Maybe now NOAA will get rid of all remaining rooftop climate monitoring stations or stations sited over asphalt, like this one. As for the carbon emissions issue, that remains to be seen.
Global Model Confirms: Cool Roofs Can Offset Carbon Dioxide Emissions and Mitigate Global Warming
Can light-colored rooftops and roads really curb carbon emissions and combat global climate change? The idea has been around for years, but now, a new study by researchers at Lawrence Berkeley National Laboratory that is the first to use a global model to study the question has found that implementing cool roofs and cool pavements in cities around the world can not only help cities stay cooler, they can also cool the world, with the potential of canceling the heating effect of up to two years of worldwide carbon dioxide emissions.
Because white roofs reflect far more of the sun’s heat than black ones, buildings with white roofs will stay cooler. If the building is air conditioned, less air conditioning will be required, thus saving energy. Even if there is no air conditioning, the heat absorbed by a black roof both heats the space below, making the space less comfortable, and is also carried into the city air by wind—raising the ambient temperature in what is known as the urban heat island effect. Additionally, there’s a third, less familiar way in which a black roof heats the world: it radiates energy directly into the atmosphere, which is then absorbed by the nearest clouds and ends up trapped by the greenhouse effect, contributing to global warming.
Today, U.S. Energy Secretary Steven Chu announced a series of initiatives at the Department of Energy to more broadly implement cool roof technologies on DOE facilities and buildings across the federal government. As part of the effort to make the federal government more energy efficient, Chu has directed all DOE offices to install cool roofs, whenever cost effective over the lifetime of the roof, when constructing new roofs or replacing old ones at DOE facilities. Additionally, the Secretary has also issued a letter to the heads of other federal agencies, encouraging them to take similar steps at their facilities.
“Cool roofs are one of the quickest and lowest cost ways we can reduce our global carbon emissions and begin the hard work of slowing climate change,” said Chu. “By demonstrating the benefits of cool roofs on our facilities, the federal government can lead the nation toward more sustainable building practices, while reducing the federal carbon footprint and saving money for taxpayers.”
In the latest study, the Berkeley Lab researchers and their collaborators used a detailed global land surface model from NASA Goddard Space Flight Center, which contained regional information on surface variables, such as topography, evaporation, radiation and temperature, as well as on cloud cover. For the northern hemisphere summer, they found that 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 (1gigaton equals 1 billion metric tons) of CO2 emissions (31 Gt from roofs and 26 Gt from pavements). That’s double the worldwide CO2 emissions in 2006 of 28 gigatons. Their results were published online in the journal Environmental Research Letters.
“These offsets help delay warming that would otherwise take place if actual CO2 emissions are not reduced,” says Surabi Menon, staff scientist at Berkeley Lab and lead author of the paper.
Co-author Hashem Akbari emphasizes that cool roofs and pavements are only a part of the solution: “Two years worth of emissions is huge, but compared to what we need to do, it’s just a dent in the problem,” says Akbari, the former head of the Berkeley Lab Heat Island Group and now Hydro-Quebec Industrial Research Professor at Concordia University in Montreal. “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 published in the journal Climate Change, which calculated the CO2 offset from cool surfaces by using a simplified model that assumed a global average for cloud cover. The earlier paper, co-authored by Akbari, Menon and Art Rosenfeld, a Berkeley Lab physicist who was then a member of the California Energy Commission, found that implementing cool roofs and pavements worldwide could offset 44 gigatons of CO2 (24 Gt from roofs and 20 Gt from pavements).
Equivalent to Getting 300 Millions Cars Off the Road
“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 Gt of CO2, but one-time only,” says Rosenfeld, who returned to Berkeley Lab this year. “However, if we assume that roofs have a service life of 20 years, we can think of an equivalent annual rate of 1.2 Gt per year. That offsets the emissions of roughly 300 million cars (about the cars in the world) for 20 years!”
In both studies, the researchers used a conservative assumption of increasing the average albedo (solar reflectance) of all roofs by 0.25 and of pavements by 0.15. That means a black roof (which has an albedo of 0) would not have to be replaced by a pure white roof (which has an albedo of 1), but just a roof of a cooler color, a scenario that is more plausible to implement.
[2]Lighter colored pavement is more reflective, resulting in a cooler surface temperature. (Photo courtesy ASU National Center of Excellence for SMART Innovations)
Roofs and pavements cover 50 to 65 percent of urban areas. Because they absorb so much heat, dark-colored roofs and roadways create what is called the urban heat island effect, where a city is significantly warmer than its surrounding rural areas. This additional heat also eventually contributes to global warming. More than half of the world’s population now lives in cities; by 2040 the proportion of urbanites is expected to reach 70 percent, adding urgency to the urban heat island problem.
