World’s hottest cities lack cooling green spaces—study

Green spaces:- Many cities in low and middle-income countries are suffering yet another effect of climate change — being more exposed to extreme heat because they lack cooling green spaces, new research shows.

City dwellers in poor countries in Africa, Latin America and much of Asia, are most at risk from extreme heat, which is projected to intensify and cost more lives unless greenhouse gas emissions fall.

Urban greenery, such as rooftop gardens and trees, “is a really effective way of tackling what can be fatal effects of extreme heat and humidity”, said Tim Lenton, a climate change specialist at the University of Exeter, in England, one of several universities involved in the study.

“Currently, the people dying due to climate change are often in the slums of cities.”

The international study, published in Nature Communications this month (September), found that cities in low- and middle income countries currently have just 70 per cent of the “cooling capacity” provided by urban greenery in the rich world.

But it found that there was “vast potential” to enhance urban cooling in these areas to reduce inequality.

The researchers used satellite data on the world’s 500 largest cities to assess their cooling capacity — the extent to which urban green spaces cool down a city’s surface temperatures.

Because global temperatures are rising, combined with the so-called “urban heat island” effects that make cities hotter than rural areas, heat-related illness and death in cities are becoming more common.

Urban green spaces, the researchers say, can help reduce this risk, by cooling down outdoor environments and providing much needed refuge for city-dwellers, who make up more than half of the world’s population.

Green spaces can cool the surface temperature in the average city by about 3°C during warm seasons, according to the analysis – “a vital difference during extreme heat,” says Lenton.

The cooling effect of urban green spaces, especially urban forests, is caused by shading and the evaporation of water, he explains.

All of the top ten cities for cooling capacity are in the United States, with Charlotte and Raleigh-Durham first, followed by Kansas and Baltimore, according to the analysis. Many US cities have low population density – leading to issues of urban “sprawl” – but this brings benefits in terms of green spaces and resulting cooling, the researchers note.

Pune, in India and Harare, Zimbabwe, were among the cities in low-and middle income countries with the highest cooling capacity. Mogadishu in Somalia was the city with the lowest cooling capacity, followed by Yemen’s Sana’a and Rosario in Argentina.

Dangerously hot

An earlier study found that current climate policies will leave more than a fifth of the world’s population exposed to dangerously hot temperatures by 2100, with the largest at-risk populations in India and Nigeria.

This new study assessed population density and location to estimate the “cooling benefit” received by the average citizen – as green areas are often found in the richer parts of a city.

Jens-Christian Svenning, a biodiversity and climate change expert at Aarhus University, Denmark, who also worked on the study, said rapid urbanization and rising extreme heat stress makes urban green space crucial.

“The good news is that this nature-based solution to cooling can be substantially improved … helping to tackle future heat stress for billions of people,” he said.

Rooftop gardens, forests

Rob Dunn, a leading ecologist at North Carolina State University, told SciDev.Net: “It won’t be easy to regreen cities.

“Yet, it will be key to making cities livable in the immediate future.”

He suggests changes could include ground-level green spaces and vertical and rooftop gardens, or even forests, to help protect city people from extreme heat.

Chi Xu, a professor in the School of Life Sciences at China’s Nanjing University, who is one of the authors of the study, said: “We need to plant more trees especially in the [low- and middle-income] cities where heat stress will be increasingly severe.”

Not only are these cities more vulnerable to extreme heat, with billions of people suffering from intolerable heat stress, but they also have less capacity to adapt, he explained.

“So far, urban green infrastructure is still the most effective means of ameliorating outdoor heat stress at large spatial scales,” he added.

Support needed

Shubo Fang, a research associate at the University of Florida, who was not involved in the study, said the research gave “a completely new perspective” on global inequalities.

Its recommendations include improving the scientific basis of urban planning, enhancing the understanding of urban design, and increasing the flexibility and adaptability of urban planning.

However, he said more knowledge was needed on historical differences in urban development and “the specific geographical factors that influence these disparities”.

Desalegn Chala Gelete, a biodiversity researcher at the University of Oslo, who was not involved in the study, said the findings underscore the urgency for policy actions and investments in urban green spaces to fight climate change impacts.

City planners must prioritize expansion and equitable distribution of green spaces and build green infrastructure into their designs, says Gelete.

“This strategy will help lower heat risks and enhance public health city wide.”

She added: “As many people in [low- and middle-income countries] still reside in rural areas, and with rapid urbanization underway, there is a unique opportunity to learn from the experiences of others and plan these emerging cities thoughtfully and sustainably.” AlphaGalileo/SP