Portsmouth, Leicester, Exeter, Bristol and Nottingham are set to become some of the warmest places in Britain by the end of the century, as global heating turns our fine cities into giant radiators, according to new research. Yes, you read that right - radiators.

An analysis from Ordnance Survey (OS), in collaboration with the Met Office and data analysis firm 4 Earth Intelligence, has cooked up a heat vulnerability index for all cities in Great Britain. And the dubious honour of being the most vulnerable to extreme heat today and into the future goes to Portsmouth, a south-coast gem home to 208,000 people. Congratulations, Portsmouth - you're the toast of the thermal town.

The science is simple: concrete, asphalt, buildings and other hard surfaces absorb solar radiation during the day, then act like giant radiators at night, refusing to cool down as rural areas do. Cities can be up to 10C hotter than their surroundings. So, think of your favourite city as a space heater with a population problem.

Unsurprisingly, areas most at risk are those with the least tree cover, green space, and waterways. Who knew that covering everything in tarmac might have downsides?

The research, which analyzed 71 royal charter cities across England, Scotland, and Wales (sorry, Northern Ireland - you're not invited to the heat party), predicts that by 2040, all 71 cities will reach high heat vulnerability. By the turn of the century, nearly two-thirds could hit the most severe level. The unlucky forecast for the worst by 2100 includes Portsmouth, the City of Westminster, Plymouth, Leicester, and Exeter.

Nick Bolton, chief executive of Ordnance Survey, said: “Cities are where millions of people live, work and spend their everyday lives. Extreme and sustained heat can affect people’s health and wellbeing, how comfortable our homes and workplaces are, and put additional pressure on transport, energy and emergency services.” He added that understanding this vulnerability is critical for planners preparing for a warmer future. Because, let's face it, we're going to need a lot of planning (and maybe some shade).

Portsmouth is already the most heat-vulnerable city today and will stay at the highest level until the end of the century. Meanwhile, Leicester is projected to have the biggest rise in vulnerability rankings, climbing from 10th now to fourth by 2100. Exeter and Nottingham also rise into the top 10, joining Bristol and Brighton and Hove. Brighton and Hove - the seaside town that's about to become a hot tub.

Leaders in Portsmouth have identified areas like Somerstown, Portsea, Southsea, and Fratton as particularly exposed due to limited green areas and hard surfaces, and noted that these areas overlap with those of deprivation. Because of course the heat hits hardest where the money is least.

The City of London and the City of Westminster rank among the most heat-retentive urban environments. The City of London has the highest built-environment score and the least effective natural cooling, given its concentration of buildings, roads, and hard surfaces, with limited green space, trees, and water. Westminster ranks third overall today and second in both future periods, right behind Portsmouth.

A study commissioned by the mayor of London put the total economic cost of the 2022 London heatwaves at a cool £1.5bn, including productivity losses, health impacts, education disruption, energy demand, transport, and emergency services. That's a lot of money to spend on being hot.

The OS index combined current observed temperatures with structural characteristics and climate projections to produce a score from 1 to 5, where 3 to 4 is high and 4 to 5 is severe.

On the flip side, Milton Keynes is the city coping best, ranking 69th out of 71 for heat vulnerability today. It has the lowest built-environment vulnerability in England and the most effective natural cooling of any city, with 46 sq km of natural surfaces against nearly 40 sq km of hard made surfaces. Who knew the concrete cow capital would be a cool oasis?

Jonathan Hendry, chief executive of 4 Earth Intelligence, noted that exposure varies street by street within a single city. “The areas consistently subjected to higher temperatures than their surroundings are the ones that overheat first during a heatwave,” he said. “We’re mapping how to turn thermal threats into actionable blueprints for urban resilience, supporting decisions on where heat refuges are sited and how emergency responses are planned in advance rather than in the middle of climate events.”

So, as Britain sweats it out, remember: your city might be a radiator, but at least we're all in it together - some more than others.