Urban growth means hotter summers; white paint can help

This post has been corrected. See the note below for details.

It’s hot out there. Hotter than it would be if instead of what I see outside my sliver of window --  roads, buildings -- there was grass and vegetation. Hotter, too, than it would be if the buildings were all covered with white paint, a la a Greek island.

This is the “heat island effect,” and it happens because the materials used to make roads and structures absorb a lot more heat from the sun than does vegetation.  They slowly release that heat through the night, keeping everything not-so-nicely cooking. The effects on local temperatures can be big, as a new study looking at the future of Arizona illustrates nicely.

Published in the journal Nature Climate Change, the study examined what will happen to summer temperatures as Arizona’s so-called Sun Corridor, an area that encompasses the cities of Phoenix, Tucson, Prescott and Nogales, becomes more developed. Almost 90% of Arizona’s residents live in this “megapolitan” area and the population is rising briskly, projected to grow more rapidly in the next few decades than any other megapolitan area in the United States.

What will things look like in 2050 as more and more of the land gets paved over? How hot will the summers be? And how could that temperature rise be mitigated?

Climate scientist Matei Georgescu of Arizona State University and his colleagues decided to take a look.

Here’s what they did. They used 2006 data on land cover in the area as a baseline. Then they compared it to two possible scenarios drawn up by the Maricopa Assn. of Governments. One was a “worst”-case scenario: pretty much blanket coverage of the region with shops, offices, roads, homes and their yards by 2050. The other was the best-case scenario: fairly limited urban growth through 2050.

And they added a further scenario: worst-case population growth, but with white paint on all of the roofs.

The scientists fed all these data into a well-tested regional climate-modeling system called the Weather Research and Forecasting Model.

The results: Under the worst-case scenario, summer temperatures would rise by 3- 4 degrees Celsius. But if the roofs were painted white, the projected rise in temperature falls in half -- down to the projected temperature rise seen for the best-case, limited-growth scenario.

(Sounds good! Let's get down to Home Depot, grab a few drums of dazzling-white primer and get our teens up on the roof doing something useful on the weekends.)

But not everything the scientists found was that promising. White paint didn’t solve the problem of water management, for example.

In undeveloped land, when it rains some water collects on the surface but a lot of it percolates into the ground, Georgescu explained in a phone conversation. “Think of the soil column basically as a sponge -- some water falls off but a lot gets sucked in,” he said. “When you lay down this carpet of built environment, you’re forbidding the sponge to act the way it normally does …. When the sun comes up, [the water] should go back into the atmosphere but [instead] it’s run off into the sewer systems.”

And if you modify the water balance in this way, he added, “you might have other consequences. You might modify regional rainfall patterns, for example.”

White paint also resulted in less temperature variation between daytime and nighttime, which might affect the environment (for example, widllife) in unforseen ways.

Georgescu said there were several key points to the findings. First off, white paint is not enough. “We need to consider additional adaptation strategies,” he said. One idea is to use permeable surfaces for roads and parking lots that would permit water to soak right into them. (This clearly might be problematic for roofs.)

Such materials exist, Georgescu said. In fact, some of it is being tested right now in a parking lot at Arizona State. “It’s this sort of off-the-cuff thinking that’s going to be required, in conjunction with technology and science” to solve these problems, he said.

The other main point is that these effects -- man-made climate change due to urban growth -- can be more significant on a regional scale than effects on global temperatures through climate change. (The study tested just that in its models and found it to be true for the Sun Corridor.)

They are also more actionable. "If I tell a farmer in the Midwest that the globe is going to be warmer by 2 degrees C by mid-century, there’s nothing he can do with that information,” Georgescu said. “But if you start to zoom in on local scales, then people can begin to make use of the information.”

[For the record, 2.59 p.m. Aug. 16: An earlier version of this post incorrectly said that nights were warmer when rooftops were painted white instead of stating that the range between maximum and minimum daily temperatures decreased.]

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