As any urban dweller who has lived through a heat wave knows, a shady tree can make all the difference. But what happens when there's no shade available?
A recent study in Nature Communications used NASA satellite data to identify a major 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 were used in the study to distinguish developed countries (mostly in the Northern Hemisphere) from developing nations (mostly in the Southern Hemisphere).
Cities tend to be hotter than nearby rural areas because of the urban heat island effect. Heat-trapping dark surfaces such as sidewalks, buildings, and roads absorb heat from the Sun's rays, which raises the temperature of the city. Extreme heat poses serious health threats for urban residents, including dehydration, heat stroke, and even death. Though not a cure-all, greenery provides shade and releases moisture into the air, cooling the surroundings.
"Cities can strategically prioritize developing new green spaces in areas that have less green space," said Christian Braneon, a climate scientist at NASA's Goddard Institute for Space Studies in New York who was not affiliated with this 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 compare their cooling capacities. They used data from the Landsat 8 satellite, jointly managed by NASA and the U.S. Geological Survey, to determine how effective green space was at cooling each city.
First, they calculated the average land surface temperature for the hottest month of 2018 for each city, as well as the average of the hottest months from 2017 to 2019. Next, the researchers used a metric called the Normalized Difference Vegetation Index (NDVI) to map how much green space each city had. The NDVI relies on the fact that healthy vegetation absorbs red light and reflects infrared light: the ratio of these wavelengths can show the density of healthy vegetation in a given satellite image.
Researchers found that cities in the Global South have just 70% of the greenery-related cooling capacity of cities in the Global North. The green spaces in an average Global South city cool the temperature by about 4.5 F (2.5 C). In an average Global North city, that cooling capacity is 6.5 F (3.6 C). This compounds an existing problem: cities in the South tend to be at lower latitudes (that is, nearer to the Equator), which are predicted to see more heat extremes in the coming years.
"It's already clear that Global South countries will be impacted by heat waves, rising temperatures, and climatic extremes more than their Global North counterparts," said Chi Xu, a professor of ecology at Nanjing University and a co-author of the study. The Global South has less capacity to adapt to heat because air conditioning is less common and power outages are more frequent.
Why do cities in the Global South struggle to stay cool? Cities in the Global South tend to have less green space than cities in the Global North. This mirrors studies of the disparities within cities, sometimes referred to as the "luxury effect": wealthier neighborhoods tend to have more green space than poorer neighborhoods. "Wealthier cities also have more urban green spaces than the poorest cities," Chi said.
It's unlikely that urban planners can close the gap between the study's worst-performing city (Mogadishu, Somalia) and the best-performing one (Charlotte, North Carolina).
Mogadishu is a dense city with a dry climate that limits vegetation growth. Still, there's a lot that each city can learn from its neighbors. Within a given region, the researchers identified the city with the greatest cooling capacity and used that as a goal. They calculated the difference between the best-performing city in the region and every city nearby to get the potential additional cooling capacity. They found that cities' average cooling capacity could be increased substantially - to as much as 18 F (10 C) - by systematically increasing green space quantity and quality.
"How you utilize green space is really going to vary depending on the climate and the urban environment you're focused on," said Braneon, whose research at NASA focuses on climate change and urban planning.
Greener cities in the U.S. and Canada have lower population densities. However, fewer people per square mile isn't necessarily good for the environment: residents in low-density cities rely more on cars, and their houses tend to be bigger and less efficient. Braneon noted that there's a suite of solutions beyond just planting trees or designating parks: Cities can increase cooling capacity by creating water bodies, seeding green roofs, and painting roofs or pavement lighter colors to reflect more light.
With a global study like this, urban planners can compare strategies for cities within the same region or with similar densities. "For newly urbanized areas that aren't completely built out, there's a lot of room to still change the design," Braneon said.
