Recent summers have sizzled. Newspapers have reported the tragic death of the poor and the aged on days when the mercury reached torrid levels. Prophets of doom forecast that rising temperatures in the next century portend a future of calamitous mortality. Scenes of men, women, and children collapsing on hot streets haunt our imaginations.

Happily the evidence refutes that dire scenario. First, however, let us review the documentation supporting the supposition that human mortality will rise with rising temperatures. Death rates during periods of very hot weather have jumped in certain cities, but above normal mortality has not been recorded during all hot spells or in all cities. Moreover, research concerned with "killer" heat waves has generally ignored or downplayed the reduction in fatalities that warmer winter months would bring.

In a 1991 paper, Laurence Kalkstein, one of the most respected and careful scholars in this field, finds that deaths are related to the length of the hot spell. In a later work (1992), he reports that heat spells early in the summer or quick rises in temperature trigger deaths; in other words unseasonal or rapid warming produces mortality. Yet his 1991 publication suggests that it takes an extended heat wave to raise the death rate. But if rapid warming causes deaths, then we should find that during heat spells most of the mortality occurs in the first day or so and then fatalities taper off rather than rise with the length of the warm spell.

Kalkstein also finds that a particular weather pattern -- characterized by high temperatures, strong south-east winds, moderate humidity, and relatively clear skies with little cloud cover -- is correlated with increased mortality in St. Louis. For other cities either no weather pattern was related to mortality or the patterns that correlated with extra deaths differed. Even in St. Louis, many of the days that exhibited the suspect weather showed no unusual number of fatalities. Moreover, very hot days, those with temperatures over 100, failed to show death rates higher than the rates on those days when the thermometer only made it to 95deg.. In fact, the number of recorded deaths in St. Louis during this particular weather pattern varied considerably more than during other periods, which reduces our confidence in the results.

Researchers analyzing hot days and deaths have not always found any relationship and even where extremes in weather and mortality are correlated the relationship is inconsistent. Cities with the highest average number of summer deaths are found in the Midwest or Northeast while those with the lowest number are in the South. Typically analysts have failed to find any relationship between excess mortality and temperature in southern cities, which experience the most heat. Other studies have found that people who move from a cold to a subtropical climate adjust within a very short period. Moreover, Kalkstein and others have reported without explanation that the "threshold" between temperatures that lead to excess deaths and those that have no effect varies significantly among the cities. In some, such as Los Angeles, San Francisco, Boston and Pittsburgh, the threshold was below 30deg.C, while in Phoenix and Las Vegas it exceeded 43deg.C.

Scholars have also reported contradictory and implausible results. Air pollution is apparently not correlated with premature deaths. Some studies have found that during hot spells mortality goes up sharply in females; other researchers have measured increased deaths among men. Blacks are apparently more susceptible in St. Louis; whites, in New York. The lack of agreement on the effects of weather and on premature deaths again raises suspicions about the robustness of the results.

Measurement error may also foul up daily figures. In 1995, for example, Chicago suffered through an extraordinarily hot July that was characterized by the press as an harbinger of global warming. The coroner reported a marked increase in deaths. What was very curious was that on Friday, Saturday, and Sunday, July 14, 15, and 16, the reported deaths were way below the normal of 78 per day -- only 14 people were reported to have died on Saturday -- but on the two next days, Monday and Tuesday, were well above normal. The previous record low deaths for any day in the last 30 years had been 46! Given that on Friday, July 14, a record temperature of 106 degrees was measured at Midway Airport, these numbers are remarkable and quite suspicious. Could it have been that most people in the coroner's office took the hot weekend off and counted bodies on Monday and Tuesday?

Researchers have attributed the absence of heat-related deaths in southern cities to acclimatization and the prevalence of housing that shields residents from high temperatures. In the North, the housing of the elderly and the poor is usually old and dilapidated. Most of these buildings will be torn down and replaced over the next hundred years, if not sooner. Should the climate warm, builders will move towards structures that protect the inhabitants from extreme heat, as housing in the South allegedly does now.

These findings may imply simply that out-of-the-ordinary high temperatures increase the mortality of those in a weakened state. Studies have found that those most likely to die during heat spells are elderly. Little attention has been paid to the question of whether excess deaths represented only premature mortality of a few days among the old or sick or whether the excess deaths shortened lives significantly. Studies examining excess deaths by months fail to find any positive correlation with high temperatures, indicating that any daily excess is offset by a reduction in fatalities over the next few days. In the south, where temperatures are routinely very high during the summer, people adjust, even the elderly. Consequently, if the climate becomes warmer, no excess deaths can be expected. Moreover, as I have shown in an earlier report, a warmer climate is conducive to lower deaths rather than higher.

Fear of killer heat waves appears to be exaggerated. If temperatures rise slowly over the next century, possibly by the 1deg. to 3.5deg.C currently predicted, people can become acclimated while housing can and, in the normal cycle, will be replaced. After all, half the housing stock in the United States has been built in the last twenty-five years. Consequently, if warming takes place, people and housing will adjust and even if extended warm spells occur, mortality should not rise sharply.

Sources:

Laurence S. Kalkstein. "A New Approach to Evaluate the Impact of Climate on Human Mortality," Environmental Health Perspectives, 96 (1991): 145-150.

Kalkstein and Robert E. Davis. "Weather and Human Mortality: An Evaluation of Demographic and Interregional Responses in the United States," Annals of the Association of American Geographers, 79(1) (1989): 44-64.

Kalkstein. "Impacts of Global Warming on Human Health: Heat Stress-Related Mortality," Chapter 26, Global Climate Change: Implications, Challenges and Mitigation Measures, eds., S. K. Majumdar, L.S. Kalkstein, B. Yarnal, E. W. Miller, and L. M. Rosenfeld, Easton, PA: The Pennsylvania Academy of Science (1992): 371-83.

Thomas Gale Moore. "Health and Amenity Effects of Global Warming," Hoover Institution, Working Papers in Economics E-96-1, January 1996.