Scorchers like the one last week are still unusual, but global warming is making them more likely, and worse when they do strike.
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The heat that demolished records in Britain last week, bringing temperatures as high as 104.5 degrees Fahrenheit to a country unaccustomed to scorching summers, would have been “extremely unlikely” without the influence of human-caused climate change, a new scientific report issued Thursday has found.
Heat of last week’s intensity is still highly unusual for Britain, even at current levels of global warming, said Mariam Zachariah, a research associate at Imperial College London and lead author of the new report. The chances of seeing the daytime highs that some parts of the country recorded last week were 1-in-1,000 in any given year, she and her colleagues found.
Still, Dr. Zachariah said, those temperatures were at least 10 times as likely as they would have been in a world without greenhouse-gas emissions, and at least 3.6 degrees Fahrenheit hotter.
“It’s still a rare event today,” said Friederike Otto, a climate scientist at Imperial College London and another author of the report. “It would have been an extremely unlikely event without climate change.”
Severe heat has become more frequent and intense across most regions of the world, and scientists have little doubt that global warming is a key driver. As the burning of fossil fuels causes average global temperatures to rise, the range of possible temperatures shifts upward, too, making blistering highs more likely. This means every heat wave is now made worse, to some extent, by changes in planetary chemistry caused by greenhouse-gas emissions.
Before last week, the highest temperature Britain had ever recorded was 101.7 Fahrenheit, or 38.7 Celsius, a milestone set in Cambridge in July 2019. This month, as temperatures climbed, the nation’s weather authority, the Met Office, warned Britons to brace for new highs.
The mercury blew past the old record on the morning of July 19 in the village of Charlwood, Surrey, and kept rising. By day’s end, 46 weather stations, spanning most of the length of England, from London in the southeast to North Yorkshire in the northeast, had logged temperatures that met or exceeded the previous national record. Other stations beat their own local records by 5 to 7 degrees Fahrenheit.
In response, trains were slowed out of fear that the steel rails could buckle in the heat. Grass fires spread to London homes, shops and vehicles in what the city described as the Fire Brigade’s busiest day since World War II. More than 840 more people may have died in England and Wales than would have been typical, according to preliminary analysis using peer-reviewed methodology.
The report on last week’s heat was produced by World Weather Attribution, an alliance of climate scientists that specializes in rapid studies of extreme weather events to evaluate the degree to which global warming was behind them. Using computer simulations, the scientists compare the existing world, in which humans have spent more than a century adding heat-trapping gases to the atmosphere, to a world that might have been without that activity.
The group’s analysis of the heat in Britain has not yet been peer reviewed or published in an academic journal, but it relies on peer-reviewed methods.
Using similar techniques, the group found that the heat wave that broiled South Asia this spring had been 30 times as likely to occur because of planet-warming emissions.
Much of Western and Central Europe had a very hot start to the summer, driven by a high-pressure area that brought in warm air from North Africa. England is having its driest July in more than a century. When the soil is parched, energy from the sun goes toward heating the air instead of evaporating water on the ground, which can contribute to even hotter temperatures.
Scientists reported this month that heat waves in Europe have grown in frequency and intensity over the past four decades, at least partly because of changes in the jet stream.
For some scientists, Britain’s recent heat called to mind last summer’s deadly temperatures in the Pacific Northwest, which smashed records in some places by 7 degrees Fahrenheit or more. That heat was so off-the-charts that it led some climate researchers to wonder whether hot extremes were appearing more quickly than their scientific models were accounting for. It was the climate equivalent, said Erich Fischer of the Swiss university ETH Zurich, of an athlete beating the long jump record by 2 or 3 feet.
So far, though, the evidence suggests that such events are surprising but not unforeseeable using current models. Dr. Fischer led a study last year that showed that global warming, with its seemingly small increases in average temperatures, was also raising the likelihood that heat records would be shattered by big margins.
The question — as with floods, droughts and other extremes — is whether policymakers will use this knowledge to start preparing better in advance.
“There are conditions which usually turn these hazards into disasters, and these conditions are human made,” said Emmanuel Raju, an associate professor of public health at the University of Copenhagen and another author of the report on the heat in Britain. These conditions include poor planning and lack of attention to vulnerable groups such as homeless people, Dr. Raju said.
Vikki Thompson, a climate scientist at the University of Bristol, led a different recent study that found that while heat extremes were growing more frequent worldwide in recent decades, most of this could still be explained by higher average temperatures caused by climate change. “They are increasing in intensity, just not any faster than the mean,” Dr. Thompson said.
Yet even this pace of increase is taxing countries’ ability to cope. Britain’s rail system was engineered to run safely only up to 80.6 degrees Fahrenheit. Most homes were designed to retain heat during freezing winters. Many Britons still see hot weather as welcome relief from the cold and damp.
In Britain, “people are still not taking it quite as seriously as maybe they will next time,” Dr. Thompson said. “A heat wave is, by most people, thought of as something great to come along. They want some heat.”
“But when it’s 40 degrees,” or 104 Fahrenheit, she said, “that starts to change.”