The problem? Those roofs and roadways collectively create what is called the "urban heat island effect" -- that is, when a city is measurably warmer than rural areas nearby.
Are reflective white roofs the answer?
Yes, according to a new study. Researchers at the Lawrence Berkeley National Laboratory say that implementing cool roofs and cool pavements in cities around the world can actually make enough of a difference to cool the world.
A quick science lesson: white roofs reflect more of the sun's rays than black ones, which absorb more of the energy as heat.
- Buildings with white roofs stay cooler.
- Air conditioned buildings with white roofs need less (and thus less energy) to stay cool.
- The heat absorbed by a black roof heats both the building and the air around it.
- Black roofs also radiate energy directly into the atmosphere, which is absorbed by clouds nearby. Trapped by the greenhouse effect, it contributes to global warming.
Better still, white roofs could cancel the greenhouse gas effects of up to two years of worldwide carbon dioxide emissions.
That's important because more than half of the world's population now lives in cities -- and by 2040, it's predicted that 70 percent of the world's population will be city slickers.
The researchers used a detailed globe land surface model from NASA's Goddard Space Flight Center to find the answer. At their fingertips: regional information on surface variables, such as topography, evaporation, radiation, temperature and cloud cover.
The researchers estimated improvement conservatively, and increased the average albedo, or solar reflectance, of all roofs by 0.25 and of pavements by 0.15.
(In other words: they didn't assume that a black roof, which has an albedo of 0, would be replaced by a pure white roof with an albedo of 1 -- just that it would be of a cooler color.)
The scientists found that, for northern hemisphere summers, increasing the reflectivity of roof and pavement materials in cities with a population greater than 1 million would achieve a one-time offset of 57 gigatons -- that's 1 billion metric tons -- of CO2 emissions.
That's a big deal, since worldwide CO2 emissions in 2006 were 28 gigatons. (The split, by the way: Thirty-one gigatons from roofs and 26 gigatons from pavements.)
In a statement, Berkeley Lab physicist Art Rosenfeld elaborates:
If all eligible urban flat roofs in the tropics and temperate regions were gradually converted to white, and sloped roofs to cool colors, they would offset the heating effect of the emission of roughly 24 gigatons of CO2, but one-time only. However, if we assume that roofs have a service life of 20 years, we can think of an equivalent annual rate of 1.2 gigatons per year. That offsets the emissions of roughly 300 million cars for 20 years.
Still, the researchers warn that it's not a remedy to reverse the effects of the Industrial Revolution.
"Two years worth of emissions is huge, but compared to what we need to do, it's just a dent in the problem,” said study co-author and Concordia University professor Hashem Akbari in a statement. "We've been dumping CO2 into the atmosphere for the last 200 years as if there’s no future."
This study is a follow-up to a 2008 paper (related presentation 1; related presentation 2; both .pdf) published in the journal Climate Change, which found that implementing cool roofs and pavements worldwide could offset the effects of 44 gigatons of CO2 emissions.
U.S. energy secretary Steven Chu is already on board with the results, announcing on Monday that the U.S. Dept. of Energy would implement cool roof technologies on its facilities whenever cost-effective over the lifetime of the roof.
In a video, Chu elaborates:
And what about cooler climes? Apparently the savings from white roofs in the summer outweigh the costs of heating such a building in the winter.
Their results were published online in the journal Environmental Research Letters.
Graphic: NASA Landsat thermal map of New York City. (Columbia University Earth Institute)