In a set of graphics two years ago, the UK's official weather and climate service produced a "plausible" weather forecast for July 2050, based on the nation's climate projections.
At the time, the Met Office stressed that the graphics were "not [an] actual weather forecast".
But early this week, the "plausible" will become a reality — 28 years early — as England braces for an "absolutely unprecedented" heatwave and, according to experts, "an insight into the future" if global greenhouse gas emissions are not reduced to net zero.
"Today, the forecast for Tuesday is shockingly almost identical for large parts of the country," atmospheric scientist at New York's Columbia University, Dr Simon Lee, wrote in a now-viral Twitter thread.
"To clarify: I don't think you can interpret this as climate change occurring 'faster than anticipated'.
"Climate models have shown that 40C is possible in the UK in the current climate, just very rare. My point is that what is coming on Tuesday gives an insight into the future.
"In the present climate, 40C represents a new extreme, which is becoming more likely due to climate change. Both the 2050 and 2022 forecasts are showing extreme events, but 40C in the 2050 forecast is less extreme than the 40C in 2022."
Temperatures in the UK are forecast to run 10 to 15C warmer than normal early this week — with a record 40C expected for southeast England on Tuesday, caused by an area of high pressure that's extended over France, Spain and Portugal, where thousands of firefighters have been battling blazes.
The Met Office said "there is a 50 per cent chance we could see temperatures top 40C and 80 per cent we will see a new maximum temperature reached". The country's hottest temperature ever measured was previously 38.7C at the Cambridge Botanic Garden in 2019.
"We hoped we wouldn't get to this situation," the Met Office's climate attribution scientist, Dr Nikos Christidis, said in a statement.
"Climate change has already influenced the likelihood of temperature extremes in the UK. The chances of seeing 40C days in the UK could be as much as 10 times more likely in the current climate than under a natural climate unaffected by human influence."
Health officials fear that "hundreds or thousands" of people could die as a result of the extreme temperatures — with the conditions likened to a 2003 heatwave in France in which 14,000 mostly elderly people died — prompting the government to trigger the first ever national emergency heat red alert.
The Level 4 alert, issued by the UK Health Security Agency (UKHSA) for Monday and Tuesday, means "illness and death may occur among the fit and healthy, and not just in high-risk groups".
"I think that, assuming the weather forecasts are roughly correct, it's very likely that there will be hundreds of thousands of excess deaths from heat in the next few days," Open University emeritus professor of applied statistics, Professor Kevin McConway, told The Guardian.
"It's possible that, because there have been so many warnings about the coming high temperatures, people and businesses will be taking more precautions than usually happen in a heatwave, which could lower the numbers of excess deaths.
"I hope that happens, but I fear there will still be excess death on quite a scale."
Residents have been urged to treat the heat like a storm warning and consider changing their plans, while operations are being cancelled in parts of the National Health Service (NHS), some schools are closing early or shutting altogether, and Network Rail has advised people not to catch trains amid fears of buckled rails.
Chief executive of the NHS Confederation, Matthew Taylor, said the service would be under intense pressure over the coming days, "with severe bed shortages, ambulance services severely stretched and several health systems around the country having to declare critical incidents."
Public health officials are therefore urging people to stay indoors with the curtains closed, and to avoid caffeine, alcohol and physical exertion in the hottest parts of the day to alleviate strain on emergency services and hospitals.
Some roadside assistance companies are also expecting an extra 1000 daily breakdowns, with the risk of car engines overheating or running out of fuel.
Met Office chief executive Penny Endersby said it could "be difficult for people to make the best decisions" in "unprecedented severe weather events … because nothing in their life experience has led them to know what to expect".
Case in point: recent reports suggest that no more than five per cent of UK homes have air conditioning to help keep residents cool.
"Here in the UK, we're used to treating a hot spell as a chance to go and play in the sun," Ms Endersby said.
"This is not that sort of weather. Our lifestyles and infrastructure are not adapted to what is coming."
Professor Richard Betts, who leads on climate impacts at the University of Exeter and the Met Office Hadley Centre, told The Guardian the situation over the coming days will not be a one-off, with a potential "heat dome" set to develop over the Mediterranean in August that could also lead to very high temperatures being pushed north into the UK.
"We are already seeing more frequent, longer and hotter heatwaves," Betts said.
"We can confidently attribute this to human-caused climate change. We can expect this to keep happening until we reduce global greenhouse gas emission to net zero."