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
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