In May, 1993, fever, headache, and respiratory distress overtook a nineteen-year-old Navajo athlete. While being sped to a medical center, his fluid-filled lungs failed his oxygen-starved body. Desperate gasps for air ceased, and he was pronounced dead on arrival. Within days, two of his neighbors had fallen mysteriously and similarly ill. Rumors of deadly epidemic brought in investigators from the Centers for Disease Control, but their searches of body tissues and fluids revealed none of the suspected respiratory illnesses, including plague. Finally Navajo elders suggested that the disease might be associated with the deer mice that seemed to be scampering impudently everywhere. Mice were trapped, their tissues and fluids examined by researchers who protectively cloaked themselves from head to boots. An organism previously unknown to science was found: a strain of a hantavirus that was capable of rapidly producing severe respiratory distress and killing about 2/3 of its victims. The virus, excreted in the feces and urine of infected mice, contaminated dust that was blown by the dry winds to the nostrils of human victims. Weather patterns had been kind to the mice that spring: several years of extreme drought, which had reduced populations of owls and other mice predators, had been followed by a mild, moist winter that provided the mice with ready burrows and abundant pinyon nuts. The mice, ten times more numerous than a year earlier, invaded human settlements, carrying with them the hantavirus that would eventually be named after the Navajo's Muerto Canyon -- the Valley of Death.

A generation ago, physicians armed with immunizations and drugs dreamed of decimating infectious disease. Now tales of new and more powerful infections speckle the mass media, as well as the minds of those in the know. Infectious agents are becoming immune to antibiotics; more virulent strains are popping from the woodwork; diseases exterminated in certain areas are returning with a vengeance; and new "emerging diseases" such as AIDS and the above-described hantavirus lurk in hidden corners, waiting to spring. Global incidences of cholera, tuberculosis, diphtheria, and even bubonic plague have all increased significantly in the last 5 years. Future massive epidemics of these or similar infections are not only possible, they are likely. The resurgence of infectious illness rises from a complex of factors: more people, crowded in larger cities, using more antibiotics, and spreading more organisms through their fast-paced worldwide travels, provide ready prey for disease-producing organisms.

Another possible contributing factor of increasing concern is climate change. Global warming and its associated precipitation alterations are likely to expand the range of some infectious organisms and the vectors that spread them, many of which are highly climate-sensitive. Widespread shifts in the distribution and number of resulting diseases will result. In addition, warmer temperatures will, in some cases, accelerate the life cycle of infectious agents or vectors, allowing their population numbers to run further rampant. Blood-feeding vectors, for example, feed more frequently in warmer temperatures, a pattern that can stimulate egg production. Increased precipitation produces more breeding sites for disease-transmitting mosquitoes and blackflies. Diseases transmitted by vectors such as mosquitoes, snails, and rodents are currently a major cause of illness and death in tropical countries. With global warming, North America would become more receptive to tropical infectious illnesses.

Malaria is a case in point. The causative plasmodium parasite is passed from person to person by blood-feeding anopheline mosquitoes. Control of malaria has increased in difficulty as these plasmodia have become increasingly drug-resistant. In addition, manifestations of the disease are becoming increasingly severe. In some regions where malaria had been almost eliminated, it has made a comeback with levels surpassing those of earlier years. In areas of Africa, over half of the people are infected, and malaria is a main childhood killer. As many as 1 person in 20 of the world's population is now infected with malaria, with 350 million new cases occurring annually; 2.4 billion people are now at risk of contracting the disease.

Global warming would favor malaria in several ways. The disease has already been shown to migrate to higher altitudes during hot, wet spells. Warming could also increase mosquito survival in temperate areas that, with warming, would no longer be subjected to cold, killing winter temperatures. In addition, even slightly warmer temperatures would greatly accelerate successful plasmodium incubation within the mosquito. The expected increases in precipitation could produce more breeding sites in local areas. These factors could increase the incidence of malaria and push the disease into currently malaria-free areas with non-immune populations, both in temperate regions and in the highlands of tropical countries. With climate change, by the latter half of the coming century, the number of people living in the potential malaria transmission zone is predicted to increase from 45% to 60% of the world's population.

Other currently emerging infectious diseases are pinpointed in figure 1. Many of these are likely to respond to global warming. Of particular concern for the US are dengue and the arboviral encephalitides, both mosquito-borne viral diseases of varying severity that can be fatal. Both are likely to extend their range northward with warming climate -- in fact dengue has already been reported in Texas. Diseases transmitted by rodents and by flies would be likely to increase as populations of those vectors rose, as would foodborne diseases and poisoning produced by consumption of fish and shellfish contaminated with biotoxins. These biotoxins are produced by marine phytoplankton which flourish when sea waters warm, and which also have been implicated in the recent resurgence and spread of cholera.

