Geoengineering, adaptation and mitigation, Part 2: White roofs are the trillion-dollar solution

Part I introduced urban heat island mitigation (UHIM). It discussed how lighter colored (or reflective) roofs and pavement, plus urban trees, can save energy, cut CO2 emissions, cool a city, and reduce smog.

But a global “cool roofs” strategy can achieve far bigger benefits — the equivalent of several trillion dollars worth of CO2 reductions — since it can increase the albedo (reflectivity) of the planet, thereby directly reducing the absorption of incoming solar radiation and hence planetary warming. The strategy proposed below “is equivalent to taking the world’s approximately 600 million cars off the road for 18 years.


[100 m2 (~1000 ft2) of a white roof, replacing a dark roof, offsets the emission of 10 tonnes of CO2.]

This is technically geoengineering, although I’d call it geoengineering light or geo-reverse-engineering, since we are mostly undoing the albedo decrease caused by all the dark roofs and dark pavement we have covered the planet with.

A forthcoming article in Climatic Change, “Global Cooling: Increasing World-wide Urban Albedos to Offset CO2,” provides the detailed calculations. A two-page non-technical summary, “White Roofs Cool the World, Directly Offset CO2 and Delay Global Warming,” has been written by two of the country’s leading UHIM experts: Lawrence Berkeley National Laboratory’s Hashem Akbari and California Energy Commissioner Arthur Rosenfeld (coauthors with me on “Paint the Town White–and Green“). I reprint it below:

As the threat of global warming becomes widely recognized, scientists have proposed using geoengineering (manipulation of the Earth’s environment) to quickly respond to this threat. Most proposed geoengineering techniques are novel and unproven. Two simple technologies that have been around for thousands of years, cool roofs and cool pavements, should be the first geoengineering techniques used to combat global warming.
Increasing the solar reflectance of urban surfaces reduces their solar heat gain, lowers their temperatures, and avoids transferring heat back into the atmosphere. This process of “negative radiative forcing” counters global warming. In a recent study to be published in journal Climatic Change, Akbari, Menon and Rosenfeld have calculated the CO2 offset, or equivalent reduction in CO2 emission, achieved by increasing the solar reflectance of urban surfaces.

Most existing flat roofs are dark and reflect only 10 to 20% of sunlight. Resurfacing the roof with a white material that has a long-term solar reflectance of 0.60 or more increases its solar reflectance by at least 0.40. Akbari et al. estimate that so retrofitting 100 m2 (1000 ft2) of roof offsets 10 tonnes of CO2 emission. (For comparison purposes, we point out that a typical US house emits about 10 tonnes of CO2 per year.) Emitted CO2 is currently traded in Europe at about $25/tonne, making this 10-tonne offset worth $250.

It is fairly easy to persuade (or to require) the owners of buildings to select white materials for flat roofs, and in California this has been required since 2005. However, the demand for white sloped roofs is limited in North America, so California compromises by requiring only “cool colored” surfaces for sloped roofs. (This rule takes effect in July 2009, see “California tightens building standards yet again.”)

Use of cool-colored surfaces increases solar reflectance by about 0.20 and yields a CO2 offset of about five tonnes per 100 m2, or about half that achieved with white surfaces. The solar reflectance of pavement can be raised on average by about 0.15, offsetting about four tonnes of CO2 per 100 m2.

Over 50% of the world population now lives in urban areas, and by 2040 that fraction is expected to reach 70%. Pavements and roofs comprise over 60% of urban surfaces (roofs 20 to 25%, pavements about 40%). Akbari et al. estimate that permanently retrofitting urban roofs and pavements in the tropical and temperate regions of the world with solar-reflective materials would offset 44 billion tonnes of emitted CO2, worth $1.1 trillion at $25/tonne.

[Note that the price of CO2 will almost certainly need to exceed $100/tonne in the 2020s if we are going to catastrophic warming (see “Must-read IEA report explains what must be done to avoid 6°C warming“). So the full benefit of this strategy would likely exceeds $4 trillion.]

