Ambient temperature in general, and systematic temperature changes in particular, have both direct and indirect impacts on human health. Direct effects include increases or decreases in heatstroke, hypothermia, and metabolic and physiological disorders. Indirect effects include increases or decreases in drought, famine, severe weather events, and disease.

The threat of possible severe climate change, including predictions of global warming from politicians, media leaders, and scientists, has aroused worldwide concern about the potential effects of such change on human health. The United Nations' Intergovernmental Panel on Climate Change (IPCC) reported in 2007 that multiple computer-model predictions were consistent with significant climate warming occurring by 2050 and potentially disastrous warming occurring by 2100 (Goklany, 2007). These predictions depended on various scenarios of increasing greenhouse gases (GHGs), especially carbon dioxide (CO2), in the atmosphere and were based on the anthropogenic hypothesis, which holds that the most important variable in climate predictions is the human contribution of atmosphericCO2 from burning fossil fuels. Less significant contributing factors include methane production by farm animals.

There is significant scientific dissent on the causation of climate change, since human historical records show temperature variation profoundly affecting agriculture and worldwide human activity for centuries before the Industrial Revolution. Furthermore, ice cores and other paleontological records show violent variations of temperature, CO2, and methane for hundreds of thousands of years before human civilization.

The IPCC computer models predict increases in average global surface temperatures of 1.0 to 1.9œ Celsius by 2050. These models calculate temperature rises of 2.0 to 4.5œ Celsius by 2100, depending on whether CO2 emissions stabilize downward at 5 gigatons per year or increase to as much as 28 gigatons per year by the end of the century.

Prior temperature records show that humans have been exposed to temperature shifts of similar magnitudes. The cooling that occurred from 1940 to 1970 was estimated at about 0.2œ Celsius, and the warming from 1970 to 1998 is estimated at 0.5œ to 0.7œ Celsius (Baer, 2009). The average surface temperature in the decade following the 1998 El Nino did not increase significantly, and physicists who study solar effects on climate have predicted possible cooling because of the decrease in solar activity (such as sunspots).

S. Fred Singer and Dennis Avery have compiled extensive reports on prior temperature fluctuations, with examples of past climate cycles showing increased temperatures of more than 1œ Celsius during the Medieval Warm Period and decreases of 0.3œ Celsius during the Little Ice Age. South African data from stalagmite temperature proxies indicate temperature increases of up to 4œ Celsius in the Medieval Warm Period.

As Earth's population continues to migrate from rural areas to more densely concentrated suburbs and cities, more people become exposed to urban heat islands. Cities are warmer than surrounding countryside, because traditional roofs and paving surfaces absorb more solar heat than do dirt and vegetation, and significant heat is generated by industry, power plants, residential heating, and air conditioning. Large cities have shown average temperature increases of as much as 3œ Celsius (Tokyo, 1876-2004) to 4œ Celsius (New York City, 1822- 2000). These data are relevant, because potential health effects may be evaluated by comparing the effects of these localized temperature changes to projected future temperature changes.

Scientists have been speculating about the health effects of climate change since the concept of global warming became widespread. In 1992 and 1995, IPCC members expressed concern that increases in the number and severity of heat waves could cause a rise in deaths. The 1992 report found that temperature increases were more prevalent in the winter and at night, diminishing the health effects of extreme cold weather; summer temperature increases have fallen, which diminishes deaths from heat waves. This could explain the IPCC's 1995 statement that global warming could result in fewer cold-related deaths.

In 1995, Thomas Gale Moore published the first of his pioneering efforts, "Why Global Warming Would Be Good for You," followed in 1998 by "Health and Amenity Effects of Global Warming." He estimated that a temperature increase of 2.5œ Celsius in the United States would cause a decrease of forty thousand deaths per year from respiratory and circulatory disease, based on U.S. mortality statistics as a function of monthly climate change.

In 1997, the Eurowinter Group published "Cold Exposure and Winter Mortality from Ischaemic Heart Disease, Cerebrovascular Diseases, Respiratory Diseases, and All Causes in Warm and Cold Regions of Europe (Baer, 2009)." This was a landmark study that elucidated the mechanisms of serious illness from cold, which are dominated by hemoconcentration, which increases blood viscosity ("sludging"). Hemoconcentration can cause death from blockage of vessels serving the heart and brain tissue, and it accounts for half of all excess cold-related mortality. The 1997 Eurowinter Group study was followed by "Heart Related Mortality in Warm and Cold Regions of Europe: Observational Study," which was published in the British Medical Journal in 2000. These two studies provided data on mortality rates as a function of mean daily temperature in Athens, Greece; London, England; and Helsinki, Finland, providing the most comprehensive collection of evidence that mortality decreases as temperature increases, over most of the current climate range in Europe.

In 2005, Robert E. Davis furnished a survey on climate change and human health, published in Shattered Consensus: The True State of Global Warming. He predicted that human adaptation "will be key in determining the ultimate impacts of climate change." He demonstrated that some adaptations are already taking place that effectively mitigate negative impacts of global warming. His data, for example, show that excess mortality due to heat waves in many U.S. cities dropped to essentially zero in the three decades following 1964. This decline in heat mortality was especially evident in Southern cities, where high heat and humidity are common, but also spread northward. This happy trend could be attributed to air conditioning, better health care, architectural changes, and public health measures such as shelters. The salutary result, however, is prevention of major death events that were previously associated with heat waves and diminution of negative effects of climate warming.

In 2006, A. J. McMichael and his colleagues published "Climate Change and Human Health: Present and Future Risks." This was an attempt at a comprehensive evaluation of the direct and indirect health risks associated with warming, including infections and vector-borne diseases such as malaria.

The evaluation assumes that the maximum daily mortality in higher-temperature periods will be equal to or greater than the maximum mortality in colder periods, resulting in heat-related deaths increasing far more than the lives saved by warming of the cold periods. This hypothesis does not stand up to previous data from the United States that showed that mortality in winter due to cardiac, vascular, and respiratory disease is seven times greater than summer (Goklany, 2007). This ratio is about 9 to 10 in Europe.

The most comprehensive data on daily mortality, from all causes, as a function of the day of the year, show a clear relationship, with maximum mortality in January and minimum mortality in the warmest months of July and August. These data strongly indicate that warming of average daily temperatures would cause a decrease in mortality in winter far greater than the slight increase of mortality from summer heat.

In early 2008, the British Department of Health released "Health Effects of Climate Change in the U.K., 2008," an update of previous reports from 2001-2002 edited by Sari Kovats. It used IPCC models that predicted an increase of mean annual temperatures in the United Kingdom between 2.5 and 3œ Celsius by 2100. They found that there was no increase in heat-related deaths from 1971-2002, despite warming in summers, suggesting that the British population is adapting to warmer conditions. Cold-related mortality fell by more than one-third in all regions. The overall trend in mortality for the warming (from 1971-2002) was beneficial. The report states, in summary, that "winter deaths will continue to decline as the climate warms."

References

1. Baer, Hans A., and Merrill Singer. Global Warming and the Political Ecology of Health: Emerging Crises and Systemic Solutions. Walnut Creek, Calif.: Left Coast Press, 2009.

2. Goklany, Indur M. The Improving State of the World. Washington, D.C.: Cato Institute, 2007.

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