Miles of pavement and clusters of towering buildings are exceptionally good at turning cities into pockets of blistering heat.
But new research from NASA suggests a surprisingly simple change could help cool growing metropolises.
Materials like concrete, asphalt, stone, and steel are dense and not very porous, so they absorb heat from the sun and constantly radiate it into the surroundings. This “heat island” effect makes cities about 2 to 5 degrees Fahrenheit warmer than neighbouring suburbs and rural areas.
A few degrees may not sound like a huge difference, but it’s enough to cause a huge uptick in air conditioning demand — and the resulting energy use. A temperature increase of about 2 degrees Fahrenheit can boost energy use from AC in the summer from 5% to 20% in the US, according to the Environmental Protection Agency.
The temperature difference is clear in the map below from NASA. The most striking differences show up along I-95 between Boston and Washington, around Atlanta and down I-85 in the south, and around big cities and roads through the Midwest and west coast.
The good news from NASA is that planting more green things in and around cities can help.
Vegetation is a natural counter to heat. Plants drink up and then release water back into the atmosphere in a process called evapotranspiration. This drops the surrounding surface temperature much the same way as sweat does: sweat evaporates from your skin taking some of your body heat with it.
“[C]ities are warmer than vegetated lands by [3.4 °F] during summer and [2.7 °F] during winter,” the researchers wrote in their study, published in Environmental Research Letters. So if a city could somehow cover itself entirely with greenery, it could theoretically cool down by that much.
A city would have to pick the right plants, however, since some are better at evapotranspiration than others. For example, broad-leafed trees are covered with more pores, so they can release more water back into the atmosphere. Urban planners can cool down cities more efficiently, drive down energy costs, and reduce the use of fossil fuels if they focus on planting vegetation that releases lots of water.
“Urbanisation is a good thing,” Lahouari Bounoua, a climate scientist at NASA who worked on the research, said in a press release. “It brings a lot of people together in a small area. Share the road, share the work, share the building. But we could probably do it a little bit better.”
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