Sustainability Climate Change

Hotter nights could lead to a 60 percent rise in global mortality: study

“Locally, heat during the night should be taken into account when designing the future heatwave warning system, especially for vulnerable populations.”
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Story at a glance


  • Numerous studies have detailed the deleterious health effects posed by rising day-time temperatures resulting from climate change.

  • Now, new research underscores the extreme toll rising night-time heat could have on human health.

  • According to models, by the year 2100, the world could see an up to 60 percent spike in mortality thanks to climbing temperatures at night. 

As the United States continues to trudge through a sweltering summer and with a warm fall on the way, new research underscores the potentially deadly ramifications of rising heat at night.

Published in The Lancet Planetary Health, data from Japan, South Korea and China show hotter night-time temperatures could lead to a 60 percent spike in mortality rates around the world by the end of the century. 

Previous research on the deadly effects of rising heat has typically focused on excessive day-time temperatures, while “the risks of increasing temperature at night were frequently neglected,” explained study co-author Yuqiang Zhang of the University of North Carolina in a statement.

Night-time ambient heat can interrupt the normal physiology of sleep and subsequently lead to a host of complications including immune system damage, chronic illness, and systemic inflammation, authors said. In urban areas, these outcomes might be exacerbated due to the urban heat island effect.

Furthermore, data show that by the 2100s, “total populations exposed to night-time heat are projected to increase four to eight times compared with the 2010s across the northern hemisphere.” 


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Researchers measured hot night excess (HNE) in the three regions between 1980 and 2015 and modeled projections for the years 2016 through 2100. Different climate change scenarios were employed and measurements were controlled for the effects of daily mean temperatures. 

A total of 28 cities with varying climates were included in the models along with daily death records from local health agencies. 

By 2090, the average intensity of hot nights in these cities will nearly double from around 69 degrees fahrenheit to 103.5 degrees fahrenheit. Severity of night-time heat worsened even under a scenario where the goals of the Paris Agreement were met. Under the models, regions with the lowest average temperatures had the largest potential for warming.

“The occurrences of HNE are projected to occur more rapidly than the daily mean temperature changes,” Zhang continued. “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.”

Co-author Haidong Kan of Fudan University in China added that governments and local policymakers should take the findings into account to better prepare for the impending consequences of climate change.

However, because the data were collected from three countries, researchers urged caution when generalizing findings to wider populations. They are currently working to develop a more broad, global dataset. 

In the meantime, “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,”  Zhang said. “Stronger mitigation strategies, including global collaborations, should be considered to reduce future impacts of warming.”