Health

One of the most well-studied impacts of climate change is the effect of temperature and, in particular, extremely hot days on mortality. While it is difficult to tie particular natural disasters to climate change, heat waves are the events scientists can most easily and robustly attribute to man-made changes in the atmosphere. Statistical analyses and climate modeling indicate that the 2010 Russian heat wave, for example, was about five times more likely to have occurred in 2010 than it would have been in the 1960s (before much of the effects of climate change were felt). An analysis conducted after the 2003 European heat wave concluded that it was four times as likely as it would have been before the Industrial Revolution and that it was likely that the excess mortality attributed to the heat wave (~15,000 deaths in France alone) was caused by anthropogenic climate change. 

Historical estimates of the temperature-mortality relationship across societies can help shed light on whether resilience measures can mitigate the risk of heat-related mortality

The level of vulnerability of populations to heat stress will depend on the severity of the temperature extremes, as well as on society’s adaptive response, all of which must be measured at a very local scale —taking into account different incomes, climates, and levels of development. Historical estimates of the temperature-mortality relationship across societies can help shed light on whether and what type of resilience measures can mitigate the risk of heat-related mortality. For example, does economic development and the resulting penetration of air conditioning have the potential to reduce mortality risk? Studying the potential effects of future climate change can help the public health community assess where risks will be the most severe and mobilize resources in local communities to improve resilience.

Lab Findings

Using the largest data set ever compiled on subnational human mortality around the world, the Lab has quantified the relationship between temperature and death rates across the globe and identified the role of income and protective adaptations, like indoor heating and cooling systems, in safeguarding public health. The researchers use these data-driven results to project the future impact of climate change on mortality rates and the costs and benefits of adaptation measures that populations are likely to undertake. They determine 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.

The Lab finds that hot days with average temperatures above 35°C/95°F prove historically 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.

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.

Previous experience living in hotter temperatures also leads to better 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 average temperature above 85°F, while Seattle experiences less than one of these days each year on average.

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 projection 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 additional 11 deaths per 100,000 people.

The benefits of greenhouse gas reductions are large. Even mitigation 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 21^st century, relative to a scenario of continued high emissions.

The 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.