonmelanoma skin cancer by 26% (44).
The Response to Ozone Depletion
Several kinds of response to the situation are warranted.
TREND ASSESSMENT Measurements of stratospheric ozone and surface-level UV radiation flux are needed, in relation to incidence rates of skin cancer and cataract; this should be feasible in selected sentinel communities. Observations of UV radiation flux would be improved by installing measuring stations both in and close to urban areas and in ecologically sensitive regions remote from population centers.
RESEARCH NEEDS More research is needed on the biological effects and on the effects on human health of exposure to UV radiation. Research is needed on attitudes toward sunbathing and use of protective measures. Protective measures, such as sunscreen ointments and creams, require rigorous evaluation. There are published reports of the protective effect of sunscreen against sunburn (63, 64), but their efficacy in preventing skin cancer and malignant melanoma remains unclear. Randomized controlled trials could, over many years, answer this question, but ethical objections will arise if sunscreens are widely believed to be efficacious.
EPIDEMIOLOGICAL STUDIES Both descriptive and analytic studies are needed and should include surveillance of incidence and mortality rates from malignant melanoma and nonmelanoma skin cancer. Cohort analyses (58) are especially useful, because they reveal incidence and mortality trends in relation to generation of birth; rising incidence rates in recently born generations would suggest increased exposure to risk. Randomized controlled trials of population screening for malignant melanoma have been proposed (54), but there are ethical concerns, comparable to ethical objections to randomized controlled trials of cervical cytology for cancer of the cervix. Because UV radiation also contributes to cataract formation, we need more information on, for example, age-specific incidence rates of cataract, particularly varieties strongly associated with UV radiation, such as postcapsular cataract. Large case-control studies and serial incidence studies in sentinel communities, where data on UV flux are also available, would help to assess the risk of cataract specifically associated with UV exposure. We should maintain rigorous surveillance of infectious diseases because of the role of UV radiation in immunosuppression.
We also need surveillance of UV-related damage to other vulnerable species, e.g. phytoplankton and mammals that live at high latitudes, such as sheep in Patagonia and Iceland. There have been press reports of eye disease among sheep in Patagonia, but such information has not appeared in scientific journals. Veterinary epidemiologists could conduct studies of animal herds and wildlife populations as sentinels of human health outcomes. Skin cancer has been reported among sheep in Australia (65), which suggests that sheep might be a useful sentinel population (65).
PUBLIC HEALTH ACTION Strategic and tactical public health responses are required (66, 67). Weather advisory messages warning about safe exposure levels have been routinely used on Australian radio and television for some years, and started in Canada in early 1992; such advisories are not part of routine weather reports in the US. Advisories should include messages about avoiding severe sunburn, especially for children, and recommendations about clothing, sun glasses, and sunscreen ointments and creams that may protect against excessive exposure to UV radiation. Suncreams and lotions are assessed for toxicity and potential carcinogenicity by government agencies, such as the Bureau of Chemical Hazards in Canada. There are no Occupational Safety and Health Act standards for protection against UV radiation, and permissible exposure limits only for short-term, high-intensity exposure. Health education might help to change attitudes back to those that prevailed in western societies from late Victorian times through the end of World War when a pale complexion, rather than a "healthy tan," was considered attractive. Should we try to convert persons with fair skins to this view and away from the notion that suntan is desirable? If so, research is needed to identify and evaluate ways to do this. An Institute of Medicine workshop in 1990 offered some ideas (68).
PUBLIC POLICY The goal is to reduce, as rapidly as possible, the contamination of the atmosphere by ozone-destroying chemicals, notably CFCs. This requires political action. Most of the industrial nations in which CFCs are produced and used have signed the Montreal protocol (69), thus agreeing to eliminate the production and use of CFCs as soon as possible. In early 1992, the Bush administration undertook to phase out CFCs by 1995; however, even if production and use of CFCs ceases worldwide, they will continue to accumulate in the atmosphere for up to 100 years (70). Many developing nations have just begun to use CFCs; their economies will suffer more than those of industrial countries if eliminating CFCs leaves them with no alternative to these useful solvents and refrigerants (71). Moreover, CFC substitutes (hydrochlorofluorocarbons) may have deleterious effects on the ozone layer that are almost as bad as CFCs (72).