The Berkeley Lab study found that global land surface temperature decreased by a modest amount—an average of roughly 0.01degrees Celsius, based on an albedo increase of .003 averaged over all global land surfaces. This relatively small temperature reduction is an indication that implementing cool surfaces can be only part of the solution to the global climate change problem, the researchers say. To put the number in context, consider that global temperatures are estimated to increase about 3 degrees Celsius in the next 40 to 60 years if CO2 emissions continue rising as they have. Preventing that warming would necessitate a 0.05 degree Celsius annual decrease in temperature between now and 2070.
Thus, even modest changes should not be dismissed. “Simply put, a cool roof will save money for homeowners and businesses through reduced air conditioning costs. The real question is not whether we should move toward cool roof technology: it’s why we haven’t done it sooner,” says Rosenfeld.
Other Studies Reach Similar Conclusions
Another recent study on cool roofs, led by Keith Oleson at the National Center for Atmospheric Research (NCAR) and published in Geophysical Research Letters, found that if every roof were painted entirely white, the CO2 emission offsets would be approximately 32 Gt for summer and about 30 Gt annually. While the NCAR study used a different model, the calculated CO2 emission offsets are similar to the results from the Berkeley Lab study and provide a useful and independent verification of the expected CO2 emission offsets from increasing the reflectivity of roofs.
Some observers have pointed out that cool roofs do not make sense in cooler climates because of “winter penalties,” since cooler buildings require more energy to heat. However, the energy savings from cooler buildings usually outweighs any increase in heating costs. Furthermore, in winter, there tends to be more cloud cover; also, the sun is lower and the days are shorter, so a flat roof’s exposure to the sun is significantly reduced.
“Cool roofs have worked for thousands of years in the Mediterranean and Middle Eastern cities, where demand for air conditioning is low,” says Akbari. “If you have a cool roof on your house, that will reduce your energy use from air conditioning and it’s a gift that keeps on giving for many, many years, for the life of the roof.”
Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research for DOE’s Office of Science and is managed by the University of California. Visit our website at www.lbl.gov/ [3].
[4]The surface of a black roof (left) heats up 78F above the air temperature, while the surface of a white roof (right) heats up only 12F. Additionally, with a black roof, far more heat flows both to the city and into the atmosphere (arrow lengths are proportional to energy radiated).
Additional information:
- Read the DOE Cool Roofs annnouncement here [5].
- Video glossary entry: Cool Roof [6]
- Read the 2010 paper by Surabi Menon, Hashem Akbari, Sarith Mahanama, Igor Sednev and Ronnen Levinson, “Radiative forcing and temperature response to changes in urban albedos and associated CO2 offsets” [7]
- Download the 2008 paper by Hashem Akbari, Surabi Menon and Art Rosenfeld, “Global cooling: increasing world-wide urban albedos to offset CO2” here [8].
Article printed from Berkeley Lab News Center: http://newscenter.lbl.gov
URL to article: http://newscenter.lbl.gov/news-releases/2010/07/19/cool-roofs-offset-carbon-dioxide-emissions/
URLs in this post:
[1] Image: http://newscenter.lbl.gov/wp-content/uploads/roof-solano-gov-center-CEC.jpg
[2] Image: http://newscenter.lbl.gov/wp-content/uploads/Cool-Pavement-ASU.png
[3] www.lbl.gov/: http://www.lbl.gov/
[4] Image: http://newscenter.lbl.gov/wp-content/uploads/White-Roof-Alliance-single-10.png
[5] here: http://energy.gov/news/9225.htm
[6] Cool Roof: http://videoglossary.lbl.gov/2009/cool-roof/
[7] “Radiative forcing and temperature response to changes in urban albedos and associated CO2 offsets”: http://www.iop.org/EJ/article/1748-9326/5/1/014005/erl10_1_014005.html
[8] here: http://www.energy.ca.gov/2008publications/CEC-999-2008-020/CEC-999-2008-020.PDF
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Daniel H says:
July 19, 2010 at 7:21 pm
“We should also consider putting more humans on rooftops by encouraging more sunbathing.”
Bad idea. Over time, the albedo of humans drops if exposed to sunlight.
FYI, here is a reasonable discussion of roof coatings in cool and warm climates — http://en.wikipedia.org/wiki/Cool_roof
The above has a link to report that evaluates the heating and cooling costs for three roof types in different climates —
http://www.energystar.gov/ia/partners/prod_development/revisions/downloads/roofs/RCMA-CommentLetter-081606.pdf
My word! Those ancient Greeks were soooooo smart! Can you imagine it, two thousand plus years later and we’re still learning things from them. Do you think they were on to warp drive, starship, enterprise technology? They must have been from another universe.
