Portsmouth ranked most vulnerable UK city to extreme heat as climate warms

Research combining data from Ordnance Survey (OS), the Met Office and 4 Earth Intelligence has produced a heat vulnerability index for all cities in Great Britain, revealing which urban centres suffer most from rising temperatures and which cope best.

Portsmouth, a compact city of 208,000 people on the south coast of England, is identified as the most vulnerable to extreme heat both today and in the future. The index examines factors that make urban areas particularly susceptible to overheating, such as building density, lack of green space, and materials that absorb and retain heat.

Alongside Portsmouth, the analysis highlights Leicester, Exeter, Bristol and Nottingham as cities likely to warm the most by the end of this century. The findings underscore the uneven impact of climate change across the country, with certain locations facing greater health and infrastructure risks from heatwaves.

The index was developed using climate projections and geographic data to assess both current heat exposure and future warming trends. No detailed breakdown of scoring methodology or specific temperature projections has been released, but the collaboration aims to inform local planning and adaptation efforts.

The research does not include cities in Northern Ireland or Scotland; Wales is part of Great Britain but its cities were not named in the initial findings. The index focuses on the urban heat island effect, where built-up areas become significantly warmer than surrounding rural areas due to human activities and infrastructure.

Why This Matters

Heat-related health risks – vulnerable populations in identified cities face increased heat stress, particularly the elderly and those with pre-existing conditions.
Infrastructure strain – rising temperatures can overwhelm power grids, transport systems and water supplies, requiring costly upgrades.
Planning urgency – local authorities need actionable data to prioritise cooling measures such as green roofs, tree planting and reflective materials.

Background

The urban heat island effect is a well-documented phenomenon where cities become warmer than surrounding countryside due to concrete, asphalt and buildings absorbing and re-emitting solar radiation. Climate projections indicate that UK summers will become hotter and heatwaves more frequent, amplifying this effect. Ordnance Survey, the national mapping agency, has previously collaborated with the Met Office on climate risk projects, but this heat vulnerability index represents a new effort to combine geospatial data with meteorological modelling. The research arrives as the UK government updates its national adaptation plan, though no specific policy links have been announced.

Key Perspectives

Local governments in vulnerable cities: Will likely use the index to justify spending on green infrastructure and to update emergency heat plans. They face competing priorities and budget constraints. Residents and community groups: May demand faster action on cooling measures and better public health warnings during heat events. Those in less vulnerable cities could feel less urgency. Critics/Sceptics: Might question the index's reliance on projection models with inherent uncertainty, or argue that adaptation funding should target existing deprivation rather than future risk. Some may see the focus on a few cities as insufficient for national preparedness.

What to Watch

Release of further details on the methodology and underlying temperature projections.
Any formal response from the UK government or affected city councils.
Summer heatwave records in the identified cities – actual impacts could accelerate or slow policy responses.

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