CONCLUSION There is convincing evidence that the stratospheric ozone layer has been damaged by human action, that the damage is progressive, and that this poses serious threats to biological systems and human health. Action to limit the damage has begun and must continue. In addition, public health action to mitigate risks to human health is needed.
In 1896, Svante Arrhenius pointed out that atmospheric CO2 permits the passage of short wavelength radiant heat to the earth's surface and traps reflected longer wavelength radiant heat (73). In 1937, Trewartha (74) used the term "greenhouse effect" to describe how atmospheric gases stabilize the earth's temperature by allowing the passage of visible and UV radiation from the sun, which warms the earth's surface, but block the escape to space of reflected infrared radiation. Thus, the biosphere preserves a temperature range that sustains life. Without the greenhouse effect, most of the radiant heat from the sun would be reflected back into space, and the surface temperature in the shade would fall to many degrees below freezing. The surface temperature does fall if solar radiation is blocked by dust or gases, e.g. SO2, in the stratosphere (76). Volcanic eruptions. such as those of Mount Pinatubo in the Philippines in 1991, can thus have a moderating effect on average global temperature; this appears to have happened in 1992.
Greenhouse Gases and Temperature Trends
The principal greenhouse gases are CO2, water vapor, oxides of nitrogen, methane (CH4), CFCs, ozone (not to be confused with stratospheric ozone), and miscellaneous others. The greenhouse gases are mainly caused by human activity, especially exhaust emissions from internal combustion engines, coal-burning electric power generators, and innumerable other industrial processes (77). There are some puzzling features of the CO2 cycle: It remains unclear where all the C2O comes from and where it goes. CH4 also comes from boreal peatlands, rice cultivation, and the flatus of cattle (78).
Since accurate recording began, the concentration of these gases in the atmosphere has risen because of the greatly increased scale of fossil fuel combustion (Table 3). Average global temperatures rose by an estimated 0.3-0.6C from 1800 to 1990, more rapidly in the last ten years than in earlier periods. However, the increase has been erratic rather than smooth, in contrast to the increase in atmospheric concentration of greenhouse gases, and it is not possible to determine whether this is part of a natural fluctuation associated with solar activity, such ocean currents as el Nino, or a consequence of the greenhouse effect.
Atmospheric scientists are concerned about the rising concentrations of greenhouse gases, and many believe that the temperature of the biosphere will rise (7, 8, 11, 23-25). There are a few dissenting views (78, 79), but the model proposed by one of these (79) was soon refuted by empirical observations (80). A temperature rise in the range of 1-5C in the next 50-100 years has been predicted; at the upper end of the range, this is greater and faster than at any time in the last 140,000 years. It could overwhelm the capacity of many species to adapt. Recent predictions (25) are for a smaller increase of 0.5-1C by 2050, but this is a global "average" that predicts a greater temperature rise, perhaps 4-5C, in mid-latitudes and a smaller increase at the equator and poles.
In this section, I offer some speculation and opinions based on several recent accounts (3, 15-21, 81-84). A temperature rise of 5C would affect local, regional, and global ecosystems; sea levels and ocean currents; prevailing winds: fresh water supplies; agriculture; forests; fisheries; industry; transport; urban planning; demographics; and human health. Some effects are mutually reinforcing, so a small increment in an existing trend could induce massive change, in accordance with the mathematics of catastrophe theory. Such a sequence would have economic and political consequences, security implications, and direct and indirect effects on human health (15-21, 81-85). The smaller temperature rise of 0.5-1C, which is more likely, will have milder but perceptible effects.
Some of these changes may already be in progress. Six of the ten hottest summers since record-keeping began occurred in the 1980s. The summer of 1988 was not only unusually hot, grain crops everywhere suffered, which led to a reduction in world grain reserves from about 100 to 55 days (86)
WEATHER Apart from temperature rise, global warming is expected to change the configuration of jet streams and ocean currents. This will alter the distribution of rainfall, thus making some regions wetter, others drier. Weather disturbances, such as hurricanes, might become more violent.
Vegetation and food supplies Global warming will change the distribution of vegetation. The capability of grain crops and trees to "migrate" from hot to cooler zones is uncertain (87). Some productive agricultural regions, the American midwest for instance, are more likely to get drier than wetter, which would reduce grain production, perhaps drastically. As grasslands and prairies get hotter, they dry out and become even hotter; the self-correcting effect of vegetation on microclimates is lost. This is one of several feedback loops that accentuate climate change. Temperate zone warming leads to a decline in soil moisture that impairs grain production. Ultimately these areas could become desertified, the topsoil lost in dust storms. The distribution of grasses, weeds, and allergens will also shift, as will the distribution of the myriad species of insects whose habitats are related to specific vegetation.