PS: And one other thing, has anyone else observed that humans seem to be getting just a little dumber with each new generation?
On a well-constructed building the roof colour will have little impact on the heating/cooling load of that building. If it does make a noticeable difference, you have too little insulation in the roof/ceiling structure. Where it does make a difference is on the background temperature, and thus the heating/cooling loads of the collection of all buildings in the urban area. In economics parlance, it’s a “public good”. The marginal benefit to each individual property owner of adjusting the albedo of his own property is unlikely to be significant. Retrofitting existing properties is probably not worth it (unless the roof needs to be replaced anyhow).
Most white or light-coloured materials are quite efficient thermal radiators – white radiator paint is especially good – but reflective metal foils are not.
For road surfaces pure white would be too glaring, but ordinary concrete would be a reasonable compromise. Light-coloured ashphalts are also available. They cost a bit more than the common black stuff, but you only need enough to cover the surface to a few millimetres depth.
It would certainly make sense to increase urban albedos this way in hot climates, and decrease them in cold climates. In cities with excessively cold winters and excessively hot summers maximising the amount of greenery would make most sense (because that mitigates both extremes), though thermosensitive coatings and adaptive structures would be fun. It should be understood that the underlying purpose should be simply to make city life more comfortable and pleasant; in no case would it have any significant impact on global temperatures, because cities comprise only a tiny fraction (~0.1%) of the globe.
I live in a temperate clime, owning a flat-roofed house. It did not take Chu to make me paint my ‘wagon’ white. I did it many years ago, basing my decision on the science that I studied during my years at school and university, and can say that it had a great effect on the temperature inside my house especially during the months of July/August. It needs maintaining of course, ’cause the trick (not CRU’s trick in hiding the decline but a real temperature decline) is in the pure whiteness and reflectance of the roof. I m sure that this has also impacted positively on my electricity (aircon) bill and am proud to having contributed to lessening carbon emmissions more than my friend Al Gore did and long before Mr. Gore founded his church of catastrophism and thermogeddon.
“but just a roof of a cooler color, a scenario that is more plausible to implement.”
This is a great statement. Using reasonable scenarios rather than pie-in-the-sky.
Here in Florida it seems to be popular to take off the tile and concrete tile roofs and replace them with shingles, usually darker in cooler. The northerners don’t seem to like the roof maintenance. Then when their houses blow away in a hurricane they wonder why. Also, when they move into canal front neighborhoods they cut down all of the trees (pines, live oaks, cabbage palms, etc) and plant little spindly palms that die when it freezes. I could go on. Sorry, I’m whining.
Paul Birch writes: “On a well-constructed building the roof colour will have little impact on the heating/cooling load of that building. If it does make a noticeable difference, you have too little insulation in the roof/ceiling structure.”
Indeed but you forget climate change. And I am not of course talking about the AGW kind but simply the real one, natural one that has increased the contrast between seasons over the past 30 years through rising atmospheric pressures: colder, sunnier winters and their equivalent in summer. Thus architects have designed homes for a certain climate only to find their buildings performing in a different one. What was appropriately designed then shows some strain and requires a bit of managing.
My complaint is why the Sec of Energy is talking about white roofs in the first place. Why isn’t he talking about the newest innovations in nuclear-plant design that improves vastly on safety, fuel availability (breeds its own fuel) & virtually eliminates radioactive by-products?
Oh wait. The American gov no longer has the courage to research these energy sources. I guess it’s up to the French, Soviets, Chinese & Indians.
TomRude says:
July 20, 2010 at 8:25 am
Indeed but you forget climate change. And I am not of course talking about the AGW kind but simply the real one, natural one that has increased the contrast between seasons over the past 30 years through rising atmospheric pressures: colder, sunnier winters and their equivalent in summer. Thus architects have designed homes for a certain climate only to find their buildings performing in a different one. What was appropriately designed then shows some strain and requires a bit of managing.
________________________________________________________________________
Certainly, architectural design needs to be appropriate for its climate. However, nowhere has climate changed enough to make previously appropriate designs inappropriate. (I don’t know what you mean by “rising atmospheric pressures”; so far as I’m aware there is no evidence of any significant change in atmospheric pressure over historical times).
Whatever the climate, insulation is good. True, it’s more important in more severe climates (eg Arctic or Sahara Desert), but unless you live in a perpetually balmy island paradise, more insulation is still useful and cost effective.