How can the reader visualize this one time offset of 44 billion tonnes of CO2? The average world car emits about 4 tonnes of CO2 each year. Permanently increasing the solar reflectance of urban roofs and pavements worldwide would offset 11 billion car-years of emission. This is equivalent to taking the world’s approximately 600 million cars off the road for 18 years.


If only roofs are changed from their current dark colors to white for flat roofs and cool colors for sloped roofs, we can offset 24 billion tonnes of CO2. If we take 20 years to implement just the cool roofs portion, it’s the equivalent of taking half of the cars in the world off the road for every year of the 20 year program (see table). The offset provided by cooling urban surfaces affords us a significant delay in climate change during which we can take further measures to improve energy efficiency and sustainability.

Akbari et al. propose an international campaign to use solar reflective materials when roofs and pavements are initially built or resurfaced in temperate and tropical regions. They point out that such an international “cool cities” program is a win, win, win proposition.

Cool roofs reduce cooling-energy use in air conditioned buildings and increase comfort in unconditioned buildings (win #1). Cool roofs and cool pavements mitigate summer urban heat islands, improving outdoor air quality and comfort (win #2). This latest research shows that cool roofs and cool pavements can cool the entire globe (win #3). Installing cool roofs and cool pavements in cities worldwide does not require delicate international negotiations about capping CO2 emission rates.

Part 3 will look at how cool roofs could fit into a green stimulus package.

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25 Responses to Geoengineering, adaptation and mitigation, Part 2: White roofs are the trillion-dollar solution

  1. Tyler says:


    Do you have any idea if a white roof would affect inside heating needs of a home or building in winter months in states and provinces with colder climates?

    I’m guessing if it does it means the building isn’t insulated properly anyway, so it’s an easy thing to deal with.

    [JR: Factor of 10 less heating impact than cooling impact, since the sun is at an angle, it is out less, and it tends to be cloudier.]

  2. Modesty says:

    1. No brainer as far as the building efficiency part is concerned.

    2. WRT the “offset” aspect from the albedo change, I wish they hadn’t called it an “offset”.

    A and R: “Installing cool roofs and cool pavements in cities worldwide does not require delicate international negotiations about capping CO2 emission rates.”

    Famous last words.

    Meanwhile, those who want to thwart the transition to the new energy economy can use the hypothetical numbers from AMR, some small fraction of which will ever be realized, to further delay the transition.

    That is, the actual “onset” caused by the delay this option may help make politically possible, ceteris paribus, may very well dwarf the “offset”.

    But since such onsets are much harder to count than so-called offsets, I guess they just don’t.

  3. Adrian says:

    Are there any proposals to deal with the 40% of surface area covered by pavement? Concrete is relatively light in colour but has lower surface friction (potentially leading to more accidents) and requires a greater initial capital cost to lay down (not sure if it’s cheaper over the expected lifespan than asphalt or not).

    Are there alternatives? I imagine it would be far too expensive to resurface existing roads so are there durable, inexpensive, reflective coatings which can be applied to asphalt?

  4. Scatter says:

    Simple, elegant, cheap.

    What are the effects of PV on albedo / warming? Suppose PV becomes cheap as chips and we cover roofs with that.

  5. Cynodont says:

    For someone who doesn’t even believe in carbon offsets, it’s hard to imagine that you would promote the effectiveness of geoengineering in terms of carbon offset equivalence. White roofs do nothing to reduce atmospheric CO2, anthropogenic GHG emissions, or ocean acidification. They would however have the identical negative effects of any other radiative forcing based geoengineering technique. At least carbon offsets, if effectively verified, actually reduce the rise of atmospheric CO2.

    [JR: My colleagues used a dumb word. The analysis stands on its merits, tho. Offsets, as you use them, are rip-offsets.]

  6. Don Monroe says:

    A linguistic quibble:

    I would avoid the phrase “geo-reverse-engineering,” since “reverse engineering” has a well defined meaning. It does not mean “engineering to achieve the opposite effect,” but “deducing from a finished product how it was engineered in the first place.”

