Cities in the Global South are more exposed to extreme heat because they lack cooling green spaces, new research shows.
The study found that cities in the Global South have only 70% of the “cooling capacity” provided by urban vegetation in the Global North.
With rising temperatures, combined with “urban heat island” effects that make cities warmer than rural areas, heat-related illnesses and deaths in cities are becoming more common.
Urban green spaces can help reduce this risk by cooling outdoor environments and providing vital shelters.
The research, led by an international team including the universities of Nanjing, Exeter, Aarhus and North Carolina, concludes that there is “enormous potential” to improve urban cooling in the global South and reduce inequality.
“Urban vegetation is a highly effective way of coping with the potentially deadly effects of extreme heat and humidity,” said Professor Tim Lenton of the University of Exeter’s Global Systems Institute.
“Currently, people who die due to climate change often live in slums in cities in the Global South, such as the hottest areas of India.
“Our analysis suggests that green spaces can cool the surface temperature of an average city by about 3°C during warm seasons – a vital difference during extreme heat.”
The cooling effect of urban green spaces, especially urban forests, is caused by shading and transpirational cooling (water evaporation).
The new study used satellite data from the world’s 500 largest cities to assess “cooling capacity” — the extent to which urban green spaces cool a city’s surface temperatures.
The top ten cities with the highest cooling capacity are all in the United States: Charlotte and Raleigh-Durham take first place, followed by Kansas and Baltimore. Many US cities have low population density, which creates problems with “urban sprawl,” but this brings benefits in terms of green spaces and the resulting cooling.
Mogadishu in Somalia is the city with the lowest cooling capacity, followed by Sanaa in Yemen and Rosario in Argentina. Chicago is the fourth city with the lowest cooling capacity, the only US city on the list with cooling capacity below 1°C.
‘Refreshing benefit’
The Global South, which includes Africa, Latin America and much of Asia, contains the areas most at risk of extreme heat.
A previous study found that current climate policies will leave more than a fifth of humanity exposed to dangerously high temperatures by 2100, with the largest populations at risk in India and Nigeria.
The new study assessed population density and location to estimate the “cooling benefit” the average citizen receives, as green areas are often found in wealthier parts of a city.
Professor Chi Xu from Nanjing University said: “As cities in the Global South lag behind in terms of cooling capacity, the cooling benefit for an average resident is 2.2°C, compared with 3.4°C for a city dweller in the Global North.
“The differences are mainly due to the amount of vegetation, but cooling efficiency is also better in the Global North, possibly due to green space management and different tree species.”
Professor Jens-Christian Svenning, from the Centre for Ecological Dynamics in a New Biosphere (ECONOVO) at Aarhus University, said: “The good news is that this nature-based cooling solution can be substantially improved across the Global South, helping to address future heat stress for billions of people.”
Professor Rob Dunn, from North Carolina State University, said: “Greening cities will not be easy. It may be expensive in the short term, but it will be essential to making cities liveable in the immediate future. It will also be critical to work to prevent the loss of green space in those cities that have it, or at least some.
“Changes could include ground-level green spaces, vertical and rooftop gardens, or even forests, to help protect city dwellers from extreme heat.”