Global North vs Global South: Climate divide in urban green spaces makes cities urban heat sinks, finds NASA study
Urban areas are often hotter than their rural counterparts due to the urban heat island effect. Dark surfaces such as roads, buildings, and sidewalks absorb heat from the sun, raising the temperature in cities. This extreme heat poses serious health risks, including dehydration, heatstroke and even death.
- Nov 29, 2024,
- Updated Nov 29, 2024 3:49 PM IST
As anyone who has experienced a heat wave in an urban area knows, a shady tree can be a lifesaver. But what happens when shade is unavailable?
A recent study published in Nature Communications highlights a significant gap in global resilience to climate change: cities in the Global South have far less green space — and therefore less cooling capacity — than cities in the Global North.
The terms ‘Global North’ and ‘Global South’ are used to differentiate between developed countries, mostly in the Northern Hemisphere, and developing nations, primarily in the Southern Hemisphere.
Urban areas are often hotter than their rural counterparts due to the urban heat island effect. Dark surfaces such as roads, buildings, and sidewalks absorb heat from the sun, raising the temperature in cities. This extreme heat poses serious health risks, including dehydration, heatstroke and even death. Greenery, though not a cure-all, helps by providing shade and releasing moisture into the air, which cools the environment.
“Cities can strategically prioritize developing new green spaces in areas that have less green space,” says Christian Braneon, a climate scientist at NASA's Goddard Institute for Space Studies in New York, who was not involved in the study. “Satellite data can be really helpful for this.”
An international team of researchers, led by Yuxiang Li, a doctoral student at Nanjing University, analyzed the 500 largest cities in the world to assess their cooling capacities. Using data from NASA’s Landsat 8 satellite, the team evaluated how effective green spaces were at cooling each city.
First, they calculated the average land surface temperature for the hottest month of 2018 and compared it with the average temperature of the hottest months from 2017 to 2019. Then, they used the Normalized Difference Vegetation Index (NDVI) to map the amount of green space in each city. NDVI works by detecting the difference between the red and infrared light reflected by healthy vegetation, which indicates the density of plant life.
The researchers found that cities in the Global South have only 70% of the cooling capacity of cities in the Global North. Green spaces in an average Global South city lower temperatures by about 4.5°F (2.5°C), while those in a typical Global North city cool temperatures by about 6.5°F (3.6°C). This disparity is compounded by the fact that cities in the Global South are usually located closer to the Equator, where they are expected to experience more extreme heat in the future.
“It’s already clear that Global South countries will face more intense heat waves, rising temperatures, and climatic extremes than their counterparts in the Global North,” says Chi Xu, a professor of ecology at Nanjing University and a co-author of the study. Furthermore, these cities have less capacity to cope with the heat due to less widespread air conditioning and more frequent power outages.
Why is this the case? Cities in the Global South generally have less green space than those in the Global North. This pattern mirrors the “luxury effect”, where wealthier neighbourhoods tend to have more greenery than poorer areas. Chi Xu explains, “Wealthier cities also have more urban green spaces than the poorest cities.”
Closing the gap between the best and worst-performing cities, such as Mogadishu, Somalia, and Charlotte, North Carolina, is unlikely. Mogadishu, for example, has a dry climate that restricts vegetation growth. However, the study shows that cities can still learn from one another. By identifying the city with the highest cooling capacity within a region, researchers set a goal for nearby cities. They found that cities could increase their cooling capacity by as much as 18°F (10°C) simply by increasing both the quantity and quality of green spaces.
“How you use green space will vary depending on the climate and urban environment,” says Braneon, whose NASA research focuses on climate change and urban planning.
In cities in the US and Canada, greener areas often have lower population densities. However, fewer people per square mile doesn't always benefit the environment, as residents in less dense cities rely more on cars and live in larger, less energy-efficient homes. Braneon points out that creating water bodies, planting green roofs, and painting roofs or pavement lighter colours to reflect more sunlight are all viable strategies to boost cooling capacity beyond just adding trees or parks.