  7. White roofs turn grey from soot. More soot in cities.

    I thought solar panels were supposed to be on these roofs.

  8. Brian Rookard says:

    Obviously, nobody in their right mind would argue that creating energy efficient buildings is a good thing. However, I’m not sure that merely making a roof white will affect the whole building (for example, of a twenty story building). At most, the top floor or two could be heated by the close proximity to the roof.

    The more obvious source of heat flux would be the windows which allow the sunlight in and which acts like a green house. Tinting the windows would seem to have the far greater impact than painting the roof.

    Furthermore, proper insulation would provide a greater boost in efficiency.

    As for pavement … not too sure what to do there … concrete is definitely cooler than asphalt.

  9. Brian Rookard says:

    oops … prior post should read “Obviously, nobody in their right mind would argue that creating energy efficient buildings is a bad thing.”

  10. Eli Rabett says:

    White roofs reflect sunlight in the summer lowering cooling loads. In the winter, assuming that the white material has an emissivity significantly less than one (this is a materials issue, aluminum for example does) it will not radiate as much as a roof with a higher emissivity, which means the heating load is lower. win-win

  11. John Mashey says:

    0) Glass: it of course is much more work & money to replace a lot of windows, and there are useful alternatives: we put in Hunter-Douglas Architella blinds [cellular with metallic inner cells], R ~7, much easier than replacing a bunch of big plate-glass single-pane windows in our house.

    Of course, for new construction and upgrades, life is easier and building codes mandate energy efficiency features.

    “Low-emissivity” windows & films have been around for years, at least some of which came from Lawrence Berkeley Labs.

    California has had rules about this for quite a while – here’s the homepage for Title 24, Part 6 on California’s Energy Efficiency Standards for
    Residential and Nonresidential Buildings
    . For gory detail, see Building Envelope and search for SHGC (Solar heat Gain Coefficient). Also, as input to the 2005 version, here were PG&E’s wishes.

    1) Pavements: See Cool Paving.

    Unsurprisingly, LBNL has studied this also, with references from American concrete pavement association.

  12. msn nickleri says:

    Of course, for new construction and upgrades, life is easier and building codes mandate energy efficiency features.

  13. Mattonym says:

    Thought I’d do a back-of-the-envelope comparison of white-roofs vs. PV-which-eliminates-coal, in response to Scatter, above.
    I used some general estimates, so if I’m off, please offer corrections.

    – – – – – – –
    From the report, painting 100 m2 (1000 ft2) of black roof white, increases reflectance, which has an equivalent effect of a reduction of CO2 emissions of 10 tonnes (if the roof was kept black).

    From Wikipedia, bituminous (thermal) coals produce about 210 lb CO2 per million BTU’s, or (210 lb CO2 per 293 kWh thermal energy).

    Coal plants are also about 30% efficient so the calculation becomes: (210 lb per 293 kWh thermal energy) x 30% electric energy / thermal energy

    …or roughly 210 lb CO2 per 88 kWh electric energy

    Temperate zones get roughly 8 kWh / m2 irradiation per day. For a 100 m2 module with overall efficiency of 10%, per-year production is:
    8 kWh / m2 x 100 m2 x 0.10 (panel efficiency) x 365 = 30000 kWh

    The amount of CO2-from-coal this could displace would be:
    (210 lb / 88 kWh electricity) x 30000 kWh electricity = 72000 lb CO2
    = 33000 kg CO2 = 33 tonnes CO2.

    On the surface then, PV’s in temperate latitudes would be about 3x as effective as painting roofs white… (33 tonnes vs. 10 tonnes CO2-equivalent) …*as long as their installation led to the closing of coal plants.*

    Accounting for the lifecycle energy cost of making the PV array doesn’t change the ratio much. Assuming an EROI of about 2.5 years for the PV and a deployment of 25 years, the effect would be about 10%. PV’s would still be three times as good (two times better) compared to the white-roof alternative. Again, providing their installation displaced coal energy.

