Today few scientists doubt that Earth's atmosphere is warming. Most also agree that the rate of heating is accelerating and that the consequences could become increasingly disruptive. Even schoolchildren can recite some projected outcomes: oceans will warm and glaciers will melt, causing sea levels to rise and salt water to inundate low-lying coastal areas. Regions suitable for farming will shift.
But less familiar effects of global warming--namely, serious human medical disorders--are no less worrisome. Many are already upon us.
Most directly, global warming is projected to double the number of deaths related to heat waves by 2020. Prolonged heat can increase smog and the dispersal of allergens, causing respiratory symptoms.
Global warming also boosts the frequency and intensity of floods and droughts. Such disasters not only cause death by drowning or starvation, but also damage crops and make them vulnerable to infection and infestations by pests and choking weeds, thereby contributing to food shortages and malnutrition. They displace entire populations, leading to overcrowding and associated diseases, such as tuberculosis.
Developing countries--where resources to prevent and treat infectious diseases are scarce--are most vulnerable to other infectious diseases associated with climate change as well. But advanced nations, too, can fall victim to surprise attack--as happened last year when the first outbreak of West Nile virus in North America killed seven New Yorkers. International commerce and travel enable infectious diseases to strike continents away from their sources.
Of course, not all the human health consequences of global warming may be bad. Very high temperatures in hot regions may reduce snail populations, which have a role in transmitting schistosomiasis, a parasitic disease. High winds--caused by parching of the earth's surface--may disperse pollution. Warmer winters in normally chilly areas may reduce cold-related heart attacks and respiratory ailments.
Overall, however, the undesirable effects of more variable and extreme weather are likely to overshadow any benefits.
Diseases relayed by mosquitoes--malaria, dengue fever, yellow fever, and several kinds of encephalitis--are eliciting particularly grave concern as the world warms. These disorders are projected to become increasingly prevalent because cold weather limits mosquitoes to seasons and regions with certain minimum temperatures.
Extreme heat also limits mosquito survival. But within their survivable range of temperatures, mosquitoes proliferate faster and bite more as the air becomes warmer. Greater heat also speeds the rate at which pathogens inside them reproduce and mature. The immature malaria parasite takes 26 days to develop fully at 68 degrees F, but only 13 days at 77 degrees F. The Anopheles mosquitoes that spread malaria live only several weeks, so warmer temperatures enable more parasites to mature in time for the mosquitoes to transfer the infection.
As whole areas heat up, mosquitoes enter formerly forbidden territories, bringing illness with them, while causing more disease for longer periods in the areas they already inhabit. Malaria has already returned to the Korean peninsula, and parts of the US, southern Europe and the former Soviet Union have experienced small outbreaks. Some models project that by the end of this century, the zone of potential malaria transmission will contain about 60% of the world's population, up from 45% now.
Similarly, Dengue (or "breakbone") fever--a severe flu-like viral illness that can cause fatal internal bleeding--has broadened its range in the Americas over the past 10 years, reaching down to Buenos Aires by the end of the 1990's. (It has also found its way to northern Australia.) Today it afflicts an estimated 50-100 million people in the tropics and subtropics.
These outbreaks, of course, cannot be traced conclusively to global warming. Other factors--declines in mosquito-control and other public health programs, or rising drug and pesticide resistance--could be involved. But the case for a climatic cause becomes stronger when outbreaks coincide with other projected consequences of global warming.
Such is the case in the world's highlands. In the 19th century, European colonists in Africa settled in the cooler mountains to escape the dangerous swamp air (" mal aria ") in the lowlands. Today many of those havens are compromised. As anticipated, warmth is climbing up many mountains. Since 1970, the elevation at which temperatures are always below freezing has ascended almost 500 feet in the tropics. Insect-borne infections are being reported at high elevations in South and Central America, Asia, and east and central Africa.
More droughts and floods due to global warming will also probably fuel outbreaks of water-borne diseases. Paradoxically, droughts can favor water-borne diseases--including cholera, a cause of severe diarrhea--by wiping out supplies of safe drinking water, concentrating contaminants, and preventing good hygiene. Lack of clean water also limits safe rehydration of diarrhea or fever sufferers.
Floods, meanwhile, wash sewage and fertilizer into water supplies, triggering expansive blooms of harmful algae that are either directly toxic to humans, or contaminate the fish and shellfish that humans consume.
The human health toll taken by global warming will depend to a large extent on us. Effective surveillance of climate conditions and of the emergence or resurgence of infectious diseases (or their carriers) should be a global priority, as should providing preventive measures and treatments to at-risk populations.
But we must also limit human activities that contribute to atmospheric heating, or that exacerbate its effects. Little doubt remains that burning fossil fuels contributes to global warming by spewing carbon dioxide and other heat-absorbing, or "greenhouse," gases into the air. Analysis of tree rings identifies fossil fuels as the source of the 30% increase in greenhouse gases over pre-industrial levels. Cleaner energy sources must be adopted, while forests and wetlands must be preserved and restored to absorb carbon dioxide, and to absorb floodwaters and filter contaminants before they reach water supplies.
None of this will come cheap. But humanity will pay a far dearer price for inaction.