Hot nights caused by climate change will likely send death toll soaring this century

CHAPEL HILL, N.C. — If you’ve been sweating through your bedsheets more often, you’re not alone. Nighttime temperatures all over the world have been excessively hot this summer, and this climate change-provoked trend shows no signs of cooling off. Even worse, scientists at the University of North Carolina, Chapel Hill predict that hot and sticky evenings will increase the mortality rate around the world by up to 60 percent by the end of the century.

Hot summer nights do more than just cause discomfort. Researchers from the UNC Gillings School of Global Public Health explain that ambient heat overnight can interrupt the normal physiology of sleep. Less sleep means a weakened immune system and a higher risk of numerous health issues such as heart disease, inflammation, mental health conditions, and inflammation.

According to the study’s calculations, the average intensity of “hot night events” will almost double by the year 2090, rising from 68.7℉ to 103.5℉ across 28 cities in East Asia.

This project is the first ever to estimate the impact of hotter nights on climate change-related mortality risk. All in all, study authors say their findings paint a worrying picture of the future; the burden of mortality may be significantly higher than originally estimated by average daily temperature increases.

“The risks of increasing temperature at night were frequently neglected,” says Yuqiang Zhang, PhD, a climate scientist in the Department of Environmental Sciences and Engineering at the Gillings School, in a university release. “However, in our study, we found that the occurrences of hot night excess (HNE) are projected to occur more rapidly than the daily mean temperature changes. The frequency and mean intensity of hot nights would increase more than 30% and 60% by the 2100s, respectively, compared with less than 20% increase for the daily mean temperature.”

Heat-related deaths likely to jump six-fold by 2100

This project was an international effort, co-authored by scientists from China, South Korea, Japan, Germany, and the United States. The team estimated mortality due to excess heat for 28 cities located in either Japan, China, or South Korea between 1980 and 2015. They then applied those estimates to two distinct climate change modeling scenarios that align with carbon-reduction scenarios enacted by the respective national governments.

Thanks to this model, study authors were able to estimate that risk of death due to excessively hot nights will increase by nearly six-fold between 2016 and 2100. That prediction is far greater than the mortality risk from daily average warming suggested by current climate change models.

“From our study, we highlight that, in assessing the disease burden due to non-optimum temperature, governments and local policymakers should consider the extra health impacts of the disproportional intra-day temperature variations. A more complete health risk assessment of future climate change can help policymakers for better resource allocation and priority setting,” explains corresponding study author Haidong Kan, PhD, a professor at Fudan University in China.

Colder regions are about to get a lot hotter

The study also notes that regional temperature differences accounted for many of the observed variances in nighttime temperature. Interestingly, projections show areas with the lowest average temperature were more likely to have the largest warming potential.

“To combat the health risk raised by the temperature increases from climate change, we should design efficient ways to help people adapt,” Dr. Zhang adds. “Locally, heat during the night should be taken into account when designing the future heatwave warning system, especially for vulnerable populations and low-income communities who may not be able to afford the additional expense of air conditioning. Also, stronger mitigation strategies, including global collaborations, should be considered to reduce future impacts of warming.”

In conclusion, since the study only included 28 cities across three countries, Dr. Zhang notes “extrapolation of these results to the whole East Asia region or other regions should be cautious. Currently, based on these findings, authors are trying to extend the analysis to a global dataset. Then we could have a global picture of the deadly nighttime heat on health under climate change scenarios.”

The study is published in The Lancet Planetary Health.

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