Food crops can be damaged in other ways. Warm climates provide a favorable habitat for insects, fungus, and microorganisms that cause diseases of grain, fruits, and vegetables.
Forests are disappearing now because of human depredation. Tropical rain forests are being cleared for agriculture or cut for commercially useful hardwood. In temperate zones, such as the high-rainfall areas of the Pacific slopes of North America, forests have been subjected to clear-cut logging. Slash-burning of tropical rain forests contributes to the atmospheric burden of CO2 and leads to the loss of many species of plants and animals, thus reducing biodiversity as well as the amount of vegetation available to metabolize CO2. In high latitudes, the warming could thaw permafrost, thus releasing frozen rotting vegetation in artic bogs and ponds. This would lead to emission of large amounts of CH4 and add to the burden of greenhouse gases.
SEA LEVEL RISE Another consequence of global warming is sea level rise, caused by melting of ice-caps and thermal expansion of the seawater mass. The extent of polar and alpine ice-melt is difficult to predict. Alpine ice-melt has been going on for many years, as shown in photographs of glaciers at intervals since the 1890s (88). In a worst-case scenario, the Antarctic ice-shelf would melt, thus causing a sea level rise of 5-7 meters. This scenario is unlikely: Recent estimates (25) predict a rise of 0.5 meters or less in the next 50 years, which would be enough to submerge many coastal wetlands and disrupt their ecosystems. This process is aggravated by other factors. For example, the annual monsoon in Bangladesh is made worse by Himalayan deforestation, which leads to torrential floods, in contrast to more gentle run-off when vegetation impedes the flow.
Many of the world's important fishing grounds are dependent on ecosystems involving coastal wetlands, so coastal flooding contributes to depletion of fish stocks. In some areas, e.g. the Humboldt current off Chile and the Newfoundland Grand Banks, fish stocks have already fallen sharply because of over-fishing in past years, perhaps aggravated by marine ecosystem changes. Ecological disasters, such as massive oil spills, are another danger to fishing grounds.
FOOD SECURITY All the above phenomena threaten food security. Severe food shortages, perhaps famines, could occur early in the twenty-first century. On the other hand, a warmer agricultural region with higher atmospheric CO2 could become more productive (89).
COASTAL FLOODING A sea level rise of 0.5 meters would flood many coastal communities, especially in the poorest developing countries, such as Bangladesh. Sea level rise, therefore, would increase the number of environmental refugees.
DRINKING WATER Fresh water supplies are threatened by salination of coastal estuaries. Potable fresh water supplies are further reduced when seawater infiltrates subterranean water tables, as in parts of Florida. Moreover, water supplies are often polluted by toxic wastes, domestic refuse, human excreta, or all of these.
VECTOR-BORNE DISEASES Insect vectors of disease become more abundant as ambient temperatures rise. Most vector-borne pathogenic organisms flourish in warm climate zones; both the pathogens and their vectors survive better in warm climates than in cool ones; as the climate warms, temperate zones become more hospitable to ticks and hematophagous insects, such as anophelene and culicine mosquitoes. The range of bats that carry rabies also extends more widely as the climate gets warmer (90, 91). With warmer ambient temperatures, such pathogens as viruses and plasmodia remain viable for longer periods, and both they and their vectors breed more vigorously. Consequently, vector-borne diseases extend over a wider range. If enhanced UV radiation impairs immune responses, humans will be more susceptible to infection, thus further increasing the probability of epidemics.
DIRECT EFFECTS OF HEAT Over the next 50 years, the average annual number of very hot days could double in the temperate zone cities of the world. The "heat island" phenomenon, which makes cities warmer than surrounding rural areas, will lead to longer and more severe heat waves than now. In temperate zones, a sustained hot-spell increases the incidence of heat-related illness: heat stroke and heat exhaustion, which affect especially the very young, the old, and those already weakened by chronic cardiovascular or respiratory disease (92, 93).
