On a hot summer day — like much of the U.S. is experiencing now — the places most likely to be a little cooler are green spaces.
Botanical gardens cool the air most efficiently, according to a 2024 study led by researchers at the University of Surrey in England. It found that the temperature difference between a botanical garden and other parts of the surrounding city was 5 degrees Celsius, on average.
That temperature difference is equivalent to about 9 degrees Fahrenheit, which might seem hard to believe, but the extent to which a green space cools air is heavily dependent on various local factors. These factors include everything from the climate of the city in which a green space is located to the size of the green space itself.
For the study, researchers analyzed existing research on urban green-blue-gray infrastructure, which refers to natural and manmade spaces that typically feature greenery or water. They wanted to know if such areas could effectively reduce summer temperatures and thus help people combat overheating in urban areas.
Besides botanical gardens, other types of green-blue-gray infrastructure found to have significant cooling effects include wetlands (4.7 degrees Celsius, or about 8.5 degrees Fahrenheit, on average) and green walls (4.1 degrees Celsius, or about 7.4 degrees Fahrenheit).
The reasons why this cooling happens vary from space to space.
For example, wetlands act as natural sponges, storing water and releasing it into the air when air temperatures are high, thereby moderating temperatures in the immediate area.
Green walls —a type of manmade structure involving a wall filled with plants — reduce heat absorption by buildings. The plants themselves also release moisture into the air, which further cools the air.
In a summary of the study findings, Prashant Kumar — a professor of air quality and health at the University of Surrey — says:
“We have known for some time that green spaces and water can cool cities down. However, this study provides us with the most comprehensive picture yet. What’s more — we can explain why.”
Some of those reasons include trees providing shade and evaporating water lowering air temperatures.
To an extent, the bigger a green space, the greater the impact on cooling, the researchers say. Thus, connecting green spaces via “green corridors” might boost cooling in a city.
The researchers say urban planners can also use green projects to remove carbon emissions from cities and to help prevent flooding.
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