Days after an intense, record-breaking heat wave sweltered wide swaths of Europe, a group of scientists released a rapid analysis estimating the extent to which climate change might have amplified the heat wave’s death toll.
The World Weather Attribution study was the first of its kind to produce a rapid assessment of deaths linked to climate change from a heat wave, researchers said. They estimate that the influence of climate change may have tripled the death toll.
Records of actual observed deaths during this heat wave will not be available for months, so researchers used historical temperature data and established mortality trends to approximate the number of excess deaths expected to have occurred because of heat. The scientists examined 12 European cities, focusing on the hottest five-day stretch between June 23 and July 2 for each.
“These numbers represent real people who have lost their lives in the last days due to the extreme heat,” said Friederike Otto, a professor of climate science at the Centre for Environmental Policy at Imperial College London and one of the study’s authors.
The researchers used historical temperature data to determine how intense the heat wave would have been in those cities without global warming, and estimated how many deaths would have been expected to occur in that scenario. They used that information to determine how many additional deaths were caused by climate change.
The analysis found that in the 12 cities, 1,500 of 2,300 estimated heat deaths could be connected to climate change, compared with a toll of roughly 770 without its effects. The researchers noted that the cities in the study represented only a sliver of the total number of excess deaths believed to have occurred throughout the rest of Europe.
“Even without climate change, many places in Europe likely would have experienced a period of hot weather in the past couple of weeks, but climate change has made it significantly hotter than it would have been, which in turn makes it a lot more dangerous,” said Ben Clarke, a researcher at the Centre for Environmental Policy at Imperial College London and an author on the study.
Global warming, caused by the burning of fossil fuels, has heated the world about 1.3 degrees Celsius over preindustrial temperatures, according to the report. Temperatures experienced by most cities during this heat wave were between two to four degrees Celsius hotter than they would have been in a world without climate change.
In turning the analysis around quickly, the researchers said they hoped to draw the attention of the public and policymakers to the dangers of extreme heat.
“When people are experiencing the heat waves, and in the immediate aftermath, is when people talk about it — when people connect their experience with the scientific evidence we can provide,” Dr. Otto said. “If we sort of wade through peer review, and wait a year, then that experience is long gone.”
A separate recent study supports that aim. It found that when people attribute extreme weather events to climate change, they are more likely to support climate policies.
Premature deaths from extreme heat can be difficult to quantify, even after the death records are available. That’s because most heat-related deaths don’t list heat as the primary cause of death. Heat exposure exacerbates underlying health conditions like heart disease and leads to premature deaths.
“This is why heat waves are known as silent killers,” said Garyfallos Konstantinoudis, an author of the study and a lecturer at the Grantham Institute-Climate Change and the Environment at Imperial College London. “Most heat-related deaths occur in homes and hospitals out of public view and are rarely reported.”
The early study has limitations that could lead to an underestimation or overestimation of the final death toll, researchers said. Those limitations include the timing of the heat wave, which took place early in the season. Early-season heat waves can be more deadly, because people’s bodies have had less time to acclimate to heat. The data also can’t fully account for adaptation strategies like local heat warnings or air-conditioning use.
“We won’t know for some time what the ultimate answer is, but this gives us a solid indication of the magnitude of what happened,” said Kristie Ebi, a professor at the University of Washington Center for Health and the Global Environment, who was not involved in the study.
Dr. Ebi said that she was confident in the study’s findings and methodology but that she believed the number of excess heat related deaths it reports are most likely an underestimate.
“This information can be used to improve early warning and response systems,” she said, “to make sure that they are effective as the world faces these much hotter and more frequent heat waves.”
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