As the world is warming up, cities are becoming more responsive to extreme heat, and greenery in cities has now emerged as the most important tool for cooling. According to a recent study in Nature Communications, there exists a very stark disparity in the city’s cooling capacities between Global North and Global South countries. From analysis of data from NASA’s Landsat 8 satellite, it was learned that cities in the less developed world had much more limited green spaces than those in other wealthy countries, thus making them so much more vulnerable to heat and rising temperature risks.
The hazards of extreme heat increase with the presence of urban heat islands-that cities are hotter than their surrounding rural environment. The energy from the sun that is absorbed by such heat-absorbing surfaces as roads, sidewalks, and buildings raises temperatures to critical levels, which present health hazards, including dehydration, heat stroke, and death. Not a silver bullet, green spaces are quite an important mitigator against such effects. Trees and other foliage shade, as well as moisture in the air, cool their immediate environment.
This study, led by Yuxiang Li of Nanjing University, will examine the world’s 500 biggest cities to calculate the cooling effect that greenery produces in the environment. It will use NDVI-a measure that estimates vegetation density- in satellite imagery to know how well foliage can decrease urban temperatures. Scientists determined the mean surface land temperature of the warmest summer months between 2017-2019 by drawing on the cool effects that green spaces have over cities around the world.
Results: From this study, an enormous difference was observed; those green spaces in the Global South cooled temperatures by 4.5°F (2.5°C) while those in the Global North scored 6.5 degrees F (3.6°C). This may be partly because most the South cities have lesser foliage thereby adding to the inherent risks there, geography. Generally, cities located farther south are much more likely to experience extremes in heat and that will get hotter in the future.
Another dimension is a socioeconomic disadvantage added only to this difference. It’s not only neighborhoods. The ‘luxury effect’ — where richer neighborhoods have more green space — is something cities, too, experience. Richer cities in the Global North are investing in and prioritizing their urban greenery. The cities in the Global South, with limited resources, cannot, Professor Chi Xu added, being a co-author. Besides regional differences, a special problem for the Global South concerning heat is access to air conditioning and reliability of electricity.
The study reveals some extreme contrasts between cities with the highest and lowest cooling capacities. Mogadishu, Somalia is considered the worst-performing city with least greenery and dry climate not favored for growth. Charlotte, North Carolina was the best performing city with full greenery and lower population density. Researchers also found great variation within regions suggesting that cities can learn from their neighbors in order to improve their cooling strategies.
There is a big potential for cooling improvement. Scientists say cities can cool themselves by as much as 10°C or 18°F by systematically expanding the quantity and quality of green spaces. However, such expansion requires a specific climate and urban settings, including arid regions using drought-resistant plants and other regions using green roofs or urban forests.
Christian Braneon is a climate scientist at the Goddard Institute for Space Studies of NASA who was not involved in the said study. He said that cities can strategically prioritize to develop new green spaces in the areas that have less greenery. Other than developing new green spaces, some things that cities can do include the creation of water bodies, painting of rooftops or pavements lighter colors to reflect heat and seeding on green roofs.
The study also reveals the broader environmental trade-offs of urban design. Cities rich in greenery in the United States and Canada tend to have lower population densities. Again, this comes with its own environmental costs- increased reliance on cars and bigger, less energy-efficient houses. Such complexities underscore the need for holistic urban planning that balances cooling strategies with sustainability goals.
The results have opened a very promising opportunity for the newly urbanized areas. “There’s still room to change the design, Braneon said, encouraging planners to work in green spaces early into the development process.”.
Ultimately, it is a wake-up call for the urban planners and policymakers across the world. When climatic extremes such as heat waves are increasing in frequency, so is the need for equitable and effective cooling solutions. For the cities of the Global South, that might be the difference between resiliency and crisis.