Cities are getting larger, and many have extensive periurban slums (94). Acts of domestic violence and civil disturbances, such as riots, occur more often in hot than cold weather. Heat waves strain utilities and essential services, such as fire departments. Heat waves increase the demand for air conditioning, but unless solar energy is used, the extra fuel consumption contributes to the burden of greenhouse gases. Climatic emergencies can be a stressor, even to stable political systems, and often contribute to a breakdown of law and order. Public health services are difficult to maintain in such situations. Other indirect effects include breakdown of sanitary services, if sewage treatment plants are inactivated by floods.
MIGRATION Depletion of resources in regions of climatic extremes, poor agricultural resources, and high birth rates have led to regional food scarcity and much population movement since the end of World War II (95). The migrants are sometimes described as environmental or ecological refugees; their predicament is often complicated by regional conflicts attributed to political, religious, or ethnic strife, but really caused by competition for living space (32). Some indirect health effects of global change are related to migration: Migrants carry the diseases of their place of origin to their destinations and, once there, they may be susceptible to diseases that they had not previously experienced. Often they live outside the established social system and may not have access to adequate health-care services.
OTHER HEALTH EFFECTS The combination of allergenic vegetation, dust, polluted water, and reduced resistance to infection (due to chronic undernutrition or UV radiation induced immune disorders) increases the risk of epidemic respiratory and gastrointestinal infections. Some commentators (16, 17, 20, 84) regard epidemics as the most serious risk associated with global change.
Overall Impact of Global Warming on Human Health
The above account conveys a sense of urgency or unreality depending upon one's point of view. The main health implications are the threat to food security, i.e. food shortages, perhaps famines' and epidemic disease. An alternative view is that warming will enhance the productivity and duration of the growing season in subarctic regions where food now grows poorly (89, 96). This sounds attractive, but assumes the presence of suitable soil, soil bacteria, and sunlight to foster more productive growing seasons. An increase in ambient temperature of permafrost to levels around 0C would not enhance plant growth. These effects or global warming are summarized in Table 4.
CONCLUSION The evidence on global warming due to greenhouse gases is confusing and equivocal. Much of it relies on models with little or no empirical demonstration that the models are valid. Uncertainty about CO2 circulation complicates interpretation of the models. However, articles in such journals as Nature and Science that present arguments and evidence to support the global warming hypothesis have greatly outnumbered those refuting the case; empirical evidence is accumulating in support of the global warming scenario. Moreover, global warming is just part, albeit a prominent part, of a wider complex of global change. I believe the situation is serious and that public health specialists would be justified in preparing to deal with the predictable health consequences.
Concern about the deteriorating global environment has evoked responses from several international agencies. The World Meteorological Organization (WMO) and the International Council of Scientific Unions (ICSU) have focused on climate change (97). UNEP has coordinated studies of atmospheric change (11, 44) and, with WMO and many national agencies, supports the Global Climate Observing System (98), an international collaborative endeavor coordinated by the UK Meteorological Office. UNEP was a prime mover in the United Nations Conference on Environment and Development (UNCED) in Rio de Janeiro in July 1992. Although political pressure from several directions influenced the agenda and resolutions, and although President Bush refused to sign the Biodiversity Treaty, UNCED was not a fiasco. All nations represented at UNCED signed the protocol on greenhouse emissions. Canada and the European community are committed to lowering emissions to 1990 levels by the year 2000. Japan will hold CO2 emissions at year 2000 levels. The US is committed to an unspecified reduction without target dates. Some statesmen, e.g. Albert Gore of the US (99), and concerned groups in many nations have proposed a more drastic reduction in carbon emissions. The arguments in favor of this are cogent; US emissions could be reduced by 10-40% at relatively low cost, perhaps even with net savings (76, 100).
Some nations have set up multidisciplinary scientific programs to monitor the effects of global change and to plan strategic and tactical responses. These include the Canadian Global Change Program (101) and similar programs in the Netherlands (102) and Australia (103). The Canadian Global Change Program is coordinated by a working group of scientists from many disciplines and sectors. Its areas of attention include the Arctic, critical zones, climate, oceans and hydrology, culture and values, data and information systems, energy, human health, ecosystem research and monitoring, past environments, renewable resources, and security. Each of these areas has a research committee that is responsible for defining research questions, setting priorities, and encouraging research programs.
Some actions, summarized in Table 5, are appropriate despite the equivocal evidence and uncertainty about scenarios.