“Today, U.S. Energy Secretary Steven Chu announced a series of initiatives at the Department of Energy to more broadly implement cool roof technologies on DOE facilities and buildings across the federal government.”
This is the glorious productivity of the green econonmy, as promised–the government coloring its roofs, with policy announcements of great solemnity.
Ah. The creation ex nihilo powers of the Federal Gov’t:
Behold:
changing its roof colors,
making solemn policy announcements, and
giving us a glorious green economy based on nonproduction
which “saves the taxpayers money!”
Apologies for the double post, had a computer problem! Sorry you had to see that. 🙂
Paul Birch please read Leroux “Dynamic Analysis of Weather and Climate”, 2010 Second edition, Springer.
I am about to have to see to the roofing needs (water-shedding) of the house I own (Phoenix, AZ), and after a good deal of study, have decided to give new white elastomeric roof coating a try. It is relatively quite inexpensive, in the range of 20 cents per sq ft, 2 coats, for materials, can be easily applied with brush, roller or brush over existing asphalt shingles, and is adequately attractive.
Cheap enough to try. I have grandsons who are more than willing enough to provide the labor.
That is brush, roller or spray-gun
Flat white roof around the Mediterranean; fine. No good to me though. Nothing to do with heat. If you live where you can expect six or seven inches of rain in two hours once every couple of years and three inches in an hour any summer, you are not going to be able to persuade your loving spouse that the flat roof was one of your better ideas.
Reducing urban heat is a desirable. I can buy the white roof idea on that basis, but I can’t buy that white roofing et al can have a global effect. Man’s constructions probably cover less than .001% of the Earth’s surface. We’re supposed to believe that coloring this miniscule percentage of the Earth’s surface area white is supposed to impact global climate?
TomRude says:
July 20, 2010 at 11:33 am
Paul Birch please read Leroux “Dynamic Analysis of Weather and Climate”, 2010 Second edition, Springer.
_________________________________________________________________
Why? What do you claim it says that would conflict with what I said above (most of which is pretty obvious)?
“Equivalent to Getting 300 Millions Cars Off the Road”
The science behind this statement looks really sound. If we offset the heating effect of the emission of roughly 24 Gt of CO2, just by guilding the roof, I think bulldozing the buildings down entirely would certainly get us more than an equivalent annual rate of 1.2 Gt per year.
It is gratifying that Environmental Research Letters has taken les than a decade to catch up with the August 21 2000 issue of Forbes magazine:
http://www.forbes.com/forbes/2000/0821/6605036a.html
Forbes 21 August 2000
Paint the town white
by Russell Seitz
Olin Institiute for Strategic Studies ,Harvard University
Al Gore’s answer to global warming is raising fossil fuel taxes. This may be as irrelevant as it is expensive and intrusive. For the greenhouse effect is not driven by fossil fuel burning, but by the power of the sun. Which is why, when a greenhouse gets too steamy, you whitewash it. This simple, passive solar cooling works so well, regardless of size, that it even applies to that largest of greenhouses, the Earth itself.
The real problem is Earth’s low albedo–it reflects only a small fraction of the sun’s energy because most of its surface is dark compared to a snowfield or a coat of white paint. Such white surfaces reflect three times as much as bare rock or desert soil, and up to ten times as much as deep water or black asphalt paving.
It is easy to calculate how much it would take to counter the retention of heat by greenhouse gases in the atmosphere. A large amount of radiant energy streams in from the sun; a small quantity of the same is trapped by carbon dioxide. The trapping effect comes to 1.62 watts per square meter of the planet’s surface, of which some 300 to 400 milliwatts is due to past fossil fuel burning. That 350 milliwatts is still only a thousandth of the incoming solar energy. Which is why enhancing the reflectivity of a corresponding fraction of the earth’s surface to the brightness of a snowfield would reverse the cumulative environmental impact of the industrial revolution. A thousandth of the earth’s surface comes to 200,000 square miles. Let’s do a little more–250,000 square miles–to allow for the fact that a white surface on the ground is not a perfect mirror into outer space.
Split 6 billion ways, brightening up 250,000 square miles is not such a formidable task: It comes to about 1,000 square feet apiece. Apart from that one-time obligation, we’d have to continue with a small annual dose of brightening to counteract the new damage from today’s fossil fuel burning. This annual obligation would be on the order of a few percent of the one-time cost.
There is plenty of space to install this cheap and passive insurance against climatic bracket creep: Even at a rate of 5,000 square miles a year, it would take centuries to exhaust the supply of treeless badlands. Compared to the draconian economics of global carbon taxation, whitewashing might be something of a bargain.
