A person shelters in the shade of the Bank of England during a heat wave in London on July 18. (Jose Sarmento Matos/Bloomberg News)

LONDON — Has it ever, in human history, been this hot in the British Isles? Maybe not.

If you want to mark an unnatural, scary, real-world data point for climate change, it is here in Britain, right now, which saw its hottest day on record Tuesday. Temperatures in six locations reached 40 Celsius or higher, with London Heathrow and St. James Park hitting 40.2 Celsius — or 104.3 Fahrenheit. It’s an extreme-weather episode, a freak peak heat, not seen since modern record keeping began a century and a half ago.

And probably not since weather observation got serious here in 1659. And maybe far longer.

Hitting 40C, for British climate scientists, is a kind of a unicorn event that had appeared in their models but until recently seemed almost unbelievable and unattainable this soon.

Cairo? Karachi? Phoenix? They are world-beating furnaces.

But London? The high-latitude city — with its recorded history dating back to the Romans — had probably never before experienced temperatures such as those on Tuesday.

Surely no Britons alive now — or their Britain-based great- or great-great-grandparents — had felt 40C without traveling abroad. Queen Victoria, William Shakespeare, Henry VIII? They probably never faced down a 40C day within the British Isles.

This nation was not built to withstand such heat. Its homes, workplaces, roads, rails, hospitals and infrastructure were constructed for temperate weather — Shakespeare’s “other Eden, demi-paradise” — not this inferno.

Britain has some of the most extensive weather records in the world, logged via diaries, observation and instruments as far back as the Age of Enlightenment, including daily records archived since the 1770s and monthly maximums and minimums dating back to the 1660s.

Until Tuesday, the highest official temperature was 38.7C (101.7F), recorded at the Cambridge Botanic Garden on July 25, 2019. The Met Office reported that at least 34 observation sites across the country topped that previous maximum on Tuesday.

Almost all the highest recorded temperatures have occurred in recent years.

“We are absolutely confident we have not recorded a 40C day going back to the mid-1850s,” Mark McCarthy, manager of the National Climate Information Center for the Met Office, told The Washington Post, referring to the beginning of the weather service’s instrument-measured temperature records.

Alexander Farnsworth, a paleoclimatologist at the University of Bristol, was willing to travel back further in time. “There is no direct evidence that the U.K. has exceeded 40C in the past 6,000 or so years,” he told The Post.

That would be back to the middle Holocene.

With caveats, Farnsworth warned.

To go deep into prehistory, before instrument data, scientists must rely on proxies that tell them average temperatures over long periods of time — looking at lake and marine sediments, ice cores, corals, glaciation, bugs in bogs, tree rings and such, to estimate past climate.

Over the past 2,000 years, it did get warmer in Britain during the Medieval Warm Period — between 750 and 1350 — but probably not as hot as the late 20th and early 21st centuries, most scientists say.

The medieval Domesday Book, completed in 1086 as a kind of census, tallied 45 vineyards in Britain, as far north as York — so it was warm enough to grow grape vines, a tradition brought to the island by the ancient Romans.

Then there was the Little Ice Age, from 1300 to 1850, when the Northern Hemisphere grew colder again. This warming and cooling was not caused by human emissions of greenhouse gases, as today, but by the subtle tilt and wobble of the planet as it faced the sun.

The Intergovernmental Panel on Climate Change, representing the authoritative consensus of world scientists, reported in 2021 that overall, and on average, Earth is now warmer than it has been in 125,000 years.

Some experts in paleoclimate studies say it’s possible that to top 21st-century heat in Britain, you would have to go back to the Miocene Climatic Optimum, about 15 million years ago, when the world looked quite unlike it does today. Back then, the continents were bumping around. There were different seas and mountain ranges. There were mammals but no humans.

Myles Allen, a professor of geoscience at the University of Oxford, suggested caution. He said it was clear that, from the 1850s onward, there had never been a day with 40C. But the further one looks back in time, the fuzzier the picture may be.

One remarkable thing, Allen said, is how accurate climate models have become — both at forecasting the future and looking backward in time.


A horse-mounted guard shelters from the sun in London on July 18. (Alberto Pezzali/AP)

Researchers at the Met Office have reported that in the “natural climate” of the preindustrial world, there might be one day in every 7,000 years that Britain could face 40C.

Today, the likelihood is once every 100 to 300 years — and growing. According to the models, a 40C day could happen once every 15 years by 2100 if countries meet their carbon emission promises — or once every three or four years if they continue to emit as much pollution as they do today.

Simon Lee, an atmospheric scientist at Columbia University, who was born and raised in North Yorkshire, England, wrote on his blog that the idea of 40C was a “seemingly unthinkable temperature for a country with an aging population which does not have widespread residential air conditioning.”

But “everything changed” on June 30, he wrote, with the publication of a Global Ensemble Forecast System model dotted with 40C across southeastern England. “Given that the UK’s previous hottest days had only seen 38°C exceeded very locally, this was unlike anything anyone had ever seen before.” Scientists were initially skeptical. No more.

Hannah Cloke, a natural hazards researcher at the University of Reading, told The Post: “We thought the models were wrong,” but today we are “sitting in the middle of a changing climate.”

“It’s unprecedented,” she said, this kind of forecast, “where we might see and feel something we’ve never experienced here before.”