Portsmouth tops heat vulnerability index for cities

Ordnance Survey has published a Heat Vulnerability Index ranking all 71 cities in Great Britain by their exposure to retained urban heat. The analysis was produced with the Met Office and 4 Earth Intelligence.

The index finds that 59 of the 71 cities, or 83%, already fall into the high heat vulnerability category. It projects that all 71 will be classed as highly vulnerable by 2040, while 45 cities, or 63%, will move into the severe category by the end of the century.

The assessment combines four postcode-level measures and rolls them up to city level: satellite readings of land surface temperatures, climate model projections, the density and form of the built environment, and the extent of natural cooling features such as tree cover, vegetation and water.

Urban heat islands form when buildings, roads and other hard surfaces absorb heat during the day and release it slowly overnight. Areas with dense development, taller buildings and limited green or blue space tend to stay warmer than nearby rural areas, especially during heatwaves.

Portsmouth ranks as the most heat-vulnerable city in Great Britain today and retains that position in the medium- and long-term projections. The City of Westminster rises to second place in future periods, while Plymouth remains in the top three throughout the forecast range.

Several cities climb the rankings over time. Leicester rises from 10th place today to fourth by the end of the century, while Exeter and Nottingham also enter the top 10 in the long-term view, alongside Bristol and Brighton and Hove.

The top 10 shifts as summer temperatures rise. Today, Portsmouth is followed by Carlisle, the City of Westminster, Newcastle upon Tyne and Plymouth. In the long term, the ranking is Portsmouth first, the City of Westminster second, Plymouth third, Leicester fourth and Exeter fifth.

London pressure

The analysis highlights inner London as one of the most heat-retentive parts of the country. The City of London records the highest built-environment score and the weakest natural cooling score in the study, while Westminster ranks second on both measures.

This reflects the concentration of buildings, roads and hard surfaces in central districts, and the relatively limited space for trees, open vegetation and water. In the overall heat vulnerability ranking, Westminster is third today and second in both future periods, while the City of London reaches the top 10 in the later scenarios.

External research cited in the analysis points to the health and economic cost of urban heat in the capital. A modelling study in The Lancet Planetary Health found that about 399 of 785 heat-related deaths in Greater London during the summer of 2018 were attributable to the urban heat island effect, with a social cost estimated at up to £987 million.

Separate work prepared for Heat Ready London estimated the total economic cost of the capital's 2022 heatwaves at £1.5 billion, including lost productivity, health effects, disruption to education, pressure on energy demand, transport impacts and strain on emergency services.

Why Portsmouth

Portsmouth's position at the top of the index reflects both its existing summer surface temperatures and the structure of its urban environment. It ranks third in Great Britain for built-environment vulnerability and 69th out of 71 for natural cooling, indicating a combination of dense development and limited vegetation or water-based cooling.

The city also ranks fifth for summer land surface temperature. Local assessments have already identified Somerstown, Portsea, Southsea and Fratton as areas particularly exposed to urban heat, partly because of extensive hard surfaces and low levels of vegetation.

Cooling contrast

At the other end of the ranking, Milton Keynes is listed as the 69th most heat-vulnerable city today, making it one of the least exposed in the current period. It has the lowest built-environment vulnerability in England and the strongest natural cooling score of any city assessed.

Milton Keynes has 46 square kilometres of natural surfaces, compared with almost 40 square kilometres of made surfaces. Even so, it is still projected to move from moderate to high heat vulnerability by 2040 and remain there through the end of the century.

The wider natural cooling ranking also places St Asaph, Durham, Dunfermline and Tyddewi among the best-performing cities. At the weakest end are Preston, London, Portsmouth, the City of Westminster and the City of London.

Nick Bolton, Chief Executive Officer at Ordnance Survey, said: "These findings matter because 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. The OS Heat Vulnerability Index shows that by 2040, every city in Great Britain is projected to reach high vulnerability, and by the turn of the century, almost two-thirds could reach the most severe level. Understanding how and where this vulnerability is changing depends on an accurate view of how the country is built, based on trusted authoritative data, which is critical for planners, developers and local authorities preparing for a warmer future."

The index scores cities on a scale of one to five, with scores from three to four classed as high vulnerability and from four to five classed as severe. The climate element of the study uses Met Office UKCP18 projections under the RCP 6.0 emissions scenario for 2040 to 2069 and 2070 to 2100.

Jonathan Hendry, Chief Executive Officer at 4 Earth Intelligence, said: "Cities are on the front lines of climate change, and while the challenge is global its impacts are often hyper-local. 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. By harnessing Earth observation data and artificial intelligence to bring it down to street level, 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."