In many mid-latitude locations it is recognized that heat is the most important weather-related killer – outpacing hurricanes, tornadoes, snow and ice, and lightning. In the US, about 1500 people are killed by heat during an average summer (Harvard Medical School 2005). During extreme heat events, such as the one that occurred in Europe in 2003, excess deaths were in the tens of thousands (Valleron and Mendil 2004). The scope of the problem is immense and large population centers around the world, from Shanghai to New Delhi to London to Toronto, are not immune (Tan et al. 2003).

The impact of a climate change could make matters worse. Several studies show that, if the climate changes as forecast by a number of climate models, the frequency of extreme heat events may double, or even triple, in many cities over the remainder of this century (Hayhoe et al. 2004b). This could lead to a dramatic increase in deaths from heat-related causes. It should be noted, however, that heat intensity is not the only major factor contributing to increased negative health responses. Probably more important is the variability of the weather. Clearly more people die from heat-related causes in cities like Philadelphia, Toronto and Chicago, than they do in Phoenix and Miami, where summer conditions are considerably warmer. This is due to the unexpected nature of extreme heat events at many mid-latitude locations, where benign weather is punctuated by heat events of great magnitude (WHO et al. 1996). Thus, if climate change brings about an increase in temperatures but a lower variability in summer weather (for example, if Philadelphia’s summer climate approaches Miami’s), heat-related mortality may not rise in a warmerworld. However, if variability stays high, as many climate models indicate, heat-related problems will likely increase in a warmer world.