Heat waves are the most prominent cause of weather-related human mortality in the United States (Changnon et al. 1996). In northern U.S. cities, human mortality increases significantly on unusually hot and humid days (Bridger et al. 1976; Davis et al. 2002, 2003; Kalkstein and Davis 1989; Kalkstein and Greene 1997; Oechsli and Buechley 1970). Mortality increases are evident in total daily deaths as well as among the elderly subgroup (Applegate et al. 1981; Greenberg et al. 1983; Henschel et al. 1969; Jones et al. 1982; Kilbourne 1997; Kunst et al. 1993; Lye and Kamal 1977; Oechsli and Buechley 1970). Although a fraction of these deaths are directly attributable to heat, the majority are ascribed to causes of death not commonly considered to be weather related, such as circulatory and respiratory diseases (Bull and Morton 1978; Ellis et al. 1980; Keatinge et al. 1986; Larsen 1990a, 1990b). Increases in total and elderly mortality have also been associated with hot weather in Eurasia (Donaldson et al. 2003; Katsouyanni et al. 1993; Keatinge et al. 2000; Kunst et al. 1993; Laschewski and Jendritzky 2002; Nakai et al. 1999).
Atmospheric concentrations of human-produced greenhouse gases have increased significantly since the onset of the Industrial Revolution (Keeling and Whorf 1994). When the effects of the most important gases--carbon dioxide, methane, chlorofluorocarbons, ozone, and nitrous oxides--are combined, the current "effective" CO2 concentration of approximately 450 ppm is more than 50% higher than the earth's natural, preindustrial background level and represents a 30% increase since 1960 (Houghton et al. 1990, 1996, 2001). Evaluations of global surface temperature histories, after accounting for urban warming biases and other influences, indicate that the globe has warmed approximately 0.67°C since 1900 (Folland and Parker 1995; Jones 1994). Some scientists argue that this increase is directly attributable to increasing greenhouse gas levels (Arrhenius 1896; Hansen et al. 1998; Houghton et al. 2001; Manabe and Wetherald 1975). Furthermore, based upon scenarios of future increases in greenhouse gas emissions, climate models estimate a globally averaged temperature rise of 1.4-5.8°C between now and the year 2100 (Boer et al. 2000; Boville et al. 2001; Houghton et al. 2001; Mitchell and Johns 1997; Stouffer and Manabe 1999). In the United States, the air temperature has increased 1.0°C since 1964 (the first year in this analysis), and model projections suggest 3-5°C of warming by 2100 [National Assessment Synthesis Team (NAST) 2000].
Given the historic linkage between high temperatures and death, these climate model temperature projections have led scientists and public health officials to forecast significant increases in mortality from greenhouse warming in the United States in the early twenty-first century (Chestnut et al. 1998; Gaffen and Ross 1998; Kalkstein and Greene 1997; NAST 2000). The ultimate impact of climate change will depend upon the extent of biophysical adaptations and the implementation of effective and widely available countermeasures (Chestnut et al. 1998; Donaldson et al. 2003; Kalkstein and Greene 1997; Keatinge et al. 2000; McGeehin and Mirabelli 2001; Seretakis et al. 1997). During the past several decades, the U.S. populace has been confronted with an increase in the annual number of heat-stress events, particularly in urban and suburban areas (Gaffen and Ross 1998). Projections of longer, more intense heat waves, more isolated hot days, higher minimum temperatures, and higher dew point temperatures arising from human influences on climate suggest a continuation of this trend. However, most, if not all, of the forecasts of increasing mortality are based on steady-state weather-mortality models that implicitly assume that weather-mortality relationships have not varied significantly over time. In contrast, we hypothesize here that mortality associated with warm and humid days has systemically declined over time (Davis et al. 2002, 2003; Donaldson et al. 2003).
The main purpose of this study was to determine if annual heat-related mortality rates have changed over the available period of record. This was accomplished by examining death rates on days in which, historically, the combination of high temperatures and humidity is correlated with significantly elevated mortality rates. Here, we explore temporal changes in the mortality characteristics of metropolitan area residents as a collective. Our specific goal was not to isolate the impact of heat alone, but to examine if, and the extent to which, the populace has adapted to increasing heat and humidity.
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