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Health Effects (Temperature - Hot vs. Cold: North America) -- Summary
What effect would a significant increase in temperature have on the health of North Americans?  In what follows, we address this question on the basis of what has been learned from studies that report pertinent real-world observations from various places throughout the continent.

Kloner et al. (1999) reviewed all 222,265 death certificates from Los Angeles County, California, which were issued from 1985 through 1996, where death was caused by coronary artery disease.  In doing so, they found, in their words, that "even in the relatively mild climate of southern California, there is a seasonal variability to coronary death, with rates in December and January 33% higher than in June through September."  This study thus provides further support for the growing body of evidence that indicates that cooler weather is considerably more conducive to the occurrence of death due to coronary artery disease than is warmer weather, even in a climate where it does not get very cold (see, for example, the many similar results that have been obtained at various locations in Europe).  Hence, it logically follows that any global warming that may occur in the future will likely reduce mortality due to coronary artery disease, which is a major cause of death the world over.

Goklany and Straja (2000) examined trends in United States death rates over the period 1979-97 due to excessive hot and cold weather.  Their work revealed there were no trends in deaths due to either extreme heat or cold in the entire population or, "more remarkably," as they say, in the older more susceptible age groups, i.e., those aged 65 and over, 75 and over, and 85 and over.  However, deaths due to extreme cold exceeded those due to extreme heat by 80% to 125%.  With respect to the absence of trends in death rates attributable to either extreme heat or cold, Goklany and Straja say this observation "suggests that adaptation and technological change may be just as important determinants of such trends as more obvious meteorological and demographic factors."  Then, again, it could also mean there has been no significant temperature trend in the United States between 1979 and 1997.  In any event, it is clear, for the United States at least, that extreme cold is a much more important cause of death than is extreme heat, which suggests that if a real warming ever were to occur, it would likely eliminate many more deaths due to cold than it would increase deaths due to heat.

Davis et al. (2002) determined changes in the impact of high temperatures on daily mortality rates over a period of four decades in six major metropolitan areas along a north-south transect in the eastern United States.  They found few significant weather-mortality relationships for any decade or demographic group in the three southernmost cities examined, where warmer weather is commonplace.  In the three northernmost cities, however, there were statistically significant decreases in population-adjusted mortality rates during hot and humid weather between 1964 and 1994.  Why?  In the words of the authors, "these statistically significant reductions in hot-weather mortality rates suggest that the populace in cities that were weather-sensitive in the 1960s and 1970s have become less impacted by extreme conditions over time because of improved medical care, increased access to air conditioning, and biophysical and infrastructural adaptations."  They further note that "this analysis counters the paradigm of increased heat-related mortality rates in the eastern US predicted to result from future climate warming."

Braga et al. (2002) "carried out time-series analyses in 12 U.S. cities to estimate both the acute effects and the lagged influence of weather on respiratory and cardiovascular disease (CVD) deaths."  The weather parameters they studied were temperature and humidity, while the cities they studied were Atlanta, Georgia; Birmingham, Alabama; Canton, Ohio; Chicago, Illinois; Colorado Springs, Colorado; Detroit, Michigan; Houston, Texas; Minneapolis-St. Paul, Minnesota; New Haven, Connecticut; Pittsburgh, Pennsylvania; and Seattle and Spokane, Washington, which were divided into two groups: hot (Atlanta, Birmingham and Houston) and cold (all the rest).  They report they could detect no clear evidence for a humidity effect on respiratory or CVD deaths.  In the hot cities, they also report that "neither hot nor cold temperatures had much effect on CVD or pneumonia deaths," although for the sub-categories of chronic obstructive pulmonary disease and myocardial infarction there were some lagged effects.  In the cold cities, on the other hand, they found that "both high and low temperatures were associated with increased CVD deaths," with the effect of cold temperatures persisting for days but the effect of high temperatures restricted to the day of the death or the day before.  Of particular interest was the finding that "for all CVD deaths the hot-day effect was five times smaller than the cold-day effect."  In addition, the hot-day effect included some "harvesting," where the authors "observed a deficit of deaths a few days later," which they say they "did not observe for the cold-day effect."  Another important finding was "the substantial [respiratory] mortality increase in cities with more variable temperature," which led them to suggest that "increased temperature variability is the most relevant change in climate for the direct effects of weather on respiratory mortality."

The driving force for Braga et al.'s study, as they describe it, was "the rapid buildup of greenhouse gases" that "is expected to increase both mean temperature and temperature variability around the world."  With respect to their concern about the expected increase in mean temperature, their findings clearly demonstrate that, in a warming world, the number of lives saved as a consequence of the alleviation of the cold-day killing effect likely will far exceed the number of lives lost as a consequence of the intensification of the hot-day killing effect.  With respect to their concern about weather becoming more variable and thereby leading to increased temperature related deaths, we note that this concern is based on a patently false assumption, at least in the United States.  Robeson (2002), for example, clearly demonstrated - from a 50-year study of daily temperatures at over 1,000 U.S. weather stations - that temperature variability declines with warming, and at a very substantial rate, so that this aspect of weather in a warmer world would lead to a great reduction in temperature related deaths at both the high and low ends of the temperature spectrum.  Consequently, as we have said so many times before in reviewing scientific papers describing studies of this nature: warmer is better, much better, than colder.

