finding 2.3 : changing-tolerance-extreme-heat

An increase in population tolerance to extreme heat has been observed over time [Very High Confidence]. Changes in this tolerance have been associated with increased use of air conditioning, improved social responses, and/or physiological acclimatization, among other factors [Medium Confidence]. Expected future increases in this tolerance will reduce the projected increase in deaths from heat [Very Likely, Very High Confidence].

This finding is from chapter 2 of The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment.

Process for developing key messages: The chapter was developed through technical discussions of relevant evidence and expert deliberation by the report authors at several workshops, teleconferences, and email exchanges. The authors considered inputs and comments submitted by the public, the National Academies of Sciences, and Federal agencies. For additional information on the overall report process, see Appendices 23.

The content of this chapter was determined after reviewing the collected literature. The authors determined that there was substantial literature available to characterize both observed and projected mortality from elevated temperatures, with sufficient literature available to also characterize mortality from cold as well as cold-related hospitalizations and illness. Populations of concern were also considered to be a high priority for this chapter. As discussed in the chapter, there were limitations in terms of the state of the literature on understanding how future adaptation will influence climate-related changes in temperature-related mortality, addressing the impact of temperature on rural populations, and examining health-related endpoints other than mortality and morbidity.

Description of evidence base: The increasing tolerance of the U.S. population to extreme heat has been shown by a number of studies.a6714dce-b324-4324-a88e-d31d31fa2d95 ad5fb3ba-9924-4df9-a68f-1e94822f78f9 6b3cd0ec-1e3e-42e8-ad82-5c12ed7ab0e8 However, there is less confidence in attributing this increase in tolerance: increased prevalence and use of air conditioning, physiological adaptation, available green space, and improved social responses have all been proposed as explanatory factors. There have been some indications (Sheridan et al. 2009)9096905c-dc99-46c1-ac2c-2e5f8d58f8d9 that tolerance improvements in the United States might have plateaued, but Bobb et al. 2014 found continuing improvements through 2005.6b3cd0ec-1e3e-42e8-ad82-5c12ed7ab0e8

Several approaches to including adaptation have been used in temperature mortality projection studies. For example, two studies used an “analog city” approach, where the response of the population to future temperatures in a given city is assumed to be equal to that of a city with a hotter present-day climate.d4613be9-fc30-44b5-afdb-e6d6daf4b490 04f852ec-7b1e-4fd0-a517-283b25468694 Another approach is to assume that critical temperature thresholds change by some quantity over time.e60cb47e-4a48-4e92-a2d3-97516836e8f3 b10f189e-fbfa-44fc-b7b5-48966591f9ba A third approach is to calculate sensitivity to air conditioning prevalence in the present, and make assumptions about air conditioning in the future.a5c47ded-9ce3-4075-b4d4-c5c3ce9036cd In general, inclusion of adaptation limits the projected increase in deaths, sometimes modestly, other times dramatically. However, approaches used to account for adaptation may be optimistic. Historically, adaptive measures have occurred as a response to extreme events, and therefore could be expected to lag warming.315edcf2-107b-449d-b694-5d5a3c87ebbb 46f2571e-7661-4163-9178-bee1d153a827 While the increase in mortality projected in these studies is reduced, the studies generally found that mortality increases compared to present day even under optimistic adaptation assumptions.e60cb47e-4a48-4e92-a2d3-97516836e8f3 d4613be9-fc30-44b5-afdb-e6d6daf4b490 04f852ec-7b1e-4fd0-a517-283b25468694 a5c47ded-9ce3-4075-b4d4-c5c3ce9036cd A limit to adaptation may be seen in that even in cities with nearly 100% air conditioning penetration, heat deaths are observed today.

New information and remaining uncertainties: While studies have been published in recent years that include adaptation in sensitivity analyses,d4613be9-fc30-44b5-afdb-e6d6daf4b490 this remains a challenging area of research. Difficulties in attributing observed increases in tolerance make it challenging to project future changes in tolerance, whether due to autonomous adaptation by individuals or planned adjustments by governments. Extrapolation of acclimatization is limited as there must be an increase in temperature beyond which acclimatization will not be possible.

Assessment of confidence based on evidence: There is very high confidence that a decrease in sensitivity to heat events has occurred based on high agreement between studies, but only medium confidence that this decrease is due to some specific combination of air conditioning prevalence, physiological adaptation, presence of green space, and improved social responses because of the challenges involved in attribution. There is very high confidence that mortality due to heat will very likely be reduced compared to a no-adaptation scenario when adaptation is included, because all studies examined were in agreement with this conclusion, though the magnitude of this reduction is poorly constrained.

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