Global cities grapple with how to cool ‘urban heat islands’

On the streets of Paris, thousands of trees are being planted, the city’s distinctive grey zinc rooftops are getting a makeover and workers are installing drinking fountains that release a cooling mist.

The changes are all part of the French capital’s “fight against heatwaves”, says First Deputy Mayor Patrick Bloche. “Heat, increasingly, is a problem for Paris.”

Paris is one of many cities around the world grappling with how to adapt and design urban areas as climate change drives more frequent and intense heatwaves, putting lives at risk and hitting economic output.

The world’s 20 most populous capital cities have experienced an overall 52 per cent increase in the number of days reaching 35C over the past three decades, according to the International Institute for Environment and Development.

As a result, heat stress and deaths linked to heat are increasing — and are predicted to rise still further. A study published in the paper Nature Medicine in January said that temperature-related deaths in European cities could increase by up to 50 per cent by the end of the century in the absence of climate change-mitigation policies, with urban areas across southern Europe worst affected.

Dimple Rana, an urban heat expert at Arup, the engineering and planning group, says the way cities are designed and built makes the situation worse. Concrete and asphalt absorbs heat, helping to drive the so-called “urban heat island” effect, where towns are hotter than surrounding areas. Even within cities, temperatures can vary by several degrees, depending on the proximity of cooling sources such as trees, soils and water.

Scientists at the European Commission’s Joint Research Centre found in 2022 that surface temperatures in cities were sometimes up to 10-15C higher than in their rural surroundings.

Arup, meanwhile, previously found that Madrid’s urban centre had air temperatures 8.5C hotter than rural surroundings, while the figure was 7C in Mumbai and 4.5C in New York and London.

“We are building with concrete, we are building high, which traps in heat during the day,” Rana says. “We are removing green spaces, water . . . The way we design transport networks, the way we are increasing air conditioning — all of that adds more heat. That means it gets hot during the day, all of that heat gets trapped and then it stays hot at night.” 

Mark Watts, executive director of C40 Cities, a global network of cities with a focus on climate change, says officials are increasingly concerned about the impact of heat on residents, infrastructure and economies. 

“When I talk to [C40 member mayors] and ask them what the thing they are most worried about now, heat would be the top answer. And that absolutely wouldn’t have been the case two years ago,” he says. “Heat and humidity is making it difficult for economies to function.”

Extreme heat can leave infrastructure such as trains struggling to function, while productivity among workers also falls. A study by the International Labour Organization, the UN agency for workers, projected that by 2030 the equivalent of more than 2 per cent of total working hours worldwide would be lost every year, because it is too hot to work or workers have to do so at a slower pace.

In Paris, where air-conditioning is uncommon in homes but temperatures exceeded 42C in 2019, Bloche says the city is trying a host of measures to protect residents. In schools, concrete and asphalt is being replaced with planting, helping to bring temperatures down by 2-3C compared with surrounding areas, says Bloche. Any new buildings will need to be much more heat resistant, with construction allowed only on the sites of existing developments as the city attempts to protect green spaces, he adds.

Existing buildings are also getting an upgrade. “Paris is famous for its zinc rooftops,” Bloche says — but these are being adapted too. Some will become “green roofs” with planting, helping reduce the heat absorption. Thermal insulation is being used for others, to prevent the rooms underneath overheating.

Cities around the world are taking similar steps. In Phoenix, Arizona, which experienced 113 consecutive days with temperatures of 100F (37.8C) or higher in 2024, the city has used “cool pavements” to lower temperatures. The pavements are painted with a coating which reflects sunlight, reducing the urban heat island effect and lowering temperatures by up to 6.6C at the hottest time of the day, according to C40 Cities.

Both Barcelona and Buenos Aires have rolled out climate shelters, where people can access cooler spaces in a heatwave.

Even London is gearing up for hotter temperatures. In the City of London, the UK’s historic financial district, green corridors are being introduced, where plants and trees are used to create cooler walking routes. In some cases, the planting has reduced air temperatures between 3-8C during heatwaves, according to officials.

Others are pushing for cities to embrace “cool roofs”, painted white. Researchers at University College London and the University of Exeter estimated last year that as many as 249 lives could have been saved during the hot summer of 2018 had London widely adopted cool roofs.

Officials are also turning to technology to understand where big temperature differences occur in cities. Anthony Baker, chief executive of SatVu, says the company has worked with cities including Darwin, using an infrared camera on a satellite to map heat differences.

“Everyone knows cities are hotter than rural areas, but we can pinpoint which buildings or car parks are . . . predominately affecting the heat of a district,” Baker says. “When we look at cities we can see the more affluent areas being more green naturally.”

Arup’s Rana says traditionally cooler cities can look to warmer peers for examples of how to adapt, such as Singapore, which has long used planting as well as shade canopies. She adds that it is vital to think about the impact of heat widely, not just building by building. “If everyone is just thinking about their own house or office, that is going to have adverse effects on neighbours.”

The increase in the use of air conditioning, for example, raises temperatures nearby by transferring heat from within buildings to the outside. Researchers at UCL said air-con warmed temperatures by as much as 1C in dense central London.

Ruth Engel, an urban heat specialist at the World Resources Institute, a non-profit organisation, says the best solutions often vary by city. Phoenix’s cool pathways, for example, would not suit European urban areas where people typically walk more, she says, as reflective surfaces increase the risk of heat exposure. In cities that lack sufficient water, shade canopies might be a better option than trees, she adds.

Local solutions will be key, Engel stresses. “It is hard to rebuild a whole city,” she says. “But heat action is achievable.”

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