Portsmouth has been identified as the UK city most vulnerable to extreme heat, according to new research from the Ordnance Survey (OS). The findings come as the UK concludes its hottest summer on record and highlight the growing impact of climate change on urban environments.
The OS Heat Risk Index, which combines OS maps, satellite temperature data from 4 Earth Intelligence, and Met Office climate modeling, ranks 71 cities across England, Scotland, and Wales. Portsmouth, with its dense urban surfaces and limited green space, experiences an amplified 'urban heat island' effect, making it up to 10°C (18°F) hotter than surrounding rural areas.
As of today, 83 percent of British cities are classified as 'highly vulnerable' to urban heat, with the remainder deemed 'moderately vulnerable'. Projections indicate a significant worsening trend: by 2040, all cities are expected to fall into the high vulnerability category. By 2100, an estimated 63 percent of cities could face 'severe' vulnerability, the highest classification.
"These findings matter because cities are where millions of people live, work and spend their everyday lives," said Nick Bolton, CEO of the OS. "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."
Shifting Vulnerability Rankings
While Portsmouth is projected to remain at the top of the vulnerability list until the end of the century, other cities are expected to see significant shifts. By 2040, London's City of Westminster is predicted to rise to second place, and Plymouth will move from fifth to third. Leicester is forecast to experience the most dramatic rise, climbing from 10th place today to fourth by the end of the century.
In contrast, cities such as Milton Keynes, St Asaph, and Durham are noted for their better natural cooling capacities and are consequently less vulnerable.
The underlying causes of the urban heat island effect include the prevalence of heat-retaining materials like concrete and asphalt, the lack of cooling vegetation and water bodies, and the way tall buildings and narrow streets can trap hot air and block cooling winds. The dense urban cores of the City of London and the City of Westminster are identified as the most heat-retentive areas due to these factors.
Health and Economic Impacts
The consequences of extreme urban heat are significant. Research indicates that in the summer of 2018, approximately 399 out of 785 heat-related deaths in Greater London were specifically attributable to the urban heat island effect. A separate study estimated that the 2022 London heatwaves cost the city £1.5 billion, factoring in losses in productivity, health impacts, and disruptions to services.
Overall figures from the Office for National Statistics show over 40,000 deaths in England attributed to extreme heat between 1988 and 2025. In London alone, there were 7,675 excess deaths on the hottest days during that period. These figures do not include the estimated 2,877 deaths during the heatwaves in May and June of the current year.
"Cities are on the front lines of climate change, and while the challenge is global, its impacts are often hyper-local," stated Jonathan Hendry, CEO of 4 Earth Intelligence. "Exposure can vary street by street within a single city, and the areas consistently subjected to higher temperatures than their surroundings are the ones that overheat first during a heatwave."
Bolton added that the index helps identify where interventions to protect communities from rising temperatures could be most effective, emphasizing the need to protect existing green and blue spaces in cities facing the greatest exposure.
