Based on present-day sensitivity to heat, an increase of thousands to tens of thousands of premature heat-related deaths in the summer [Very Likely, High Confidence] and a decrease of premature cold-related deaths in the winter [Very Likely, Medium Confidence] are projected each year as a result of climate change by the end of the century. Future adaptation will very likely reduce these impacts (see Changing Tolerance to Extreme Heat Finding). The reduction in cold-related deaths is projected to be smaller than the increase in heat-related deaths in most regions [Likely, Medium 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 2–3.

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: An extensive literature examines projections of mortality due to increasing temperatures. In particular, nine studies were identified that provide heat mortality projections in the United States for at least 10% of the U.S. population.d4613be9-fc30-44b5-afdb-e6d6daf4b490 e805bfdc-c4c2-43a0-b2e5-5a66945c74e4 48d08f16-c2bb-4591-831d-22c262d869fb 6b3cd0ec-1e3e-42e8-ad82-5c12ed7ab0e8 14e7930d-648f-4a7a-a41d-2eb61b7f9894 fe80908b-c181-4da8-8163-5abaade99c7e 18e73954-8b13-4c4a-acd1-9687b8d811d2 638cccd2-9ae4-469e-a1d7-6123abec2780 49407e4c-351c-4240-9c91-50e36a83849d Each of these studies projected an increase in heat-related mortality due to projections of future warming, though several noted the potential modification effect of adaptation (discussed in Key Finding #3). In general, the magnitude of projected increases in annual premature deaths in these studies was in the hundreds to thousands by mid-century, and thousands to tens of thousands by the end of the century, when scaled to the total U.S. population. These conclusions are further supported by studies at the city, county, and state level.e60cb47e-4a48-4e92-a2d3-97516836e8f3 9e1170c5-e68e-4aab-bf70-33e4b69e46bf 315edcf2-107b-449d-b694-5d5a3c87ebbb b3760069-032c-4bb6-9d34-9771c08f0171 04f852ec-7b1e-4fd0-a517-283b25468694 97024533-f9d5-4ee9-abb6-56733744a4d0 ba31f764-b679-480d-adc9-a66cd2354c10 6168163c-2f61-4593-810e-fe389a8f7834 a5c47ded-9ce3-4075-b4d4-c5c3ce9036cd

The Third National Climate Assessment (2014 NCA) found that “While deaths and injuries related to cold events are projected to decline due to climate change, these reductions are not expected to compensate for the increase in heat-related deaths,”61fd6e32-63d0-4f5a-bbbb-f68262376a37 and studies published since that time have further supported this finding. Of those studies that examine both heat and cold at the national scale, only Barreca found that the reductions in cold deaths would more than compensate for the increase in heat deaths.d4613be9-fc30-44b5-afdb-e6d6daf4b490 e805bfdc-c4c2-43a0-b2e5-5a66945c74e4 48d08f16-c2bb-4591-831d-22c262d869fb 14e7930d-648f-4a7a-a41d-2eb61b7f9894 18e73954-8b13-4c4a-acd1-9687b8d811d2 Barreca’s study was novel in terms of its treatment of humidity, finding that weather that was both cold and dry, or both hot and humid, was associated with higher mortality. However, this treatment of humidity was not the cause of the difference with other studies, as leaving out humidity actually showed a greater benefit from future climate change. Instead, the author stated that the reduction in net deaths was a result of relying on counties with over 100,000 inhabitants, and that using a state-level model covering all U.S. deaths would lead to a prediction of an increase of 1.7% in mortality rates rather than a decrease of 0.1%. The finding by the majority of studies at a national scale that heat deaths will increase more than cold deaths will decrease is consistent with studies at smaller spatial scales.9e1170c5-e68e-4aab-bf70-33e4b69e46bf Moreover, several studies provide rationales for why heat mortality is expected to outpace cold mortality,bdea0759-701d-4183-9966-cee3ce977e08 d4613be9-fc30-44b5-afdb-e6d6daf4b490 e805bfdc-c4c2-43a0-b2e5-5a66945c74e4 and some studies suggest that cold mortality may not be responsive to warming.2cee671a-e17f-4e66-b37d-0c29a35f7210 de50864a-6264-4114-bef0-399a41bc44a8 fbceb8d9-2504-47a6-a3df-50e173f578c9 Barnett et al. (2012) showed that cold waves were not generally associated with an increase in deaths beyond the mortality already associated with cold weather, in contrast to heat waves.5b3848ca-bd86-4549-a937-dea708342827

New information and remaining uncertainties: The largest remaining uncertainties concern questions of future adaptation, which are discussed in Key Finding #3. A related uncertainty involves the link between the temperatures measured at weather stations and the temperatures experienced by individuals. As long as the relationship between the weather station and the microclimate or indoor/outdoor difference remains constant, this should not impair projections. However, as microclimates, building construction, or behavior change, the relationship between recorded weather station temperature and actual temperature exposure will change. This is related to, but broader than, the question of adaptation. Additionally, there are uncertainties regarding the non-linearities of heat response with increasing temperatures.

Assessment of confidence based on evidence: There is high confidence that heat deaths will very likely increase in the future compared to a future without climate change, based on high agreement and a large number of studies as well as consistency across scenarios and regions. Because there are fewer studies on winter mortality, and because studies exist that suggest that winter mortality is not strongly linked to temperatures, there is medium confidence that deaths due to extreme cold will very likely decrease. The majority of the studies that examine both heat and cold deaths find that the increase in heat deaths due to climate change will likely be larger than the decrease in cold deaths in most regions, but there are a limited number of such studies, leading to an assessment of medium confidence.

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