Key Takeaways
- Portsmouth is currently the most heat‑vulnerable city in Great Britain and is projected to remain at the highest risk level through 2100.
- Leicester will experience the steepest rise in vulnerability, moving from 10th place today to 4th by the end of the century.
- Exeter, Nottingham, Bristol, and Brighton & Hove are also expected to join the top‑10 most vulnerable cities by 2100.
- The City of London has the strongest built‑environment heat retention, while Westminster ranks highly for both present and future risk.
- Tree cover, green space, and waterways are critical mitigators; cities with abundant natural surfaces (e.g., Milton Keynes) show the lowest vulnerability.
- The 2022 London heatwaves cost the economy roughly £1.5 bn, illustrating the financial stakes of urban heat.
- Ordnance Survey’s heat vulnerability index combines observed temperatures, urban form, and climate projections to guide planners in targeting heat refuges and emergency planning.
Introduction
A new analysis by Ordnance Survey (OS), in partnership with the Met Office and 4 Earth Intelligence, has mapped heat vulnerability across 71 British cities granted royal charter. The study reveals how urban heat islands—areas where concrete, asphalt, and buildings trap heat—are intensifying under climate change, placing millions of residents at greater risk of heat‑related health impacts and strain on infrastructure.
Methodology and Heat Vulnerability Index
The OS heat vulnerability index blends current observed temperature data with the physical characteristics of each city’s built environment (building density, road coverage, surface materials) and future climate projections. Scores range from 1 to 5, where 3–4 denote high vulnerability and 4–5 indicate severe vulnerability. This composite metric allows a city‑by‑city comparison of present risk and projected trajectories to 2040 and 2100.
Current Vulnerability: Portsmouth Tops the List
Portsmouth, a coastal city of roughly 208,000 inhabitants on England’s south coast, emerges as the most heat‑vulnerable city today. Its limited tree cover, sparse green spaces, and extensive hard surfaces—particularly in districts such as Somerstown, Portsea, Southsea, and Fratton—cause it to absorb and retain solar radiation more effectively than surrounding rural areas, yielding temperatures that can be up to 10 °C higher than the environs.
Projected Changes by 2040 and 2100
According to the index, all 71 cities will reach at least high heat vulnerability by 2040. By the turn of the century, nearly two‑thirds could fall into the severe category (score 4–5). Portsmouth is expected to stay at the highest vulnerability level throughout this period, while other cities will see their rankings shift dramatically as urban form and climate trends interact.
Leicester’s Rapid Rise in Rankings
Leicester is projected to show the most pronounced increase in vulnerability, climbing from 10th place in the present ranking to 4th by 2100. The city’s current moderate risk stems from a mix of residential and commercial development, but anticipated growth, coupled with insufficient green infrastructure, will amplify its heat‑island effect, pushing it into the upper echelons of at‑risk areas.
Other Cities Gaining Vulnerability: Exeter, Nottingham, Bristol, Brighton
Exeter and Nottingham are also forecast to enter the top‑10 most vulnerable cities by the end of the century, joining Bristol and Brighton & Hove. These cities share common traits: expanding urban footprints, limited vegetative cover, and a high proportion of impervious surfaces that impede nighttime cooling, thereby elevating their future heat‑stress profiles.
City of London and Westminster: Built‑Environment Heat Retention
The City of London records the highest built‑environment score in the analysis, reflecting its dense concentration of buildings, roads, and hard surfaces coupled with minimal green space, trees, and waterways. Consequently, it exhibits the least effective natural cooling of any city studied. Westminster ranks third for overall heat vulnerability today and second for both future periods, trailing only Portsmouth, due to similarly dense development and limited cooling landscapes.
Role of Green Infrastructure in Mitigation
Tree canopy, parks, and water bodies serve as natural cooling agents, reducing surface temperatures through shade and evapotranspiration. The research underscores that cities with abundant green infrastructure—such as Milton Keynes, which boasts 46 km² of natural surfaces versus nearly 40 km² of hard surfaces—experience markedly lower heat vulnerability. Expanding and preserving such features is presented as a key strategy for urban resilience.
Economic Costs of Recent Heatwaves
A study commissioned by the Mayor of London estimated that the 2022 London heatwaves imposed an economic burden of approximately £1.5 bn. This figure encompasses lost productivity, health‑care expenditures, educational disruption, heightened energy demand, and strains on transport and emergency services. The findings highlight the tangible financial implications of inadequate heat‑adaptation measures.
Resilient Cities: Milton Keynes as a Model
Milton Keynes ranks 69th out of 71 cities for heat vulnerability today, benefitting from the lowest built‑environment vulnerability in England and the most effective natural cooling among the surveyed cities. Its extensive network of green spaces, lakes, and tree‑lined avenues provides a benchmark for how thoughtful urban planning can mitigate heat‑island effects while accommodating growth.
Implications for Planners and Policy Makers
The OS heat vulnerability index offers a practical tool for local authorities, developers, and emergency planners. By pinpointing neighbourhoods that consistently overheat—often those with scarce greenery and high deprivation—authorities can prioritize the siting of heat refuges, target tree‑planting initiatives, and refine emergency response protocols ahead of extreme heat events. Integrating such data into long‑term spatial plans is essential for safeguarding public health and maintaining urban functionality in a warming climate.
Conclusion
The research paints a stark picture: without concerted efforts to expand green infrastructure and reduce impervious surfaces, many British cities—especially Portsmouth, Leicester, Exeter, Nottingham, Bristol, and Brighton & Hove—will face escalating heat risks that threaten health, productivity, and urban resilience. Conversely, cities like Milton Keynes demonstrate that strategic investment in natural cooling can markedly lower vulnerability. Leveraging detailed heat‑vulnerability assessments will be vital for shaping adaptive, equitable, and sustainable urban futures across Great Britain.