20th century. Associated with that temperature rise have been observations that heat waves have become longer and more extreme and that cold spells have become shorter and milder. For example, the western Europe heat wave of 2003 was responsible for upwards of 70,000 deaths and was the warmest summer there in more than 600 years (Robine et al., 2008). No single event like that can be reliably attributed to climate change, but it is consistent with expectations for the future. Within the United States, hot days, hot nights, and heat waves have become more frequent in recent decades and were the leading cause of weather-related morbidity and mortality during 1970–2004 (USGCRP, 2009).

On an urban scale, the heat-island effect contributes to local temperature increase. For example, the urban heat island around Phoenix, Arizona, raises minimum nighttime temperatures by as much as 12.6°F (7°C) (Brazel et al., 2000). When increased ozone events occur simultaneously with heat waves, mortality can rise by 175% (Filleul, 2006). As extremely hot days tend to be associated with high pressure and stagnant air-circulation patterns, ground-level ozone, PM2.5, particulate sulfate, and organic carbon have been found to correlate strongly in summer months (NRC, 2008).

Measurements of rainfall indicate that moist regions of the globe are getting wetter and semiarid regions are becoming drier; this is consistent with an intensification of the hydrologic cycle. In situ and space-based precipitation observations indicate that both global precipitation and extreme rainfall events are increasing. Total runoff is increasing but shows substantial regional variability (cf. USGCRP, 2009). In the United States, the amount of precipitation falling in the heaviest 1% of rain events increased by 20% in the past century, and total precipitation by 7%. Over the past century, there was a 50% increase in the frequency of days with precipitation of more than 10 cm in the upper Midwest. Heavy rains can lead not only to flooding but to a greater incidence of sewage overflows, contaminated drinking water, and waterborne diseases, such as cryptosporidiosis and giardiasis. Rivers and lakes are freezing later and thawing earlier with serious implications for flooding. The manner in which increased temperature and decreased rainfall covary in the western United States has led to a 400% increase in western wildfires in recent decades (Westerling et al., 2006). Drought and possible changes in irrigation practices could induce more frequent windblown-dust storms, which constitutes an air-quality effect with potential public-health consequences.

Literature Regarding Projected Climate Change

Observations like those summarized above needed to be supplemented with models that project potential conditions. Such predictions are essential

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