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What's Your Temperature?

How a changing climate will affect health

   Almost daily and in every part of the world, new health hazards arise from modern technology. Some of these hazards make an immutable public impact… Others attract less attention because they lack the drama and are not obvious in their effects… Such is the case for the dangers posed by certain pollutants of air, water and food, which remain almost unnoticed despite their potential importance for public health… Hardly anything is known of the delayed effects of pollutants on human life, even though they probably constitute the most important threats to health in the long run.—Rene Jules Dubos

 
  When Rene Jules Dubos, a professor of tropical medicine at Harvard and Pulitzer Prize winner, wrote those words in 1965, the world had never heard of the terms “global warming” and “climate change.” Today, with "climate change" a prominent political, social and scientific issue, it behooves us to pay heed to the warning Dubos issued nearly 50 years ago. Future policy debates should incorporate the potential effects and future likelihood of the international health consequences of global climate change.
   When it comes to the health of individuals and societies, the relationship between man and nature has always been a tenuous one. Massive plagues and epidemics have devastated entire societies and have continually changed the course of history. Disease played a prominent role in the Peloponnesian War, striking down nearly a third of the population of Periclean Athens (including Pericles himself) during its war with Sparta in 429 B.C. In the Middle Ages, the Black Death (bubonic plague) killed off one third of the population of Europe. The 16th century Spanish conquest of Mexico by Cortes, which changed the face of Mesoamerica, may not have been possible had it not been for a smallpox epidemic that killed thousands of Aztec warriors during the battle in 1520 for the Aztec capital, Tenochtitlan. In the last century, an influenza epidemic at the end of World War I took as many as 30 million or more lives throughout the world in one year, far more than all the combat casualties of the actual war itself. Since the 1970s, AIDS has been an international scourge killing millions, and even today millions of people die every year of AIDS and other communicable diseases on the African and Asian subcontinents.
   Nor are infectious diseases the only natural disasters to have killed millions and radically altered societal patterns. The European Little Ice Age of the 13th and 14th century was devastating to the indigenous inhabitants of northern Europe and caused mass migrations across the continent. China has historically been afflicted with terrible river flooding that has killed millions over the centuries. Drought, a natural calamity in much of central Africa and Australia, has periodically decimated and displaced entire populations of those continents.
    Today, the world faces another potential threat—the adverse health consequences of climate change and global warming.  At the outset, it should be noted the climate variability that many claim will result in a warmer world in the next century is certainly a possibility, but far from a foregone conclusion. Despite the mantra that “the debate is over,” there remains significant controversy about whether global warming is truly a long-term trend. Further, if it is, the debate extends to whether that warming is the result of rising concentrations of atmospheric greenhouse gases due to the burning of fossil fuels and other human activities (and ultimately what measures should be taken).
   While that debate is beyond the scope of this article, it can be said there is general agreement that the Earth’s surface temperatures have been warming steadily for the past three decades (although indications are the trend has slowed or even reversed in the last decade). If the warming trend of the current generation does continue unabated in the next century, regardless of cause, it presents a number of serious potential health consequences. Anticipating these health risks will allow us to take action to prevent the catastrophic social, demographic and economic upheaval that could result worldwide.
  Because climate is such a complex phenomenon, it is difficult to predict precisely how climatic changes could affect human health. Moreover, all of the changes may not be adverse; some may even be beneficial, e.g. the health effects of milder winters in some areas might mitigate some of the effects of hotter summers in others. Assuming overall warmer temperatures worldwide, there will be associated changes in precipitation, humidity and wind patterns. We can anticipate the concomitant health effects as likely to fall into three categories—increases in extreme weather events, infectious diseases primarily due to water and insect transmission and changes in the ecosystem that affect food supply. Not surprisingly, the most profound effects of all of these would probably be seen in the world's poorer populations. This is living proof of the quote by the famous columnist and gambler Bat Masterson. Simply paraphrased he said, “Because the rich man gets ice in the summer and the poor man gets it in the winter there are those who argue things are breaking even for both.”