    – – – – – –

    Of course, PV’s are likely to be a bit more than 3x as expensive as white paint. :)

  14. Bob Wallace says:

    How about “low risk geoengineering”?

    That would set it apart from blasting chemicals into the atmosphere, seeding the oceans.

    If a problem is discovered just get out there with some dark paint…. ;o)

  15. Bruce says:

    Note to Eli Rabett.

    Low emissivity roofs are not cool roofs. In spite of their high reflectivity, they get almost as hot as dark-colored roofs. That is, there are essentially no cool-roof benefits for aluminum-based roofing products.

    On a different subject, the paving solution proposed (15 or more years ago) by researchers at LBNL was the use of white aggregate in the paving mix. As the asphalt is abraded off of the surface by traffic, the white aggregate is exposed, increasing the albedo of the pavement. I don’t know if this theoretical approach has ever been tested on an actual roadway.

  16. Bob Wallace says:

    Bruce – can you explain why high reflective roofs would be as hot as low-reflective roofs?

    I need another conceptual tool before that makes sense to me.

    I can understand that over time reflective roofs might manage to absorb enough energy to raise their temperatures to that of dark roofs. But that still leaves room for earlier in the heating day coolness. And it would argue for a net decrease in heat passed through to the building below.

    Of course the building heat issue can be solved via either insulation or ample airflow under the roof. The real issue here is about bouncing some heat back out of our atmosphere.

  17. Bruce says:


    Good point about bouncing light and heat waves out of the atmosphere to reduce global warming. It’s possible that low-E reflective roofs do that as well as high-E reflective roofs. There, however, a number of economic reasons for high-E reflective roofs.

    But first, let me try to explain why low-E reflective roofs get as hot as low-reflectivity roofs. In spite of the fact that the reflectivity is high, the low emissivity traps heat and causes the material to warm. Consider, for example a reflective crescent wrench left out in the sun on a hot day – the handle gets hot, really hot. I have been on low-E reflective roofs that are every bit as hot as low reflectivity (dark) conventional roofs.

    Why are low-E, high-reflectivity roofs bad? In the summer, the low emissivity negates the value of the reflectivity to the building owner. From the point of view of commercial building owners, electric utilities, and all of the entities involved in managing electrical transmissions systems, the most important aspects of cool roofs are peak-load reduction in cooling and reduction in peak demand for electricity. Any early-day reduction in cooling loads (if any) would be small and have little economic benefit for building owners and electric utilities.

    Other factors that favor high-E reflective roofs are: (1) the typical location of air-distribution systems in commercial buildings; and (2) the fact that rooftop, packaged HVAC units with air-cooled condensing are mounted on rooftops. Air-cooled condensing is negatively impacted by higher air temperatures, and, at least in California, about 75% of the commercial building stock has roof-mounted, packaged HVAC units with air-cooled condensing.

    The suitability of low-E roofing materials was debated endlessly when the original research on cool roofs was being performed at LBNL and the Florida Solar Energy Center. It was concluded, by virtually everyone involved except for the manufacturers of low-E aluminum-based roof coatings, that low-E roofing materials were not acceptable for cool roofs.

    Yes, building heat loads can be reduced by increasing insulation. However, you cannot ignore the immense existing stock of commercial and residential structures, where, in many cases, a cool roof is less expensive than adding insulation, particularly if the building needs re-roofing. Put most simply, if you’re going to put a roof on a building, it might as well be a cool roof.

    Emissivity and reflectivity are surface conditions so, in my experience, low-E high reflectivity roofs heat up as fast as dark roofs. I never checked monitoring data to verify this assumption, but I have been on a number of roofs with metallic surfaces in the summer in California and I can tell you that they get darn hot long before noon.

  18. Bob Wallace says:

    There seem to be a couple of confused issues here.

    First there is the problem of global warming which is producing climate change. Part of that warming is due to the fact that the Earth now bounces less light (heat) back out into space because we have removed a lot of the stuff that used to reflect light (snow). Joe’s original post was about replacing some of that lost reflectance via white/light colored roofs and pavements.