Davis et al. (2003) evaluated "annual excess mortality on days when apparent temperatures - an index that combines air temperature and humidity - exceeded a threshold value for 28 major metropolitan areas in the United States from 1964 through 1998."  In doing so, they learned that "for the 28-city average, there were 41.0 4.8 excess heat-related deaths per year (per standard million) in the 1960s and 1970s, 17.3 2.7 in the 1980s, and 10.5 2.0 in the 1990s."  In further analyzing these results together with various types of ancillary data, they concluded that "this systematic desensitization of the metropolitan populace to high heat and humidity over time can be attributed to a suite of technologic, infrastructural, and biophysical adaptations, including increased availability of air conditioning."  Consequently, because "all-causes mortality during heat stress events has declined despite increasingly stressful weather conditions in many urban and suburban areas," Davis et al. but state the obvious when saying "heat-related mortality in the United States seems to be largely preventable at present."  Indeed, the technology and infrastructure advancements made possible by the economic progress of the past few decades have more than compensated for the increasing heat stress that has been experienced during this period of what climate alarmists typically describe as "unprecedented" global warming.

In a final study of this same topic, Davis et al. (2004) examined the seasonality of mortality due to all causes using monthly data for 28 major US cities from 1964 to 1998, after which they calculated the consequences of a future 1C warming of the conglomerate of those cities.  At all locations studied, they report that "warmer months have significantly lower mortality rates than colder months."  Specifically, they calculate that "a uniform 1C warming results in a net mortality decline [our italics] of 2.65 deaths (per standard million) per metropolitan statistical area."  Since the annual death rate of about 9500 deaths (per standard million) is so much larger, however, the "death benefits" of the warming are extremely small, i.e., a reduction in the annual number of deaths of less than 0.03%, which also pales in comparison to the nearly 20% reduction in annual mortality that has occurred as a consequence of technological advancements experienced between the 1960s/70s and the 1990s.  On the other hand, the result is extremely important for the specific people included in that particular 0.03% of the population!  Nevertheless, the primary implication of Davis et al.'s findings, in their words, "is that the seasonal mortality pattern in US cities is largely independent of the climate and thus insensitive to climate fluctuations, including changes related to increasing greenhouse gases."  Hence, it would appear that we in the developed world should be concentrating our efforts on fostering the same technological advancements we have experienced in all other parts of the planet, if we are truly serious about improving the quality - and length - of people's lives everywhere.

In conclusion, North American studies of temperature effects on human health, which at this point are comprised solely of reports from the United States, tell essentially the same story as that told by studies from other continents: the net effect of an increase in temperature is a reduction in human death rate.  Likewise, the reduction in temperature variability that is often seen to accompany warming (see Temperature (Variability) in our Subject Index) also tends to mitigate against human mortality.  However, the controlled climates of the buildings within which most people in the United States both live and work probably provide the best protection of all against the various maladies that are enhanced by the vagaries of outdoor weather, including temperature.

Braga, A.L.F., Zanobetti, A. and Schwartz, J.  2002.  The effect of weather on respiratory and cardiovascular deaths in 12 U.S. cities.  Environmental Health Perspectives 110: 859-863.

Davis, R.E., Knappenberger, P.C., Michaels, P.J. and Novicoff, W.M.  2003.  Changing heat-related mortality in the United States.  Environmental Health Perspectives 111: 1712-1718.

Davis, R.E., Knappenberger, P.C., Michaels, P.J. and Novicoff, W.M.  2004.  Seasonality of climate-human mortality relationships in US cities and impacts of climate change.  Climate Research 26: 61-76.

Davis, R.E., Knappenberger, P.C., Novicoff, W.M. and Michaels, P.J.  2002.  Decadal changes in heat-related human mortality in the eastern United States.  Climate Research 22: 175-184.

Goklany, I.M. and Straja, S.R.  2000.  U.S. trends in crude death rates due to extreme heat and cold ascribed to weather, 1979-97.  Technology 7S: 165-173.

Kloner, R.A., Poole, W.K. and Perritt, R.L.  1999.  When throughout the year is coronary death most likely to occur? A 12-year population-based analysis of more than 220,000 cases.  Circulation 100: 1630-1634.

Robeson, S.M.  2002.  Relationships between mean and standard deviation of air temperature: implications for global warming.  Climate Research 22: 205-213.

Last updated 5 October 2005