The simple ways cities can adapt to heatwaves
12 July, 2023 / Articles
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Ribbons of blue snake through the bird’s eye view of Prague, a cool relief from the intense patchwork of hot red, vivid orange, and bright yellow that dominates the satellite images.
The blue marks the Vltava River, offering cool respite to the Czech Republic’s capital from the blazing heatwave that hit the city in June 2022, while interspersed green patches symbolise parks, another relief from the hot land surface temperatures, which measured up to 45C (113F) last summer.
These are urban heat map images captured from space. They show the dramatic impact of green spaces, white road surfaces and water on cities, helping them cool in a natural way and resist the rise of deadly heat waves.
Mapping these extreme hotspots are a vital asset to city planners as the world warms up and heat-related deaths continue to rise. In 2022, more than 20,000 people died of heat-related causes in Europe, with temperatures in the UK surpassing 40C (104F) for the first time in history. In June this year, areas in Spain registered 44C (111F) on the thermometer – and city dwellers across the pond in the US aren’t faring much better. There have been at least 13 heat-related deaths in Texas over the past two weeks, as meteorologists predict the extreme heatwave will spread over the southern US, and that temperatures exceeding 43C (110F) will be commonplace.
Images that are constantly being captured by the International Space Station not only show extreme land surface temperatures in cities, but also dramatically cooler areas that are the result of having parks or water features in densely populated areas. One study found that neighbourhoods within a 10-minute walk of a park are as much as 3C cooler than areas outside that range.
“The images can definitely be a resource,” says Glynn Hulley, an atmospheric research scientist at Nasa’s Jet Propulsion Laboratory, who is heading up the image-capturing project, called the Land Surface Temperature Monitoring mission. “The data can be used to identify hotspots, vulnerable regions, and assess the cooling impacts of heat mitigation approaches.”
Hulley and his team, who are based in Los Angeles, work routinely with the local city council, who used the data in these Ecostress maps in its sustainability and mitigation plan.
“They use the images to pinpoint areas in the city that are very hot, and very vulnerable from a human perspective,” Hulley explains. “We helped them zero in on areas that were hottest, and officials went in and painted the hottest streets with a special cool pavement coating.” The paint reflects the sun, rather than absorbing it and retaining the heat like conventional asphalt surfaces.
Hulley then quantified the effects by using the satellite to recapture the same area – and could show that the coating had cooled the streets by 1-2C. The city council used Hulley’s data to receive a further $6m (£4.72m) in funding from California’s state governor to paint more roads.
This kind of collaboration isn’t commonplace, but it could be if cities were to use the satellite images in their climate resilience planning.
The Ecostress instrument collects data year-round, capturing US cities once every four days, and European cities around once every three to five days. The team focuses on 10 cities around the globe, including London, Paris, Athens, Mumbai and Melbourne. Last June, they worked with the European Space Agency to collect data from Paris, Prague and Milan during a particularly extreme heatwave.
The maps show the temperature of densely populated areas, which appear a deep red, compared to areas that are more sparsely populated, which are shaded yellow. Areas with forest or water cover appear green, demonstrating that more vegetation or water features correspond to cooler temperatures, by up to 12C in some regions.
Scientists in Beijing used the data in these satellite heat maps to explore the cooling effects of urban parks in a 2022 study. The report concluded that parks heat up more slowly than urban regions during the day, and that green public spaces containing water had a better cooling effect both inside the park and in the surrounding area.
City officials can use these maps to better plan for heatwaves, as they’re a clear indicator just how efficient simple adjustments to the landscape can be, and as deadly heatwaves are set to become more frequent due to climate change, these heat maps will be increasingly more important in cooling down cities, says Marta Olazabal, head of the adaptation research group at the Basque Centre for Climate Change in Spain. “These maps are extremely useful to diagnose the location of specific urban spaces that are highly exposed to heat,” she says, adding that it is “essential” to combine the information with data on vulnerable populations and infrastructure – a method that Austin, Texas, is already using.
Olazabal recently co-authored a study published in May, which assessed adaptation plans of 167 European cities, and found planning measures to tackle heat are actually improving.
“I see many good examples in Global North cities with resources,” she says, citing New York’s roof painting, Berlin’s promotion of greenery and biodiversity, and Barcelona’s experimentation with urban design to reduce traffic and integrate green spaces within the city. “But there is clearly a lack of resources and action in small and medium cities and Global South cities, where most vulnerabilities and less resilient infrastructures concentrate,” she says.
Installing more green spaces, planting more trees along streets, and painting concrete surfaces white are just a few of the simple adjustments that cities have been rolling out to combat trapped heat, a phenomenon known as “urban heat islands”. These islands are areas that are significantly hotter than the surrounding rural regions, sometimes as much as 15C (59F), and are created as heat in cities dissipates slower due to dense infrastructure. They’re especially dangerous for vulnerable populations such as the elderly, young children, low-income populations and manual labourers.
Planning officials have been forced to think outside the box when it comes to innovative ways to combat the heat. Paris, where citizens have the highest risk of heat-related deaths in Europe, has created an interlinked network of “cool islands” where Parisians can seek refuge – from parks and forests to swimming pools and museums. These islands are linked by naturally cool tree-covered walkways and can be accessed via a mapping app called Extrema, which the city developed in 2019. Some parks will also be open 24/7, and the city is tripling its underground cooling network. These heat action plans can be extremely effective; one study looked at a plan Montreal in Canada implemented between 2004 to 2007, and found the strategy led to a reduction of 2.52 daily deaths on average.
In Seville in Spain, the first city to start naming its heatwaves, the mayor has ordered the installation of more awnings across the city as part of a “policy of shade”, as well as planting 5,000 trees a year and building more public fountains. City officials in Rotterdam, Netherlands, are hoping to plant green vegetation on more than 900,000 square metres (9.6 million sq ft) of rooftops, a tactic which helps cool cities down. In Athens, Greece, meanwhile, the city’s chief heat officer Eleni Myrivili is leading a project to renovate an historic aqueduct dating to the Roman era to funnel water through the city; whilst in Acharnes, another Greek municipality, specially developed cool roofs and pavements are being implemented, with promising results.
Hulley hopes that a new technology his team is developing will aid city planning even further. In 2028, the team plans to launch a satellite that will produce higher resolution images that could be used to pinpoint at an even more detailed scale – for example, the impact on green roofs.
“I cannot imagine a future where heat monitoring and planning is not one of the most relevant policy areas in any city worldwide,” says Olazabal. “The climate emergency in cities has already become a health emergency and appropriate resources and planning needs to be put in place, without any delay.”
