Posted by: chrismaser | October 4, 2012

CURRENT CRISES: OUR GROWING HEAT STRESS

It is imperative that people become aware of the long-term effects of their decisions. I say this because children are one of the two silent parties in all social-environmental decisions; the land and its productive capacity is the other. Leaders, at whatever level of society, must understand the social, environmental, and economic circumstances to which they are committing future generations through their decisions of today. If the outcome of their decisions and actions is a deficit in terms of the future options for humanity, the productive capacity of ecosystems to serve human necessities, or both, it is analogous to “taxation without representation,” and that goes against everything our American democracy stands for.1

In keeping with this statement, those decision makers who deny the measurable social-environmental changes wrought by a warming climate to protect economic interests serve as illustrative of the irreversible consequences of ill-informed decisions. This irreversibility is best understood within the historical context of humans and a warming climate.

OUR FIRST AIR CONDITIONER

Millions of years ago, when hominids became omnivores by including meat in their diet, they needed to range farther and wider than their herbivorous counterparts if they were to procure a sufficient amount of food. The animals they hunted were also moving targets, save for the windfall of an occasional carcass, which meant they had to expend a lot more energy to obtain their meals. In the case of early hominid hunters and scavengers, who still spent some time in the trees, natural selection transformed their apelike proportions into a long-legged body that was built for sustained striding and running. But, these elevated levels of activity came at a price—a greatly increased risk of overheating.

Keeping cool became problematic, especially for those early hominids who lived in hot places and thus generated abundant heat from prolonged walking or running. They had to regulate their core body temperature carefully because overheating damaged their tissues and organs, specifically their brain.

Consequently, we humans evolved an effective strategy to avoid overheating. In addition to lacking fur, we possess between 2 million and 5 million eccrine glands, which are the major sweat glands of the human body. These glands are found in virtually all skin, where they reside relatively close to the surface and discharge up to 12.7 quarts of thin, watery sweat a day through tiny pores. This combination of naked skin and watery sweat, which sits directly on top of the skin rather than collecting in fur, allows humans to eliminate excess heat effectively.

When did this metamorphosis occur? About 1.6 million years ago, an early hominid evolved body proportions that were essentially modern, which would have permitted prolonged walking and running. Moreover, details of the joint surfaces of the ankle, knee, and hip clearly demonstrate that these hominids actually exerted themselves in this way. Thus, according to the fossil evidence, the transition to naked skin and an eccrine-based sweating system must have been well under way by that time to offset the greater heat stress that accompanied our forbearers’ increasingly strenuous way of life.

The evolutionary loss of fur was just a partial answer to the avoidance of overheating from a strenuous lifestyle. As with many other animals that live in hot climes, hominid evolution also increased the surface area of the naked skin to the volume of the body, thereby making it easier to expel deleterious heat, although this same evolutionary trait makes it more difficult for humans to retain the necessary body heat in cold climes.

Another clue to when hominids evolved naked skin has come from investigations into the genetics of skin color. A specific gene variant always found in Africans with dark skin pigmentation originated 1.2 million years ago. Early human ancestors are thought to have had pinkish skin covered with black fur, much like that of chimpanzees, so the transition to permanently dark skin was presumably an evolutionary requisite to the loss of our sun-shielding body hair.2

OUR BODY’S INVIOLABLE TEMPERATURE REGIME

In addition to naked skins, maintaining a closely regulated, high body temperature of 98.6° Fahrenheit characterizes people and approximates that of other mammals. Although maintaining such a high inner temperature is costly in the amount of energy that must be garnered through the consumption of food, the trade-off is the prevention of pathogenic fungi from invading the body because they cannot tolerate the heat.3

Today, despite the uncertainty about the impacts of climate change, it is often assumed that we humans could adapt to any possible warming. This assumption is not necessarily true, however, because all life has a limited tolerance when it comes to temperature—both hot and cold. This said, the peak heat stress is today similar across diverse climates.

The wet-bulb temperature never exceeds 87.8° Fahrenheit. The wet-bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin and exposed to the flow of air. It is a measure of the evaporation of water from the thermometer and the cooling effect the evaporation has. The rate of evaporation depends on the humidity of the air, which means the higher humidity, the more water vapor the air contains, the less effective evaporative cooling is. The second law of thermodynamics does not allow an object, such as a person, to lose heat to an environment when the wet-bulb temperature exceeds the person’s or object’s temperature, no matter how wet or well ventilated. In effect, the wet-bulb temperature is a measure of the lowest body temperature a person, or other mammal, could achieve solely by evaporative cooling in a given circumstance without the aid of technology. If the wet-bulb temperature were to exceed 95° Fahrenheit for extended periods, it would induce hyperthermia in humans and other mammals because the dissipation of body heat through evaporative cooling becomes impossible. (“Hyperthermia” is from the Greek hyper, above, and therme, heat.)

While the wet-bulb temperature does not currently exceed 95° Fahrenheit for extended periods, it would begin to occur if the world warmed an average of about 47° Fahrenheit—a temperature that would make habitability of some regions doubtful. Moreover, with a warming of 52–54° Fahrenheit, such regions would spread to encompass the majority of the human population as currently distributed. A global climate warmed by 54° Fahrenheit is possible from the burning of fossil fuel.

