Projected regional temperature increases, combined with the way cities amplify heat, will pose increased threats and costs to public health in southwestern cities, which are home to more than 90% of the region’s population. Disruptions to urban electricity and water supplies will exacerbate these health problems.

This finding is from chapter 20 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: A central component of the assessment process was the Southwest Regional Climate assessment workshop that was held August 1-4, 2011, in Denver, CO with more than 80 participants in a series of scoping presentations and workshops. The workshop began the process leading to a foundational Technical Input Report (TIR) report.17ad4429-1321-4e7c-9cd5-3554eb0c3b38 The TIR consists of nearly 800 pages organized into 20 chapters that were assembled by 122 authors representing a wide range of inputs, including governmental agencies, non-governmental organizations, tribes, and other entities. The report findings were described in a town hall meeting at the American Geophysical Union’s annual fall meeting in 2011, and feedback was collected and incorporated into the draft. The chapter author team engaged in multiple technical discussions through more than 15 biweekly teleconferences that permitted a careful review of the foundational TIR17ad4429-1321-4e7c-9cd5-3554eb0c3b38 and of approximately 125 additional technical inputs provided by the public, as well as the other published literature and professional judgment. The chapter author team then met at the University of Southern California on March 27-28, 2012, for expert deliberation of draft key messages by the authors. Each key message was defended before the entire author team prior to the key message being selected for inclusion. These discussions were supported by targeted consultation with additional experts by the lead author of each message, and they were based on criteria that help define “key vulnerabilities, which include magnitude, timing, persistence and reversibility, likelihood and confidence, potential for adaptation, distribution, and importance of the vulnerable system.”3277e83c-e374-4ed5-b0a2-0adadfaf118d

Description of evidence base: There is excellent agreement regarding the urban heat island effect and exacerbation of heat island temperatures by increases in regional temperatures caused by climate change. There is abundant evidence of urban heat island effect for some Southwest cities (for example, Sheridan et al.3a2a968a-aed1-49a5-baca-bd2a406f3481 e023de37-cf71-434d-82db-9d1349b28e89 cc220439-2490-486e-89c2-c2acfcc8d627), as well as several studies, some from outside the region, of the public health threats of urban heat to residents (for example, Ch. 9: Human Health,a5c47ded-9ce3-4075-b4d4-c5c3ce9036cd 22344c1d-cee2-4f9d-91c0-60ceb6e9ca57). Evidence includes observed urban heat island studies and modeling of future climates, including some climate change modeling studies for individual urban areas (for example, Phoenix and Los Angeles). There is wide agreement in Southwest states that increasing temperatures combined with projected population growth will stress urban water supplies and require continued water conservation and investment in new water supply options. There is substantial agreement that disruption to urban electricity may cause cascading impacts, such as loss of water, and that projected diminished supplies will pose challenges for urban cooling (for example, the need for supplemental irrigation for vegetation-based cooling). However, there are no studies on urban power disruption induced by climate change. With projected surface water losses, and increasing water demand due to increasing temperatures and population, water supply in Southwest cities will require greater conservation efforts and capital investment in new water supply sources.9efcee0b-03a0-4d58-8cc9-d83dace385b8 Several southwestern states, including California, New Mexico, and Colorado have begun to study climate impacts to water resources, including impacts in urban areas.16e78234-963a-4af1-8b6d-fc9c3734776d 2b9cf7c5-f966-42aa-b52f-82492bd43d2a The interdependence of infrastructure systems is well established, especially the dependence of systems on electricity and communications and control infrastructures, and the potential cascading effects of breakdowns in infrastructure systems.edfad9f8-de47-42f3-bf4e-e8041bc1fbc1 aa3b16fa-abb7-45b0-afd3-a20cf5ea53bd The concentration of infrastructures in urban areas adds to the vulnerability of urban populations to infrastructure breakdowns. This has been documented in descriptions for major power outages such as the Northeast power blackout of 2003, or the recent September 2011 San Diego blackout.db326164-e496-4d91-9e71-7b680407ff80 A few references point to the role of urban power outages in threatening public health due to loss of air conditioningf34ea0cd-8d4a-4725-b08f-d3bf687734d2 71d9445f-170b-46f7-afb6-78275b3bc1c4 and disruption to water supplies.db326164-e496-4d91-9e71-7b680407ff80

New information and remaining uncertainties: Key uncertainties include the intensity and spatial extent of drought and heat waves. Uncertainty is also associated with quantification of the impact of temperature and water availability on energy generation, transmission, distribution, and consumption – all of which have an impact on possible disruptions to urban electricity. Major disruptions are contingent on a lack of operator response and/or adaptive actions such as installation of adequate electricity-generating capacity to serve the expected enhanced peak electricity demand. Thus a further uncertainty is the extent to which adaptation actions are taken.

Assessment of confidence based on evidence: Assessment of confidence based on evidence The urban heat island effect is well demonstrated and hence projected climate-induced increases to heat will increase exposure to heat-related illness. Electricity disruptions are a key uncertain factor, and potential reductions in water supply not only may reduce hydropower generation, but also availability of water for cooling of thermal power plants. Based on the substantial evidence and the remaining uncertainties, confidence in each aspect of the key message is high.

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