A global study like this allows urban planners to compare strategies across cities in similar regions or with similar population densities. “For newly urbanised areas that aren’t fully built out, there’s a lot of room for design changes,” says Braneon.
As anyone who has experienced a heat wave in an urban area knows, a shady tree can be a lifesaver. But what happens when shade is unavailable?
A recent study published in Nature Communications highlights a significant gap in global resilience to climate change: cities in the Global South have far less green space — and therefore less cooling capacity — than cities in the Global North.
The terms ‘Global North’ and ‘Global South’ are used to differentiate between developed countries, mostly in the Northern Hemisphere, and developing nations, primarily in the Southern Hemisphere.
Urban areas are often hotter than their rural counterparts due to the urban heat island effect. Dark surfaces such as roads, buildings, and sidewalks absorb heat from the sun, raising the temperature in cities. This extreme heat poses serious health risks, including dehydration, heatstroke and even death. Greenery, though not a cure-all, helps by providing shade and releasing moisture into the air, which cools the environment.
“Cities can strategically prioritize developing new green spaces in areas that have less green space,” says Christian Braneon, a climate scientist at NASA's Goddard Institute for Space Studies in New York, who was not involved in the study. “Satellite data can be really helpful for this.”
An international team of researchers, led by Yuxiang Li, a doctoral student at Nanjing University, analyzed the 500 largest cities in the world to assess their cooling capacities. Using data from NASA’s Landsat 8 satellite, the team evaluated how effective green spaces were at cooling each city.
First, they calculated the average land surface temperature for the hottest month of 2018 and compared it with the average temperature of the hottest months from 2017 to 2019. Then, they used the Normalized Difference Vegetation Index (NDVI) to map the amount of green space in each city. NDVI works by detecting the difference between the red and infrared light reflected by healthy vegetation, which indicates the density of plant life.
The researchers found that cities in the Global South have only 70% of the cooling capacity of cities in the Global North. Green spaces in an average Global South city lower temperatures by about 4.5°F (2.5°C), while those in a typical Global North city cool temperatures by about 6.5°F (3.6°C). This disparity is compounded by the fact that cities in the Global South are usually located closer to the Equator, where they are expected to experience more extreme heat in the future.
“It’s already clear that Global South countries will face more intense heat waves, rising temperatures, and climatic extremes than their counterparts in the Global North,” says Chi Xu, a professor of ecology at Nanjing University and a co-author of the study. Furthermore, these cities have less capacity to cope with the heat due to less widespread air conditioning and more frequent power outages.
Why is this the case? Cities in the Global South generally have less green space than those in the Global North. This pattern mirrors the “luxury effect”, where wealthier neighbourhoods tend to have more greenery than poorer areas. Chi Xu explains, “Wealthier cities also have more urban green spaces than the poorest cities.”
Closing the gap between the best and worst-performing cities, such as Mogadishu, Somalia, and Charlotte, North Carolina, is unlikely. Mogadishu, for example, has a dry climate that restricts vegetation growth. However, the study shows that cities can still learn from one another. By identifying the city with the highest cooling capacity within a region, researchers set a goal for nearby cities. They found that cities could increase their cooling capacity by as much as 18°F (10°C) simply by increasing both the quantity and quality of green spaces.
“How you use green space will vary depending on the climate and urban environment,” says Braneon, whose NASA research focuses on climate change and urban planning.
In cities in the US and Canada, greener areas often have lower population densities. However, fewer people per square mile doesn't always benefit the environment, as residents in less dense cities rely more on cars and live in larger, less energy-efficient homes. Braneon points out that creating water bodies, planting green roofs, and painting roofs or pavement lighter colours to reflect more sunlight are all viable strategies to boost cooling capacity beyond just adding trees or parks.
A global study like this allows urban planners to compare strategies across cities in similar regions or with similar population densities. “For newly urbanised areas that aren’t fully built out, there’s a lot of room for design changes,” says Braneon.