Hot days and hot nights have become more frequent in recent decades (Trenberth et al., 2007), and the frequency, intensity, and duration of heat waves are projected to increase in the decades ahead, especially under higher warming scenarios (CCSP, 2008a). By applying the magnitude of the 2003 European heat wave (see Box 12.1 in Chapter 12) to five major U.S. cities, Kalkstein et al. (2008) concluded that a heat wave of the same magnitude could increase excess heat-related deaths by more than five times the average. Projected excess deaths in New York City associated with such a heat wave, for example, would exceed the current average number of heat-related deaths nationwide each summer, with a death rate approaching that for all accidents.
There is also the potential, however, for warming temperatures to reduce exposure and health impacts associated with cold winter temperatures, although this potential is projected to vary by location (CCSP, 2008a). For example, research has shown that regions with milder winters actually have higher mortality rates during cold weather than regions with colder winters (Curriero et al., 2002; Davis et al., 2004). Seasonal variations in death rates in the United States are well documented, with more deaths occurring during winter than during summer months (Curriero et al., 2002; Macken-bach et al., 1992). However, mortality rates are influenced by a range of factors other than temperature, including housing characteristics and personal behaviors, which have not been extensively studied in the context of future climate projections. Thus, determining whether warming temperatures could alter winter temperature mortality relationships is complex and requires understanding all of the factors involved.
There have been several attempts to project future heat-related health impacts of climate change, and this is an active, albeit not large, area of current research. Figure 11.3 shows a schematic illustration of the expected impacts of warming temperatures and increased number of hot days on human health. Figure 11.4 shows a projection of total increases in heat-related deaths for a major U.S. city (Chicago) experiencing a