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316 assessment of climate change in the southwest united statesFigure 15.2 Projected ozone response to climate in three urban areas of California, 2050.A recent EPA-funded study at the University of California, Berkeley evaluated the effects of variablesassociated with anticipated changes in climate and ozone precursor emissions for cities in California’sCentral Valley (Fresno and Sacramento) and San Francisco Bay Area. In the set of bars at left, expectedreductions (due to existing and projected control measures) in ozone precursor emissions in 2050 wouldreduce ozone levels, assuming no change in climate. In the middle set of bars, changes in climatevariables (such as higher temperatures) and expected increases in nitrogen oxide and hydrocarbonemissions lead to higher regional ozone concentrations. When these two effects are combined onthe right, the benefits of the emissions reductions are partially or completely offset by climate-relatedincreases in ozone, especially in the San Francisco Bay Area. Thus, climate change will make ozonestandards more difficult to attain and maintain, and will increase control costs. Adapted with permissionfrom American Geophysics Union (Steiner et al. 2006; California Air Resources Board, http://www.arb.ca.gov/).Heat extremesHeat stress is the leading weather-related cause of death in the United States (CDC 2006;Kalkstein and Sheridan 2007; Sherwood and Huber 2010). Based on death certificates, anestimated 400 deaths each year are directly attributed to heat-related causes (CDC 2006),with the largest number occurring in Arizona (CDC 2005). However, both heat mortalityand morbidity are believed to be significantly underreported (CDC 2006). Moreover,heat exposure can cause morbidity directly and also through exacerbation of preexistingchronic disease, particularly of the circulatory system (reviewed in Drechsler 2009).