    Then there’s the problem of undesired solar heating of buildings. A summer problem. It might well be that simply painting a roof white has little value in keeping the interior cooler in the summer. I find no reason to believe that painting an existing flat roof white or installing/replacing light color shingles on a pitched roof isn’t going to bounce away some of the heat striking it.

    I can see that attaching a metal roof, even a shiny one, directly to the structure with no air circulation or insulation might leave the building still hot. But I don’t think anyone is suggesting that we follow that practice. Metal such as aluminum is an excellent conductor of heat and will readily transfer any solar gain to whatever substance with which it is in contact.

    I think there’s a problem with your Crescent wrench analogy. While the surface is shiny, there’s lots of mass and metal is an excellent conductor of heat. If the wrench had a small piece of wood attached the handle for you to grasp could still store as much heat but it would not be passed quickly to your hand by flesh/metal contact. That’s what insulation does.

  19. Bruce says:


    It’s not too confusing. There are indeed two problems – global warming and cooling commercial buildings in the summer. Cool roofs help to address both problems. Metallic roofs help with global warming but don’t keep buildings cooler.

    There are several viable ways of applying a cool roof to a building. Ordinary paint is not adequate. Instead, there a number of white roof coatings that work quite well.

    A fair amount of monitoring has been done on buildings with cool roofs. Much of the monitoring included measurements taken before and after the cool roof was installed. Reductions in peak demand for electricity and electrical energy usage were repeatedly demonstrated. I refer you to LBNL and the Florida Solar Energy Center for information. You might also try Oak Ridge National Laboratory. They have done interesting work on reflective roof coatings. Most of the work at FSEC was performed on residential structures, I think, but the results are applicable to come types of commercial buildings.

    It’s not likely that anyone will install shiny metal roof on a building just to reduce global warming and, in general, shiny metal roofs are rare. On the other hand, synthetic coatings loaded with aluminum particles are very common and are inadequate for cool roofs. I have observed that the exposed surfaces aluminum particles oxidize very quickly. I’m guessing that this significantly reduces their reflectivity, while emissivity stays very low. Not good.

    Of course if you insulate the handle of the Crescent wrench, it won’t get as hot. You have not only insulated the handle – you have also increased the emissivity of the surface. The point is this – the uninsulated Crescent wrench gets hot because the surface of the wrench has low emissivity, in spite of the high reflectivity. If the surface of the Crescent wrench had high emissivity, the wrench would stay much cooler.

  20. Bob Wallace says:

    “…the uninsulated Crescent wrench gets hot because the surface of the wrench has low emissivity, in spite of the high reflectivity. If the surface of the Crescent wrench had high emissivity, the wrench would stay much cooler.”

    Doesn’t the shiny wrench have low emissivity because it is shiny? The more an object approximates a black body the higher the emissivity.

    I guess I’d like to see some data. My common sense (which has been know to be uncommonly wrong at times) tells me that because the shiny wrench reflects heat away it will take longer to warm to hot.

    Given a low emissivity (along with high mass) will mean that heat will be stored longer after the heat source is withdrawn.

    The rapid transfer of stored heat from the wrench to your hand would have to do with conductance. I’m unaware that shiny/black has any relationship to rate of conductivity.

  21. Bruce says:


    As I recall, the low emissivity is more a surface property of metals and is not necessarily restricted to shiny metallic surfaces. I think that the manufacturers of solar collectors have been selling collectors with low-E surfaces for years and these surfaces are not shiny.

    I’ve pretty much forgotten all of the heat transfer that I learned in college, but, as I recall, surface properties are dependent on wavelength. Metallic surfaces are highly emissive at the wavelength of incident solar radiation but have low emissivity at the wavelengths associated with heat radiated at perhaps 140°F.

    I don’t know if the shiny wrench will take longer to heat up, but I’m pretty certain that over the longer time periods experienced by a roof over a single day, the differences between a conventional (i.e. dark and absorptive) roof and low-E roof are pretty insignificant.

