Preserving a 鈥済reen belt鈥� of countryside around urban areas helps keep cities cool during the hottest times of the year, suggesting that new UK plans to build on currently unoccupied land could affect urban temperatures.
We already know that many cities experience an urban heat island effect, as buildings and other human-made objects retain more heat than rural land. But as warm air rises in a city, it creates a layer of low pressure near the ground, which draws in cooler air from the surrounding countryside.
at the University of Surrey, UK, and his colleagues have now quantified this cooling effect by studying 20 years of satellite data up to 2020, across 30 Chinese cities. They found that having a city encircled by a band of countryside can reduce urban temperatures by up to 0.5掳C (0.9掳F). 听
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The cooling effect is most powerful when a rural band extends outwards for at least half of the city鈥檚 diameter. Other effective actions to enhance cooling include joining up patches of green space, planting more woodland around cities and incorporating a few large lakes on a city鈥檚 outskirts. 鈥淭丑e number and adjacency of rural landscape patches emerge as pivotal factors,鈥� says Cao.
Although the team only looked at cities in China, the findings were robust when controlling for a city鈥檚 local climate, meaning they should apply to urban areas elsewhere, says Cao.听听
at the University of Manchester, UK, says the work underscores the value of creating large areas of green or blue space, rather than small patchworks of rural land. The important consideration is the size and quality of the habitat. 鈥淲hether these are green belt or not is not of itself that important,鈥� he says.
The research could play into plans by the UK鈥檚 new Labour government to reconsider the nation鈥檚听approach to rural construction. For decades, UK cities have been thronged 鈥� or throttled, depending on who you ask 鈥� by protected听green belts听designed to stop settlements from endlessly expanding.
Now, in an effort to alleviate a nationwide housing shortage and accelerate economic growth, the UK government has said it will build 1.5 million homes in England over the next five years, allowing construction on parts of the green belt. 听
at UK architecture firm Building Design Partnership has argued for planners to allow 鈥渃onfident bites鈥� to be taken out of the green belt. He says construction could proceed without losing green belt benefits such as urban cooling, because developers should only need relatively small amounts of land.
鈥淭丑e cooling effect may well exist, we have long known about urban heat islands, which are more intense in larger cities,鈥� he says. 鈥淏ut our confident bites will have a minimal impact and it is important to get things into proportion.鈥�听
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