By Andrew Nusca
Posting in Cities
Researchers say that it's smart to implement white roofs and pavements in the world's biggest cities. The benefits: saved energy, cooler cities and an easy way to fight global warming.
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)
Jul 23, 2010
We have addressed the "black vs. white" issue for flat / low-sloped roofing installations in northern areas in our blog: http://www.duro-last.com/blog/?p=215 Personal opinion: I believe it's naive to think that over 6 billion carbon-swilling and spewing inhabitants are not having some effect on the temperature of the earth.
Couple questions... 1. What are the maintenance sosts associate with a white roof versus a more traditional roofing? White is a "color" that in all other instances requires much more maintenance in order to keep it up (painting, cleaning, fading, etc) so how does the additional maintenance stack up against the savings both economically and environmentally is a secondary question as well. 2. White pavement? How is this going to be acheived and maintained in an environmentally friendly manner? Not sure how this will really work or the ability to maintained it. Jim
I had a co-worker who put Al foil under his roof in his attic and it amde his electric bill so much lower the electric company had to replace his meter to ensure he wasn't cheating, same goes for the gas company. White roofs are not the all in all answer, just part. When I flew to California, with a stopover in Phoenix, I noticed a majority of the roofs there were white. We all know it gets hot in Phoenix. I have a light colored roof, but in the summer it is too hot to get into the attic until it has cooled down outside, I suspect it might be similar even with a white roof. However a white roof along with other measures--Al foil, for instance could make quite a bit of difference. There has to be an outlet for the hot air, of course, so it is a design probelm. If iwere to have anew house built, I would have some kind of reflective material added to accomplish this goal.
@bdubya - Throughout the whole article I was wondering about those of us in the north that have to heat during the winter. I was a bit let down too that they didn't finish the research.
...or at least the 2/3rds of it that are not visible from the street. I'm doing this not because any concerns I have about my "carbon footprint" or "global warming", but for the simple reason that I believe that it will take 25% off of my summer air conditioning bill, which would likely pay for the effort in a single year. Hopefully by the end of next summer, I'll have some useful data on this experiment.
I've known this for a long time, when I had my current home built 18 years ago, I had the lightest colored roof I could installed, Its a very light sandstone color to go with the stained wood siding. I just cringe every time I see someone get their homes re roofed using coal black shingles. People really should educate themselves before making such large purchases... Response to above question about the solar panels.. yes the solar panels would be better because they cover the roof (making the color choice a non-factor) and convert the sunlight to electricity instead of heat.
Don't forget the color of the interior of the car as well - that makes a big difference - perhaps even bigger than the paint on the car, due to the trapped air collecting the heat, driving the need for A/C way up.
I think those who believe that man-made global warming is real, and mankind is the thing that is wreaking havoc on the world, should do the earth a favor. 1. Do Not Have Children 2, Kill yourself. That way you won't cause any more global warming. Idiots!
"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." Apparently? They didn't complete the loop and research whether it would outweigh heating?
The White Roof is a sane, relatively easy and inexpensive way to reduce energy consumption, especially in climates where one of the major electrical uses is for air cooling inside buildings. As common sense as a white roof is, it still has its detractors. In some cities, and in some home-owner associations, white roofs are actually ILLEGAL/PROHIBITED - Example: Tangerine, Florida.