The greatest benefits would arise from whitening or brightening dark unvegetated landscapes in sunny climates, in effect providing “snow” that would not disappear from rocky uplands with the coming of summer. The bright surface need not be artificial–introducing and propagating any of many species of bright-leafed vegetation, natural or genetically engineered, can have the same effect.
Or we could lighten up the hundreds of thousands of square miles of roofs and roadways that already exist, starting with those expanses of asphalt that already contribute to cities being warmer than the countryside. Titanium dioxide is the best pigment for producing this “White House Effect”: enduring, utterly nontoxic and dirt cheap–one penny’s worth will cover a square foot nicely. China clay, or kaolin, is less efficient as a reflector, but an order of magnitude cheaper. Applying a paint film of either can suffice to raise the reflectiveness of most natural surfaces from 25% or less to 75% or more, at a materials cost on the order of around $250,000 per square mile. America’s annual per capita share of such a global solution would cost as much as a couple of gallons of gas.
Once you understand that the sun power entering our atmosphere is nearly 100,000 times the power of the whole human energy economy, you can comprehend how, at one and the same time, carbon taxation would be both a fiscal enormity and an environmental irrelevance. After two generations of solar-cell R&D it still costs $3 million a megawatt to convert sunlight into electrical power. But $300 worth of aluminum foil will reflect a megawatt of solar energy back into space.
It will grieve the green and frugal alike to learn that some things are cheaper than energy conservation. Just saying no to the passive solar heating of the Earth is one of them.
(Edited by Tim W. Ferguson, now Editor In Chief of Forbes )
The use of light colored surfaces in hotter climates is nothing new and not rocket science. In Florida, virtually all cars delivered to dealer lots are painted with light colors and have interiors of light colored fabrics. Anyone who buys a black car in Florida or similar warm State deserves the thrill of jumping into a microwave oven.
Charles S. Opalek, PE says:
July 20, 2010 at 6:58 pm
The use of light colored surfaces in hotter climates is nothing new and not rocket science. In Florida, virtually all cars delivered to dealer lots are painted with light colors and have interiors of light colored fabrics. Anyone who buys a black car in Florida or similar warm State deserves the thrill of jumping into a microwave oven.
______________________________
Tell me about it.
My pickup is black and the A/C quit a couple of years ago. At least NC is not as hot as SC so I do not burn my hands on the steering wheel if I am not wearing gloves.
I am planning on painting the roof white. At 18 yrs old I do not care what it looks like any more.
Paul Birch wrote: “Why? What do you claim it says that would conflict with what I said above (most of which is pretty obvious)?”
Here is what you said.
“(I don’t know what you mean by “rising atmospheric pressures”; so far as I’m aware there is no evidence of any significant change in atmospheric pressure over historical times).”
I am simply encouraging you to read and learn from a climatologist who knows and explains. It’s worthy of your time.
TomRude says:
July 21, 2010 at 12:07 am
Paul Birch wrote: “(I don’t know what you mean by “rising atmospheric pressures”; so far as I’m aware there is no evidence of any significant change in atmospheric pressure over historical times).”
I am simply encouraging you to read and learn from a climatologist who knows and explains. It’s worthy of your time.
_________________________________________________________________________
What a snarky remark. If this alleged climatologist claims that atmospheric pressure has been changing significantly over the past thirty years, as you imply, then I’d say either he knows something vital that no one else does or he knows squat. My money’s on the latter. It is physically impossible for global atmospheric pressure to change significantly unless there are major net sources or sinks of nitrogen or oxygen. Fossil fuel burning is roughly neutral until the CO2 is sequestered, but would in any case be equivalent to a change in altitude <10cm a year (bear in mind that pressure fluctuations due to weather fronts can be up to the equivalent of ~500m in altitude). A warmer climate could put more water vapour into the air, but this would only amount to the equivalent of ~1-2 m of altitude per centigrade degree (depending on assumptions of average relative humidity, lapse rate, distribution of temperature globally, etc.). This is quite insignificant. There would be no impact on architectural designs unless the change were enormously greater, say equivalent to ~1km. Furthermore, although increasing altitude normally reduces temperatures in line with the adiabatic lapse rates, this would not occur with a global reduction in atmospheric pressure; the base temperature would barely change.
From some posts here, I’m getting the impression that we have to be careful in how much we paint the planet white, ’cause the albedo effect will be so great the planet may freeze. But the cientific truth lies elswhere. Our effect on the planet is irrelevant, and one would easily reach this conclusion when one considers what could have caused such a fast freezing, and re-heating of the planet during the most recent (in geological history) event, that is, the Younger Dryas Period.
Painting one’s wagon white is highly effective, but it stops there.