Extreme Weather Events
  Climate change is often misunderstood to mean simply warming of the environment. Weather variability, that is shifts in day-to-day conditions, is also a possible consequence of climate change. Even a small increase in mean temperatures in the coming decades, if accompanied by an increase in the variability of weather patterns, could result in a rise in the frequency of extremely hot days to dangerous levels. (Consider what would happen in Chicago if the average summer high temperature, currently 85 degrees, increased almost imperceptibly to 87, but the number of days at least five degrees higher than average doubled from the current 18. Instead of 18 days with temperatures above 90, there would now be 36 days with temperatures above 92. Dangerous heat waves would become a routine occurrence).
   An increased number of heat waves could pose a serious threat to many countries, especially those where populations are aging and moving to urban areas. Currently, roughly half the world’s population is located in cities, but by 2030 this figure is expected to be 60%. Because of roads, buildings and industry, large cities tend to be “heat islands” that can reach temperatures 10-12 degrees higher than the surrounding countryside (in Chicago, Lake Michigan modifies this effect). The Chicago heat wave of 1995 killed over 700 people, and a 2003 heat wave in Southern France may have killed as many as 15,000 people, with mortality occurring primarily in the elderly, the infirmed and those socially isolated. There is no evidence that either of these heat events was the result of global warming; both were almost certainly isolated weather events. Nevertheless, they illustrate the devastating potential for heat-related mortality in future extreme weather events.
   Besides heat-related mortality, climate change poses the threat of deaths from the increased frequency and severity of storms, especially hurricanes in the Atlantic, typhoons in the Pacific and cyclones in the Indian Ocean, as well as smaller-scale storms such as tornadoes in the American Midwest. Deaths from flooding, wind damage and waterborne disease are all potential consequences. Hurricane Katrina in 2006, a storm that was an isolated weather event and not the apparent consequence of global warming, did immense damage as a result of its strike over a populated land mass and the resultant storm surge.  Global predictive models are still not sophisticated enough to forecast the likelihood of similar future storms, but the threat, while uncertain, remains.

Infectious Diseases
  Even a mild increase in temperatures, especially in the tropics, will increase the incidence and seasonal transmission of various infectious diseases by extending the geographic range of vector hosts such as insects, ticks and watersnails. These disease carriers are the intermediate hosts for a number of serious conditions including malaria, Lyme disease, sleeping sickness, yellow fever, viral encephalitis and West African river blindness.
  Malaria is a case in point. Mathematical models have demonstrated that changes in rainfall patterns and temperature could allow the mosquitoes that transmit the disease to flourish and drastically increase the number of people exposed in endemic areas of Africa. Malaria was common in Europe in the 19th century and was still seen in southern Europe as recently as the 1950s. It is conceivable that, with climate change, the European continent could see a recurrence, especially if the mosquito vector demonstrates resistance to the insecticides currently used for control.
  After a period of decline in the middle of the last century, African sleeping sickness, trypanosomiasis, now kills 40,000 people in central Africa annually. A warming of less than 5 degrees (Fahrenheit) would permit the tsetse fly, the insect vector, to disperse southward. Were the fly able to extend its geographical boundary as far south as South Africa, the disease could threaten the large livestock reserves and population centers of that country.
  Another potential infectious disease threat is cholera, which can be rapidly fatal in its most severe forms and is currently found in Asia, Africa, South America, Central America and Mexico. There were several serious cholera outbreaks in the United States in the 19th century. The causative organism may be harbored in oceanic coastal waters and some believe that if sea surface temperatures were to increase, the bacterium would proliferate causing major cholera epidemics worldwide.

Food Supply
  Changes in precipitation patterns and temperature could have a profound effect on regional food yields and water supplies (imaginethe implications if climate change caused a dislocation of the abundant Midwestern American corn and soybean production several hundred miles north to southern Canada). Any shifts in food production would be unpredictable, but in some areas drought and heat would likely result in widespread malnutrition and contaminated water supplies. Diseases like tuberculosis and typhoid could become endemic. Refugees from affected areas would flood toward areas with more food, cleaner and more accessible water and better opportunities for employment. It is not hard to imagine population migration scenarios resulting in the increased spread of disease and even the possibility of armed conflicts.
  Nor would the problem be confined to land-based agriculture. Countries such as Japan that depend primarily on fish as their dietary staple might be affected by changes in fluctuations in ocean temperatures.  Changes in ocean currents and the warmth of the water could change the locations where fish reside. Moreover, there is some evidence these factors would also increase the uptake of pollutants like mercury in the fish that constitute a vital part of the food chain.
   There are a host of uncertainties over whether climate change and global warming will actually occur. Despite the most complicated mathematical models currently available, the future remains a mystery. But even if a mild degree of global warming should take place over the next generation, the possibility of the aforementioned effects on human health suggests certain strategies to mitigate or avoid worst-case scenarios. Every scientific disciple has a role. Epidemiologists should be concentrating on case surveillance for serious infectious diseases while the public health community strengthens public health defenses and adopts strategies such as mosquito control and netting campaigns. Meanwhile, medical scientists must develop new vaccines and prevention campaigns for serious infectious diseases. Plant biologists must work on developing new crops resistant to drought and disease while meteorologists work on more precise models to predict severe weather events such as hurricanes and heat waves. Local and national governments should be involved in issues such as water conservation and purification, as well as measures to protect their citizens in the event of health emergencies.
  Ironically, Rene Dubos was prescient in articulating the threat to human health from the delayed effects of pollutants long before the current debate on climate change. So perhaps it is not surprising that Dubos, who almost certainly knew nothing of the issue of global warming, is credited by many as the original author of one of the famous maxims that still applies to the debate—“Think globally, act locally.”

Published: April 04, 2009
Issue: 2009 Spring Green Issue