Heat stress is associated with not only warm nights and sleep deprivation but also hot days, which alter the lifestyles and productivity of people living at low latitudes. Although both impacts will worsen in warmer, more humid climates, most believe people will simply adapt because humans already tolerate a very wide range of climates. But, when measured in terms of peak heat stress—including humidity—this turns out to be a bogus assumption. Even modest global warming could expose large fractions of the human population to unprecedented heat stress, and severe warming would make such stress intolerable.

As mentioned, humans maintain a core body temperature near 98.6° Fahrenheit, which varies slightly among individuals but does not adapt to local climate. Human skin temperature is strongly regulated at 95° Fahrenheit or less under normal conditions, which is critical because the skin must be cooler than the core temperature of the body for the body’s internal temperature to be regulated by conducting excess heat to the skin for evaporative cooling. Sustained skin temperatures above 95° Fahrenheit imply elevated core body temperatures (hyperthermia). If the temperature of the skin is sustained at 98.6–100.4° Fahrenheit, the core temperature can reach the lethal values of 107.6 to 109.4° Fahrenheit, even for acclimated and fit individuals. Thus, sufficiently long periods where the wet-bulb temperature would remain at 95° Fahrenheit would most likely prove to be intolerable.4

CAN WE BUY OUR WAY OUT OF EXCESS HEAT?

Although people with sufficient monetary means in the industrialized countries could, and already do, turn to technology for such things as air conditioning, it is a temporary, symptomatic fix. The biophysical costs extracted from the environment would ultimately catch up and not only be catastrophic ecologically but also affect everyone without regard to monetary riches. There would be no opportunity for a person to “buy” his or her way out of the consequences. Moreover, poor people in both industrialized and non-industrialized countries would be the first to suffer greatly—to say nothing of large mammals, whose body mass is greater than the surface area of their skin, making them prone to fatal heat stress, as well dramatic changes in the availability of their normal food. Finally, genetic adaptation to a warmer world would not be readily possible given the speed of the potential changes in temperatures.

What today’s and tomorrow’s decision makers must understand is that human-induced climate change depends not only on the magnitude of the change but also on the potential reversal of the process of change, which will take about 1,000 years after the emissions cease due to the slower loss of heat to the world’s expanding and warming oceans.5 Even with the potential for a reversal of global warming as a process, the current and ongoing biophysical consequences of climate change are already and forever irreversible.


Related Posts:

• Tropical Crisis: “Bushmeat”

• Easter Island: A Lesson In Over-Population

• Global Water Crisis—Urban Sprawl

• Global Water Crisis—Agriculture

• Current Crises: Our Inner Vs Outer Landscapes

• Current Crises: On The Eagle’s Wing

• Current Crises: The Choice Is Ours


ENDNOTES

1. Chris Maser and Carol Pollio. Resolving Environmental Conflicts. Second Edition. CRC Press, Boca Raton, FL. (2011) 241 pp.

2. Why humans became naked is based on: (1) Shannon P. McPherron, Zeresenay Alemseged, Curtis W. Marean, and others. Evidence For Stone-Tool-Assisted Consumption of Animal Tissues Before 3.39 Million Years Ago At Dikika, Ethiopia. Nature, 466 (2010):857–860; (2) Nina G. Jablonski. The Naked Truth. Scientific American, 302 (2010):42-49; (3) Alan R. Rogers, David Iltis, and Stephen Wooding. Genetic Variation at the MC1R Locus and the Time since Loss of Human Body Hair. Current Anthropology, 45 (2004):105–108; and (4) Nina G. Jablonski and George Chaplin. Skin Deep. Scientific American, 287 (2002):74-81.

3. Aviv Bergmanand and Arturo Casadevall. Mammalian Endothermy Optimally Restricts Fungi and Metabolic Costs. mBio 1(5):e00212-10. doi:10.1128/mBio.00212-10.

4. The preceding discussion of heat stress is based on: (1) Steven C. Sherwood and Matthew Huber. An Adaptability Limit To Climate Change Due To Heat Stress. Proceedings of the National Academy of Sciences, 107 (2010):9552-9555; (2) A.B. Alley, J. Marotzke, W.D. Nordhaus, and others. Abrupt Climate Change. Science, 299 (2003):2005–2010; (3) R.Sair Kovats and Shakoor Hajat. Heat Stress and Public Health: A Critical Review. Annual Review of Public Health, 29 (2008):41–55; (4) Thomas R. Karl, Richard W. Knight, Kevin P. Gallo, and Thomas C. Peterson. A New Perspective On Recent Global Warming-Asymmetric Trends of Daily Maximum and Minimum Temperatures. Bulletin of the American Meteorological Society, 74 (1993):1007–1023; (5) A. P. Sokolov, P. H. Stone, C. E. Forest, and others. Probabilistic Forecast For Twenty-First-Century Climate Based On Uncertainties In Emissions (Without Policy) and Climate Parameters. Journal of Climate, 22 (2009):5175–5204; and (6) Anthony J. McMichael and Keith B. G. Dear. Climate Change: Heat, Health, and Longer Horizons. Proceedings of the National Academy of Sciences, 107 (2010):9483-9484.

5. Susan Solomon, Gian-Kasper Plattner, Reto Knutti, and Pierre Friedlingstein. Irreversible Climate Change Due To Carbon Dioxide Emissions. Proceedings of the National Academy of Sciences, 106 (2009):1704–1709.


Text © by Chris Maser 2012. All rights reserved.

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If you want to contact me, you can visit my website. If you wish, you can also read an article about what is important to me and/or you can listen to me give a presentation.



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