Future population exposure to US heat extremes

Extreme heat events are likely to become more frequent in the coming decades owing to climate change1,2. Exposure to extreme heat depends not only on changing climate, but also on changes in the size and spatial distribution of the human population. Here we provide a new projection of population exposure to extremeheat for the continentalUnited States that takes into account both of these factors. Using projections from a suite of regional climate models driven by global climate models and forced with the SRES A2 scenario3 and a spatially explicit population projection consistent with the socioeconomic assumptions of that scenario, we project changes in exposure into the latter half of the twenty-first century. We find that US population exposure to extreme heat increases fourto sixfold over observed levels in the late twentieth century, and that changes in population are as important as changes in climate in driving this outcome. Aggregate population growth, as well as redistribution of the population across largerUS regions, strongly a ects outcomes whereas smaller-scale spatial patterns of population change have smaller e ects. The relative importance of population and climate as drivers of exposure varies across regions of the country. Climate change risks are a function of both the nature of physical hazards related to climate and the vulnerability of society to those hazards4. Research has focused on characterizing potential changes in the frequency and magnitude of physical hazards, whereas possible changes in future vulnerability have received less attention. However, recognition of the importance of this dimension is growing as evidenced by the treatment of risk and vulnerability in the Intergovernmental Panel on Climate Change (IPCC) Special Report on Extremes4, the recent Working Group II report of the IPCC Fifth Assessment Report5, the third National Climate Assessment6, and the new set of socioeconomic scenarios in production for use in climate change research that explicitly recognize the role of vulnerability in determining climate change risk7. Vulnerability itself can be viewed as a function of the exposure and sensitivity of society to hazards and its capacity to adapt4. These three aspects of vulnerability will change over time, potentially having a substantial influence on the magnitude of the risk from extreme events. To better prioritize research and inform risk management strategies, it is important to integrate this influence with projected change in climate to estimate future risks, evaluate the relative importance of different drivers of risk, and quantify uncertainty and its different sources in potential outcomes. Extreme heat is responsible for more deaths in the United States than any other weather-related event8,9, and its frequency and intensity is expected to increase over this century10,11. The physical effects of extreme heat on human populations are well documented12–14, and certain demographic/socioeconomic factors heighten vulnerability to heat-related health problems12,14. Anticipating changes in exposure to future heat extremes is a key component of understanding future vulnerability and therefore to adequate planning and mitigation15. Most attempts to quantify future climate-driven changes in mortality lack consideration of explicit population scenarios16. In many cases constant population is assumed,which is not adequate for projecting future exposure or vulnerability16,17 as these outcomes are heavily influenced by demographic change. In the few existing studies considering spatial population dynamics it has been found that, for example, assumptions regarding internal migration patterns are a strong driver of future exposure/vulnerability and mortality17. Not surprisingly, the recently completed third National Climate Assessment identifies as a key research goal ‘understanding how climate uncertainties combine with socioeconomic and ecological uncertainties and improve ways to communicate the combined outcomes’18. Here, we focus on systematically quantifying the exposure component of vulnerability to extreme heat in the US as a function of both climate and population change. Our results represent a first step towards understanding how patterns of exposure emerge as a result of the interaction between changes in population structure and regional climate. Here we use projections of future climate change according to the Special Report on Emissions Scenarios (SRES) A2 scenario (see Supplementary Discussion 1) based on general circulation models (GCMs) downscaled to 50-km resolution using regional climate models (RCMs) as part of the North American Regional Climate Change Assessment Program (NARCCAP). NARCCAP includes 11 GCM–RCM combinations (see Supplementary Discussion 2), allowing us to address the uncertainty in spatial climate change outcomes.We combine thesewith a recent spatial population projection for the US (ref. 19) consistent with the A2 scenario (see Methods). There are many indices for measuring extreme heat, and it has been found that the best predictor of heat-related mortality for specific age groups, seasons and geographic regions can vary significantly20. However, averaged over larger population groups and regions, no single variable has significantly stronger predictive capabilities and alternative measures of heat extremes are highly correlated20. It has also been found that excess mortality related to extreme heat events can be effectively described as the independent effect of daily temperatures rather than as a function of multi-day heat waves21. Similarly, there are many approaches to quantifying exposure and vulnerability, and a number of studies have attempted to estimate/project changes in heat-related mortality that can be attributed to climate change at the city/regional22 andnational scales17.