Madrid’s urban centre has the most extreme urban heat island (UHI) “hot spot” of six major cities around the world, with temperatures 8.5°C hotter than rural surroundings, according to new research by global sustainable development consultancy, Arup.
Arup’s Urban Heat Snapshot is designed to help cities understand how heat is impacting them from one neighbourhood to another and what can be done to reduce temperatures.
Using AI and satellite images from space, Arup’s Urban Heat Snapshot mapped the most extreme hot spots in a 150km2 sample of the urban centres of a diverse range of cities: Cairo, London, Los Angeles, Madrid, Mumbai and New York. Arup used its digital analytics tool UHeat to understand the difference in air temperatures experienced from neighbourhood to neighbourhood, on the hottest day in each city in 2022. The survey also showed Mumbai had the second most severe hot spot at 7°C, while New York and London both saw hot spots of 4.5°C (see full table of results below). The report authors want to highlight how city design is driving up urban temperatures – with nature often pushed out, streets asphalted, and tall buildings made of steel and glass.
UHeat draws on an advanced climate model by the University of Reading in the UK and demonstrates how advanced digital tools can bring academic models to real-word scenarios to find the causes of the UHI effect. It can then rapidly model solutions to show how the strategic deployment of nature and other interventions can help cities reduce the impact of hot spots. In the majority of cities, the hottest spots had less than 6% vegetation cover, while the coolest spots in most cities had over 70% and were found almost entirely in parks, away from residential and commercial areas.
This contributed to massive temperature swings within cities, with Madrid’s built up downtown experiencing heat almost 8°C hotter than El Retiro Park a short distance away.
The authors are calling city leaders, urban designers and planners to better understand how their designs can mitigate hot spots, particularly for the most vulnerable communities.
Dima Zogheib, Nature Positive Design Lead at Arup, said: "We’ve inadvertently designed many of our cities to be hot. We’ve pushed out nature – concreted our streets, built high in steel and glass. And we’ve largely confined our green spaces to grand parks, away from where most people live. Ponds, lakes, trees, grasses, soils and other surfaces that allow water to permeate into the ground have to be seen as vital infrastructure, essential to helping us adapt to climate change. Our challenge as designers is to think creatively to strategically and equitably deploy nature throughout our cities. Today we have the advanced digital tools to help us rapidly pinpoint where investment in solutions can make the greatest impact.”
Cities are getting hotter due to climate change, with the number of cities exposed to extreme temperatures, 35°C and above, expected to triple by 2050.* The authors looked at the hottest day in each city in 2022, one of the hottest years on record. With six months left of 2023 it looks like many of these records will be surpassed. Extreme temperatures are proving lethal, with a recent study by the European Health Institute estimating that extreme heat killed more than 61,000 people in Europe alone in 2022. ** It is increasingly understood that poorer neighbourhoods are also more at risk to heat exposure, due to factors such as a lack of trees and a lack of air conditioning. ***
Worryingly, three of the cities studied experienced the worst UHI hot spots during the evening or night-time. Urban heat is a particular problem at night, due to materials like cement absorbing heat in the day then slowly releasing it when the sun goes down. This causes stress and health issues and acutely impacts vulnerable citizens – including children and the elderly.
Within the survey area in Madrid, severe UHI hot spots meant researchers found 500,000 children and elderly people living with evening UHI heat spikes of 7°C or more. In London’s urban centre, almost a quarter of a million elderly people and children saw heat spikes of 4°C compared to rural surroundings. A study has suggested that by 2100, the risk of death from excessively hot nights could grow six-fold compared to 2016. ****
Rapid heat modelling for cities
Arup’s Urban Heat Snapshot is one of the few international comparisons of the UHI effect on air temperatures during both day and night-time. This method is much closer to people’s actual experience than cruder satellite analysis of land surface temperatures, which can be skewed by the dramatic highs of heat-absorbing materials on roofs and roads.