Assuming global warming scenarios are valid (or even if they are not), it would be useful to monitor the distribution and abundance of insect vectors, such as anophelene and culicine mosquitoes, ticks, and sandflies, and of the pathogens that they carry. Surveillance would work best in combination with comprehensive reporting of diseases caused by these pathogens. Monitoring of food supplies is also required, in combination with development of national food policies similar to those of Britain during World War II. Surveillance of heat-related illness, using a method similar to that used by the Centers for Disease Control for surveillance of influenza, could reveal outbreaks of heat-related illnesses and deaths.
Health Protection of Migrants
Whether because of changing global climate or other factors, considerable international migration is occurring. This has predictable health implications, aiming to cope with the increasing impact of these migrations.
Weather disturbances could become more violent, e.g. severe floods could occur if stable weather systems are disrupted, as predicted in global warming scenarios. The mass movement of environmental refugees is a human disaster, and the scale of this movement will increase if more habitats, especially in developing countries, become unable to sustain their populations. The public health response to such emergencies is better worked out ahead of time than during a disaster.
Education, Awareness, and Advocacy
Scientists (104) and health workers (105) in many countries have discussed the implications of global change. Some have advised their national leaders to take action to limit the burden of greenhouse gases. So far however action has been limited and localized.
The worrisome aspects of global change result from human activity. The world's population is increasing now at a rate of more than 90 million a year and is expected to pass 6 billion before the year 2000. This proliferation has been likened to a cancer of the planet: Aerial photographs and maps of urban areas at intervals resemble the appearance of neoplastic growths (106). The rate of global change is enhanced by population growth which cannot continue indefinitely in a closed, finite system, the biosphere. Like bacterial colonies, flour beetles, field mice, shoals of fish and roving herds of caribou, humanity must strike a balance between reproductive rates and available supplies of nutriment and other essential resources. The exponential population growth of the past eight to ten generations must be followed eventually by one of only two outcomes: leveling off or crashing into reverse. A population crash is more likely if we take no action now.
Aspirations to have children are encouraged by favorable economic conditions by deeply held beliefs that are integral parts of the world's religions, by almost universally held moral and social values, and in some nations by pronatalist policies. There are troubling ethical dilemmas. Endeavors by health workers from industrial nations to establish family planning programs in some of the world's poorest and most crowded nations may be perceived locally as genocidal (71). But the hopes of people in nations that are emerging into industrialization to achieve the level of affluence enjoyed by Western Europeans and North Americans with automobiles and other heavy users of fossil fuels for every family, threaten further detrimental global change.
Anxiety and concern are expressed in many mass media discussions of global change. Scientific journals are more restrained: the evidence does not warrant the conclusion that the effects of global change will be calamitous. The caution of scientists in the absence of certainty has encouraged governments and many influential commercial interest groups to take minimal action aimed at reducing harmful emissions, despite campaigns by environmental interest groups. Governments in the industrial nations have mostly resisted pressure from these quarters, with backing from the energy industry. I believe there is an element of denial in this, akin to a person's reluctance to accept a diagnosis of incurable cancer or an adolescent's faith in immunity from the consequences of risky behavior.
It is helpful to recall the evolution of attitudes toward the public health that have shaped public health practice and law in the past 100 years (107). The sequence is penetration of scientific knowledge about the nature and cause of a health problem through mass media to the general public, followed by a change in values, thus leading to political pressure for legislation aimed at control of the underlying cause of the health problem. Examples of the sequence include the nineteenth century sanitary revolution and changes in social values that led to increasingly stringent regulation of tobacco use in many countries in the second half of the twentieth century.
This sequence may be beginning with growing public concern about the implications of global change for humanity. There have been proposals for a code of environmental conduct (108, 109) and for laws dealing with crimes against the environment (110). Public health organizations are mobilizing for advocacy (111), which may encourage a shift in values and might lead to the evolution and adoption of an environmental ethic that recognizes the need for sustainable development (112, 113).
If population pressure is the ultimate cause of all the other problems, we have to limit population growth. The ethical and moral conflicts involved are very difficult to resolve. After the development of the atom bomb, Albert Einstein remarked that everything had changed and that humanity required an entirely new way of thinking. The same can be said about the problems of population and the environment, the most challenging problems we face in the future. Public health workers, along with all other thoughtful members of the human race, must wrestle with these difficult issues.
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