Главная страница «Первого сентября»Главная страница журнала «Английский язык»Содержание №29/2003
 
NATURE & ECOLOGY

 

Climate and the Human Body

HEAT, COLD, AND THE HUMAN BODY

Although most climatologists are reluctant to draw broad conclusions about climate’s effects on human societies, the study of its impact on individuals has flourished since World War II. “Human health, energy, and comfort are affected more by climate than by any other element of the physical environment”, observes Howard Critchfield of Western Washington University.

“Bioclimatology” attracts researchers from a variety of specialties, with markedly different interests – industrial psychologists, physicians, space scientists, They have linked climate to everything from homicide to human fertility to mental acuity.

At the extremes of hot and cold, climate’s effects are relatively easy to measure – and to avoid. In the United States, heat stroke and hypothermia together claim only about 325 lives each year. Yet W. Moulton Avery, of the Center for Environmental Physiology, contends that it “would be front page news” if federal researchers has actually collected data on the thousands of heat-related deaths (e.g., from stroke) among elderly Americans last summer.

Climate exerts its influence in subtler ways. According to one study, for example, “excessive aggressiveness” begins to manifest itself between 82.4 and 86 degrees Fahrenheit, when the relative humidity is 100 percent. The Federal Bureau of Investigation (FBI) lists climate as one of a dozen factors that influence crime rates.

Some researchers have tried to link climate-induced physiological changes to physical and intellectual performance. As temperatures rise above 86 degrees, they note, the body cools itself by increasing blood flow to the skin and reducing the flow to the brain and muscles. One result: a loss of energy and ability to concentrate. When the thermometer drops below 68 degrees, the body conserves warmth by restricting blood flow to the skin. Yet, some groups, such as Eskimos, may have developed different tolerances through evolution; individuals undergo short-term adaptations to harsh climates.

Most studies suggest that comfort and mental vigor are not entirely synonymous. Andris Auliciems of the University of Toronto found that English schoolchildren performed best on a variety of tests at temperatures of 58.5 to 62.9 degrees. Some bioclimatologists have put the optimum temperature as high as 82 degrees; others dismiss such correlations as worthless.

Science does confirm much folk wisdom. Winter in the temperate zones of the world means more flu, partly because the cold depresses the body’s immune system, but mostly because it drives people indoors, where microbes spread easily. Other sicknesses plague the tropics because certain disease-bearing organisms flourish in heat and humidity.

Climate has other, unexpected, effects. Wolf H. Weihe, a Swiss biologist, reports that the fertility rate of women in Bombay, India, drops by more than 50 percent during the monsoon season. In the United States, he says, statistics indicate that fertility is lowest during the winter – except, for some reason, in Kansas, where it jumps when the temperature drops to 18 degrees below zero.

CLIMATE CHAOS

For the past several years scientists have issued ominous warnings about the future of the earth’s climate. Predictions of dramatic global change arising from the continued dumping of industrial by-products into the atmosphere and forest loss of massive scale can no longer be ignored. Compelling scientific evidence now strongly suggests that world climate patterns, previously regarded as reliably stable, could be thrust into a state of turmoil. Emissions of natural and synthetic gases are increasing the heat-trapping capacity of the atmosphere through a phenomenon known as the “greenhouse effect”.

The projected effects of this worldwide climatic disruption dwarf many of the environmental problems of the past and augur political, economic, and social disruptions on an enormous scale. Global warming could have catastrophic consequences for the habitability and productivity of the whole planet. The accompanying strain and upheaval on the international scene in turn could have serious foreign-policy consequences for all countries.

Broad scientific agreement exists on the underlying theory of climate change, although the nature and magnitude of future effects from greenhouse warming as predicted by computer models remain in debate. Some of these, such as a rise in the sea level, have been established with greater certainty than others. Nonetheless, the range of consequences is sufficiently clear and the magnitude of the resources at stake so enormous that policy action is required sooner rather than later. Once a crisis has been reached, it will be too late to act.

The effects of a greenhouse-driven climate disruption will be characterized with complete certainty only after significant damage has already occurred. However, among the most dramatic effects likely to ensue from greenhouse warming is an unprecedented rise in sea level resulting from thermal expansion of the oceans and melting of glaciers and polar ice. Over the past century the average global sea level has increased less than 6 inches. By contrast, the sea level will have accelerated considerably, producing a total increase of up to 1–7 feet by 2075, depending on the degree of global warming that occurs.

The impact of sea-level rise in the United States is likely to be severe. The anticipated increase in the elevation of the oceans could permanently inundate low-lying coastal plains, accelerate the erosion of shorelines and beaches, increase the salinity of drinking-water aquifers and biologically sensitive estuaries, and increase the susceptibility of coastal properties to storm damage. An increase of 5–7 feet in sea level would submerge 30–80 percent of America’s coastal wetlands, which are crucial to the productivity of commercially important fisheries. Extensive existing coastal development may prevent the widespread formation of new wetlands. Even in undeveloped coastal areas, the rapidity of the predicted sea-level rise will mean that existing wet-lands would be lost faster than new ones can be created.

