Tamma A. Carleton, Amir Jina, Michael T. Delgado, Michael Greenstone, Trevor Houser, Solomon M. Hsiang, Andrew Hultgren, Robert E. Kopp, Kelly E. McCusker, Ishan B. Nath, James Rising, Ashwin Rode, Hee Kwon Seo, Arvid Viaene, Jiacan Yuan, Alice and Tianbo Zhang
Communities are already experiencing a changing climate as temperature extremes become a familiar trend around the globe. How much is tempera-ture to blame when hospital visits increase during heat waves and cold spells? What role do adaptations like indoor heating and cooling systems play in blunting these effects? And, at what cost? Empirical evidence of the risks extreme temperatures pose to human health is limited since fatalities often come from periods of heat or cold that worsen underlying conditions. Individual deaths are rarely attributed to temperature surges, so public health of-ficials and policymakers often invest less in addressing and responding to climate change. The answers to these questions would inform policymakers, city planners, business leaders, and a range of stakeholders who are preparing to mitigate and adapt as the climate becomes more unstable.
1. Using the largest data set ever compiled on subnational human mortality around the world, this study quantifies the relationship be-tween temperature and death rates across the globe and identifies the role of income and protective adaptations, like indoor heating and cooling systems, in safeguarding public health.
2. The authors use these data-driven results to project the future im-pact of climate change on mortality rates and the costs and benefits of adaptation measures that populations are likely to undertake. The study reveals the net effects of hot and cold temperatures on global health and the economy, dividing the world into 24,378 distinct regions and calculating impacts for each.
3. Hot days with average temperatures above 35°C/95°F prove histori-cally worse for global public health than cold days below -4°C/25°F. On average, a single hot day increases mortality rates by 4 deaths per 1 million people, while cold days increase the mortality rate by 3 deaths per 1 million people. But substantial differences exist between places, depending on how wealthy the population is and how warm the climate is.
4. As temperatures rise, the damages to society grow with death rates increasing most among today’s poorest populations. By 2099 under a scenario of continued high emissions growth (SSP3-RCP8.5), climate change increases death rates in low-income countries by 106.7 deaths per 100,000. Meanwhile, high-income countries are projected to see death rates decrease by 25.2 deaths per 100,000, while spending significantly to prevent more deaths. Overall, today’s rich countries pay nearly three times more than poor countries to adapt to rising temperatures and prevent additional deaths.
5. Previous experience living in hotter temperatures also leads to bet-ter outcomes. For example, Houston’s population fares better than that of Seattle, Washington, on a hot day because these two wealthy U.S. cities have differing levels of experience with extreme heat. Houston each year experiences at least eight days with a daily aver-age temperature above 85°F, while Seattle experiences less than one of these days each year on average.
6. Both income growth and protective investments to adapt to long-term climate change improve outcomes. In a future with continued high emissions growth, climate change’s impact on temperatures will cause an additional 73 deaths per 100,000 in 2100. This projec-tion accounts for adaptations to climate that populations are likely to make, given historical patterns of adaptation. The benefits of adapting—reducing the death rate 29% from an average of 104 per 100,000—outweigh the costs, which would be equivalent to an ad-ditional 11 deaths per 100,000 people.
7. The benefits of greenhouse gas reductions are large. Even mitiga-tion efforts that fall short of the long-term targets of the Paris Agreement would cut the projected mortality costs of warming. Under moderate emissions (SSP3-RCP4.5), those costs fall by about 84 percent at the end of the 21st century, relative to a scenario of continued high emissions.8. The new analysis calculates that the mortality cost to society of each additional ton of CO2 is $36.6 per ton (using a 2 percent annual discount rate and a valuation that takes into account the age of those affected) under a scenario of continued high emissions and $17.1 per ton under a moderate emissions scenario. In other words, it would be worth it for society to pay roughly $36.6 per ton of CO2 to avoid the mortality impact from climate change.
These results provide essential information for policymakers forming strategies to reduce the impacts of climate change because they capture the benefits, in terms of mortality, of reducing CO2 emissions. Further, the results provide estimates of the changes in mortality risk for 24,378 unique regions that together account for the entire world. Having this localized information provides key insights for policymakers at all levels of government to debate the importance and value of investing in adaptation measures. It is also important to note that the researchers’ estimate of the Partial Mortality Social Cost of Carbon suggests taking a second look at the U.S. government’s previ-ous best assessment of future climate damages.