    You are correct – as soon as you skin comes into contact with the wrench, conductivity takes over and the surface properties are no longer a factor. And yes, because of the low emissivity, heat will be stored longer, which in the case of low-E roofs, means that more of the heat will transferred into the conditioned space.

  22. The physics of reflectance, absorptance, and transmittance of light is very well understood. Each material (let us assume opaque for the remainder of this note) has a reflectance that depends on wavelength of the light.

    The incoming solar radiation in United States is about 3 to 5% UV (wavelength of 200 to 350 nm), 43% visible (350 to 700 nm) and 52% near infra-red (700 to 2500 nm). Over 99% of the incoming solar radiation is in the range of 250 to 2500 nm. When one is referring to solar reflectance and solar absorptance of materials, it refers to an average value of those properties over the range of 250 to 2500 nm.

    When the light is absorbed by a surface it makes the surface hot and the surface emits that energy back to the environment. The characteristics emitted radiation has a wavelength of 10,000 nm. When one is referring to thermal reflectance and thermal absorptance of materials, it refers to an average value of those properties over the range of 8000 to 20000 nm. A low-E material is a surface that has low thermal emittance (= thermal absorptance) and a high thermal reflectance. In principal, solar reflectance and thermal emittance are independent of each other.

    The reason that metals are shinny is that it reflects light specularly.

    In conclusion: A white surface stays cooler under the sun than a shinny metal surface with the same solar reflectance. The reason is that a white non-metallic surface has a high thermal emittance (it emits the thermal radiation freely) and a metallic surface has a low thermal emittance. To emit the thermal radiation, the temperature of the metallic surface will rise to a level that it can emit the absorbed solar radiation. The lower the thermal emittance, the higher the surface temperature.

  23. shop says:

    In conclusion: A white surface stays cooler under the sun than a shinny metal surface with the same solar reflectance. The reason is that a white non-metallic surface has a high thermal emittance (it emits the thermal radiation freely) and a metallic surface has a low thermal emittance. To emit the thermal radiation, the temperature of the metallic surface will rise to a level that it can emit the absorbed solar radiation. The lower the thermal emittance, the higher the surface temperature.

  24. wookiemeister says:

    a quick explanation of why white paint works to cool the planet

    1 around 50 percent of sunlight is visible light

    2 around 50 percent of sunlight is heat

    3 visible light is a frequency of light NOT absorbed by the atmosphere

    4 some HEAT (infra red) from the sun IS absorbed by the atmosphere directly (greenhouse gasses which makes the atmosphere warmer).

    5 HEAT from the sun also warms the surface of the planet and this HEAT is then radiated back into the atmosphere again being absorbed by greenhouse gasses.

    6 visible light is poorly absorbed by the atmosphere, some of it gets reflected back by the atmosphere.

    7 the visible light that doesn’t get reflected back by the atmosphere is either reflected by the surface (eg a snow field) or absorbed by the surface (an asphalt carpark/ road). this absorbed light heats the dark surface. this heat is then radiated into the atmosphere and is absorbed by greenhouses gasses makiig the atmosphere warmer.

    8 by painting your roof white and other surface you reflect around 50 percent of the visible energy from the sun, a significant amount of this is reflected back into space WITHOUT heating the atmosphere.

    9 by painting enough roofs you make buildings cooler and cool the atmosphere. air conditionig works easier to cool houses because they are cooler, perhaps you wouldn’t even need air conditioning?

    if you are thinking of using white paint to cool your roof you will need a special white insulative paint that both reflects the visible frequencies of light and stops infra red frequencies from heating your roof.

    the main thing to understand is that white paint reflects visible frequencies that would otherwise be transformed into heat (infra red) that is absorbed so readily by the atmosphere. stuff painted with white paint would still get warm but a significan percent of the suns energy would be reflected back into space without heating the atmosphere.

  25. senior hamburger says:

    so what happens when all the light reflected from the roof goes back into the atmosphere and heats up the planet more, rather than being absorbed by black roofs?

    [JR: You’re not serious, I assume. Black bodies radiate back in infrared — which is what CO2 traps!]