The increase in elevation of the oceans will also seriously affect the approximately 50 percent of the earth’s population that inhabits coastal regions. Entire countries, such as the Maldives, could disappear.

By David A. Wirth

CLIMATE AND EVOLUTION

Climate clearly has put its imprint on the tint of our skin, the size of our noses, and other physical traits.

The study of skin color and racial differences has been a touchy matter in Western science ever since the mid-19th century, when naturalist Louis Agassiz asserted that whites had bigger brains than others, and promptly concluded that they must therefore be more intelligent. Today, scientists differ over what a “race” is. During the late 1960s, for example, anthropologist Grover S. Krantz of Washington State University distinguished between “climatic races” – groups that share traits, such as skin color, which can change over generations in response to climate – and “descent groups”, which share ancient and immutable genetic traits, such as blood type. Climatic races have evolved separately from descent groups, Krantz maintained, and the two should not be considered “inherently connected”.

Amid such discussions, however, a consensus exists that climate has influenced the evolution of the human physique.

Skin color, for example, is determined largely by the amount of melanin, a dark pigment, in the outer layer of the skin. (Carotene imparts a yellow tint.) In sunny climates close to the Equator, natural selection has favored dark, melanin-rich skin, which protects its owner by absorbing harmful ultraviolet rays before they penetrate to lower layers. But some ultraviolet light must penetrate the skin so that the body can produce Vitamin D. Thus, at higher latitudes, where sunlight is less intense, pale skin with little melanin is the norm.

Among dark-skinned people, moreover, there are great variations in skin color. The drawback of dark skin is that, like dark cloth, it absorbs more heat from the sun than does lighter skin. In prehistory, anthropologists explain, those who roamed the savannah “traded off” some protection from ultraviolet rays for the reduced heat retention of lighter skin. For forest-dwellers, living in less extreme heat, a darker complexion was an evolutionary advantage.

Melanin also determines eye color. The human eye appears blue when there is no melanin in the outer iris, and turns darker as melanin increases. In the iris, as in the skin, melanin absorbs light, protecting the eye from glare. Thus, dark eyes are generally favored by nature. In Europe, however, almost 50 percent of the population has blue, green, or gray irises, Such people may see further in dim light, but scientists still do not fathom the evolutionary logic of blue eyes – or blond hair, for that matter.

The eyes of the Chinese, Japanese, Eskimos, and other people of Mongoloid descent – one third of the world’s population – are protected by epicanthic folds. These folds, composed of fatty tissue, probably evolved among their forebears inhabiting the Arctic in order to insulate the eye against freezing, and to provide an additional shield against glare from snow and ice.

Even the human nose adapts to climate. Inside the nose, a series of wet, mucus-lined air chambers “conditions” inhaled air before it reaches the throat and the delicate air sacs of the lungs, warming it to about 95 degrees Fahrenheit and raising its relative humidity to 95 percent. Humans in cold climates – or in hot, dry ones – thus have the greatest need to condition the air they breathe. Natural selection in such climes generally favors larger noses with more mucus lining: flattened, to protect against frostbite, in frigid environments; long and narrow in arid regions.

In a like manner, the size and shape of the human form help the body regulate internal temperature. Over thousands of years, cooler climates tend to produce larger people. The reason: Their extra mass helps them retain heat. Although large people also have more skin surface from which heat can escape, the tradeoff still works to their advantage. As the body grows larger, mass becomes greater relative to skin area.

The Alakaluf Indians on the frigid southern tip of South America, for example, are 25 percent taller than the Ituri Pygmies of Central Africa. Yet, the Alakalufs are more than two times heavier – and thus store much more body heat.

Variations in body shape complicate the picture. A tall, skinny man has more surface area – and heat loss – than does a shorter, huskier man of the same weight. Thus, cold territories closest to the North and South Poles tend to be populated by stocky folk.

In southern Africa, Pygmies, the world’s shortest people, dwell very near the Nilotic tribes (e.g., the Dinka), the tallest. But the Nilotic tribes live in the dry, open savannahs, the Pygmies, in the shaded forests. The Nilotics’ environment puts a premium on having more skin surface to release heat, thus their extremely tall, slender build. And, occasionally, there appear uniquely adapted humans, A notable example: the Khoikhoi women of the open African savannah, who have thin torsos suited to the hot climate, but also protruding buttocks (steatopygia) containing storehouses of fat to draw upon in times of famine.

Just as it is difficult to prove a correlation between past climatic change, and, say, the demise of an ancient civilization, so today’s anthropologists are not certain that all their inferences about climate and human evolution are well founded. Very few of the world’s peoples in all their variety now inhabit the same territories where, long ago, their ancestors presumably developed certain characteristics in response to climate. Often, notes Grover Krantz, anthropologists resort to “pulling people out of areas where their... traits don’t fit the environment and putting them back where they do fit”.

Wilson Quarterly