New index ranks Portsmouth as Britain’s city most vulnerable to extreme heat

Portsmouth leads a nationwide heat-risk ranking

Portsmouth is the British city most vulnerable to extreme heat now and in future projections, according to a new index described by the Guardian. The assessment covers 71 chartered cities in England, Scotland and Wales and combines observed temperature, the structure of the built environment and climate projections. Northern Ireland was not included, so the findings should be read as an assessment of Great Britain rather than the entire United Kingdom.

The index was developed using work from Ordnance Survey, the Met Office and geospatial-analysis company 4 Earth Intelligence. It projects that all 71 cities will reach high heat vulnerability by 2040 and that almost two-thirds could enter the most severe category by the end of the century. Portsmouth remains at the highest level, while Westminster, Plymouth, Leicester and Exeter form the projected top five at century’s end.

Urban design determines who can cool down

Cities retain more heat than surrounding countryside because concrete, asphalt and buildings absorb solar energy during the day and release it slowly overnight. Tree cover, parks and waterways provide cooling, while dense hard surfaces intensify exposure. In Portsmouth, areas with limited greenery overlap with neighbourhoods experiencing deprivation, indicating that physical heat risk and social vulnerability can reinforce one another.

Leicester shows the largest projected climb, moving from tenth place today to fourth by the end of the century. Exeter and Nottingham also enter the ten most vulnerable cities in the projections. By contrast, Milton Keynes ranks near the least vulnerable end of the present index, helped by extensive natural surfaces and a built environment that allows more effective cooling.

The planning consequences extend beyond health

Extreme heat affects hospitals, care homes and housing, but it also strains transport, power systems and emergency services. UK government research published in 2025 examined overheating in Manchester’s housing stock and barriers to adapting vulnerable buildings. Although that study does not validate every city ranking in the new index, it independently confirms that urban building design and housing conditions are central adaptation problems.

The next test is whether national and local authorities convert the index into investment decisions. Priorities include tree planting, shaded public space, cooling centres, reflective materials and protection for residents in poorly ventilated homes. Because the index maps relative vulnerability rather than guaranteeing a specific future temperature, planners will need to combine it with local health, housing and infrastructure data before allocating resources.