Human Health and Altered Food Production

Drought is normal from October to June in the Sahel region of sub-Saharan Africa. But when the rainy season does not follow the dry as it should, food production plummets and the situation becomes dire. That's what happened between 1969 and 1973, and in the last year of that extended drought, a hundred-thousand people died as a result. A quarter of the region's cattle either succumbed or were slaughtered. The following year, 200,000 people in Niger became dependent on food aid and 200,000 Mali citizens became refuges because of drought. Drought, overcultivation, and overgrazing continue to turn the region into desert and to wound agricultural productivity. Climate change that magnifies drought cycles threatens to intensify these problems.

An adequate supply of quality food is imperative to health. Without it, children's physical and mental growth is stunted. The immune system declines, infection increases. At the extreme, inadequate food leads to starvation.

Since agricultural plants and animals are, like all living creatures, responsive to their environments, they will respond to changing climates. Ranges of some crops may expand; those of others may shrink. Changes in precipitation and temperature would affect productivity. Agricultural pests, pathogens, and weeds too would mirror the changing climate; aphids increase under dry conditions, locusts proliferate when it's wet. Pests (with their short life cycle) are quick to react to changes in climate with altered numbers and ranges. Changing patterns of rainfall might invoke or alleviate drought, enhance soil erosion and flooding, or alter the replenishment of freshwater aquifers. Decreased aquifer replenishment and changes in river flows could, in turn, grossly impact crop production, since about a third of the world's crop supply is currently produced by irrigated agriculture. Fisheries -- which today provide one-fifth of the human-ingested animal protein -- also would be altered as sea levels rose, water temperatures and currents fluctuated, and salt water penetrated previously fresh waters. Rising seas and associated salinization may seize some agricultural lands, and geographic shifts of productive croplands or fishing grounds could lead to political conflicts. Stratospheric ozone depletion may compound losses by disrupting photosynthesis in plants, impacting livestock health, and playing havoc with ocean plankton (and thus with oceanic balances dependent on these tiny organisms).

These and other biological considerations -- not to mention the interplay of social, economic, and technical factors -- make the modeling of climate-related changes in food production extremely complicated. Add in predictions that higher temperatures and increasing CO2 concentrations might induce more efficient water use and could actually stimulate the growth and productivity of some plants -- or conversely could decrease yield by shortening the required growing season -- and the situation becomes even more complex.

Despite these complexities, and although general predictions are made with caution, some can be stated more firmly. The aggregate effects of climate on agricultural productivity are likely to be adverse. Their impacts may be modest in scale when integrated around the globe, but will be regionally significant, with poorer tropical and semi-tropical areas -- where many large populations already suffer from malnutrition -- being the hardest hit. Semi-arid and arid regions dependent on rainfed agriculture -- such as those in Africa, southern and eastern Asia, and some Pacific islands -- would be particularly vulnerable. In 1994, 700 million people already were lacking sufficient food to lead healthy and productive lives, and a sizable proportion of the earth's soils and fisheries had been degraded through misuse. Climate change that magnifies the current inability of the human community to feed its own would join other environmental stressors in decreasing human resistance to infection and disease.

Today, although researchers are increasingly recognizing the types of problems outlined above, they admit that the quantitative aspects of these impacts remain vague. "Global and especially regional surprises can be expected," concluded the Task Group mentioned earlier. The number of persons to be affected would depend on the exact amount of global warming, the density and distribution of humans, and other such variables. Just as some towns were obliterated in the Middle Ages by the Black Death and others were spared, the effects of climate change may hit some regions hard while others remain relatively intact. In addition, our response to changing health patterns will influence the number of ill or dead in each locale, with the most vulnerable being persons who are overcrowded, lack quality food and shelter, or are stressed in other ways. But regardless of the sophistication of technological and social responses to probable health threats, all will be affected to some degree; the search for immunity will be as illusory as it was in the 14th century, when Florentine nobility attempted to flee the Black Death by locking themselves in their high-walled palaces.

However, just as we have the power to exacerbate problems of global change, so we have the ability to minimize them. The exact consequences of global warming will depend on the response of individuals, societies, and governments to both the threats and the consequences of warming. CGRER, for example, has just published an Iowa Greenhouse Gas Action Plan, which is intended to help minimize global warming by outlining policies that could be utilized to mitigate our states emission of greenhouse gasses. Better understanding of the complexities of changing disease patterns will allow more appropriate public health planning programs and responses. Such understanding might also invigorate movement toward the ultimate response: prevention of future actions that would further stimulate climate change with all its concomitant problems. CGRER will continue in its efforts to encourage the interdisciplinary interactions needed to untangle the complexities of health-related global change problems and to focus creative minds on proactive solutions of these impending problems.