Survey contributor, Dr Ting Sun, UCL, Institute for Risk and Disaster Reduction, said: “Modelling urban heat has been the reserve of the few, mainly those in academia. UHeat bridges this gap, drawing on the urban climate science and models developed by academia and combining them with increasing amount of city data available through remote sensing. It gives those on the ground shaping cities the tools they need to rapidly understand the impacts of their design on urban heat.”
The authors of the report recommend cities focus on:
1. Increasing tree canopy cover: Trees have proven to lower temperatures in cities and reduce heat-related mortality. As shown by a recent study in the Lancet of European cities, increasing city tree coverage to 30% could have prevented 2644 excess deaths.***** In Tanzania’s largest city, Dar es Salaam, UHeat has been used to evaluate the city’s future urban heat profile based on projected increases in population. This has helped show the potential of nature-based solutions to tackle the UHI effect, reducing temperatures across the city.
2. Creating more permeable surfaces: Permeable surfaces, such as bare or planted soil, tend to absorb less heat compared to impermeable surfaces like concrete or asphalt. Increasing permeable surfaces and allowing water to infiltrate into the ground will cool the surrounding environment. Sustainable Urban Drainage schemes such as Arup’s design for Greener Grangetown a residential Cardiff neighbourhood in the UK, are slowing down water runoff during heavy rainfalls, increasing areas of green space, and cooling neighbourhoods.
3. Using every space possible: More than half of the space in cities (including roofs and streets) is open space, providing a large canvas for building resilience. Measures can include greening building facades and roofs or using white paint to change the reflectiveness of surfaces and reduce the amount of heat absorbed from the sun. Released alongside the Greater London Authority's Climate Resilience Review, Arup produced the Roofs Designed to Cool report, assessing the benefits of mass roof retrofit, as part of the Mayor’s Climate Resilience Review. Using reflective and solar PV can help cool buildings themselves, reducing the air conditioning as well as capturing energy to help decarbonise its energy use.
4. Establishing cool islands: With cities set for heatwave conditions every summer we need to create a network of cooling spaces in cities for people to take refuge. For example, in London Arup worked with the Greater London Authority to map cool spaces - where locals could find opportunities to shelter during hot days in a bid to reduce risk to health from hot weather. Also, something as simple as bringing back drinking water fountains to cities could improve health for citizens, becoming the main access point to water during a drought.
5. Sparking behaviour change: People will also need to change the way they live in cities within the next decade. Hot countries around the world have been adjusting their lives to this for centuries, and it’s time to learn from them – incorporate siestas, re-consider office hours, introduce shop and restaurant closures over peak heat.
Will Cavendish, Global Digital Services Leader at Arup, said: “This survey shows that many more people in our major cities are experiencing worrying levels of heat. But now we have AI and advanced digital tools to help us rapidly understand the heat profile of our cities at a street level. When overlayed with social factors to understand need, this allows cities to be far more precise in building resilience against the effects of climate change. They are allowing designers to show decision makers the exact impact nature-based solutions will have on cooling cities and enhancing city resilience.”
* Around 200 million city-dwellers in over 350 cities live with summer temperature highs of over 35 degrees Celsius. The number of cities exposed to extreme temperatures will triple by 2050. C40 Cities – Heat Extremes
** Ballester, J., Quijal-Zamorano, M., Méndez Turrubiates, R.F. et al. Heat-related mortality in Europe during the summer of 2022. Nat Med 29, 1857–1866 (2023).
*** A study by Yale School of Forestry and Environmental Studies (T Chakraborty) of 25 cities around the world found that in most (72%) of cases, poorer neighbourhoods experience elevated heat exposure due to factors such as lower vegetation density.
**** Study published in the Lancet Planetary Heath Journal, The effects of night-time warming on mortality burden under future climate change scenarios: a modelling study.
***** Study published by The Lancet: Cooling cities through urban green infrastructure: a health impact assessment of European cities