Forget global warming. Well, don’t forget it completely, because it’s probably going to kill us all. But forget it for a moment, while we take a look at another climate-related effect: city warming. A new NASA study shows that trees and plant-life are essential to keeping our cities cool and not just because they’re nice to sit under on a sunny day.
Cities are like giant storage heaters made up of what NASA calls "impervious surfaces"—roads, buildings, concrete. This causes what's known as the urban heat island effect, where a city’s temperature is set a few degrees above the surrounding countryside thanks to the heat retention of all that city infrastructure.
But the new study shows that the amount of green space in the city has a big effect on just how hot our urban heat islands get.
"Everybody thinks, 'urban heat island, things heat up.' But it's not as simple as that," said research scientist Kurtis Thome, co-author of the paper. "The amount and type of vegetation plays a big role in how much the urbanization changes the temperature."
What the team found is that trees and other vegetation are essential for keeping our cities cool. Trees naturally cool the air by a process called evapostranspiration. It's a process plants undergo that's a little similar to sweating, in that released water vapor carries off heat.
In their model, based on observation of real cities using satellite and temperature data, the researchers found that when the area of the impervious surfaces reaches 35% of the total (the other 65% being vegetation cover), things go haywire. Up until that point, the urban area stays at a constant 1.3˚C above the surrounding area. Above it, that difference increases as the vegetation is stripped away, "reaching 1.6°C warmer by 65% urbanization." That might not sound like much, but one degree is enough to push up air-conditioning use by up to 20%.
Noteworthy is the fact that fancier, tree-lined neighborhoods with gardens and well-tended lawns can be a couple of degrees lower than less-wooded areas. The effect is so strong that in cities built in the desert, "the urban area actually has a cooling effect because of irrigated lawns and trees that wouldn't be there without the city."
As our cities grow, and green spaces are replaced with more impervious surfaces, their temperatures rise too. The answer is pretty straightforward: more trees and plants means cooler cities. And more trees also mean a more pleasant city environment for those living there. We just have to figure out how to build cities and plant trees at the same time.