Cities are warming 29% faster than rural areas. Could urban greening  fix this?

Extreme heatwaves have affected millions of people around the world this year. By 2030, a total of 1.9 billion people are expected to be exposed to heat stress, and this will hit city dwellers particularly hard. The city of Shush in Iran recorded temperatures of 53.6°C in August, the hottest ever in Asia.

A simple way we could be helping cool our cities is through a process known as “urban greening” – adding vegetation such as trees, rooftop gardens and other green spaces into cities.

It’s a solution that’s becoming increasingly urgent. This year is on track to be the sixth hottest on record, with global temperatures in January-September running 0.86°C above the 20th-century average.

Cities are warming faster than rural areas partly because of “urban heat islands”. These are areas full of concrete, asphalt roads and other man-made materials, all of which absorb and release more heat than natural features.

Scientists have now discovered that cities and urban areas are warming 29% more quickly than rural areas on average.

The international group of researchers analysed data from more than 2,000 cities, and then compared surface temperatures with those in rural areas between 2002 and 2021. They found that daytime temperatures in cities are warming by an average of 0.56°C per decade, while nighttime temperatures are rising by 0.43°C.

This warming is happening even faster in megacities – those with populations of more than 10 million – with the increase averaging 0.69°C a decade for daytime temperatures.

The study also reveals that these temperature rises are more pronounced in Asia, which has a considerable amount of megacities. The lowest daytime warming trend is in Europe, while the lowest nighttime trend is in Oceania.

Climate change is the greatest contributor to rising temperatures in 90% of the cities studied, being responsible for an average of 0.3°C of warming per decade, the scientists say.

However urban expansion is also a contributor. It is estimated to be increasing temperatures by more than 0.23°C per decade in some cities in China and India.

Urban greening has offset an average of 0.13°C of surface warming per decade in European cities, the research shows. This cooling effect appears to be particularly noticeable at night.

The scientists say their results “support the use of urban greening as an effective strategy to mitigate urban surface warming”.

They point to efforts in the American city of Chicago to expand street tree coverage following a severe heatwave in 1995. They say this decreased the rate of warming by around 0.084°C per decade, they say.

Elsewhere, Amsterdam’s RESILIO project is repurposing rooftops to help cool the city and reduce the risk of flooding.

RESILIO’s smart ”blue-green” roofs have a “buffer layer” that can store rainwater and reduce the chance of damage to houses and their surroundings during heavy downpours. At the same time, this allows the plants and vegetation in the green part of the roof to grow, helping to reduce the urban heat island effect.

Launched in 2018, the project is moving towards having installed 10,000 square metres of its smart rooftops.

RESILIO is one of 15 innovations getting scale-up support through the BiodiverCities Challenge on Uplink, the World Economic Forum’s crowdsourcing platform for solutions to some of the world’s biggest problems. The challenge is looking for innovations to help make cities greener, healthier, wilder and fairer through nature.

Zhuhai in China is part of a pilot project developing “sponge cities”. This involves using permeable and porous bricks and concrete in roads and pavements to store water and help prevent flooding, but it is estimated that these porous surfaces can also lower air temperatures by up to 1°C.

Singapore is increasing the extent and density of its green spaces – including by using walls and roofs – to try and keep temperatures lower.

In the UK, Gloucestershire council’s climate change manager has suggested using rainwater gardens to reduce city temperatures. Rainwater gardens are shallow areas of ground filled with water-tolerant plants that receive water runoff from paved areas.