Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Paint your roof white to reduce global warming

  1. Jul 28, 2009 #1

    Ivan Seeking

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    http://www.timesonline.co.uk/tol/news/environment/article6366639.ece

    That should read that it would yield the same benefit as removing all of the cars for 11 years...

    https://www.youtube.com/watch?v=pLlxjYACa5U
     
  2. jcsd
  3. Jul 28, 2009 #2
    I'm surprised (and somewhat skeptical) that the effect would be that significant. As for painting roads how about using concrete instead of asphalt.
     
  4. Jul 28, 2009 #3

    Ivan Seeking

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Given that this comes from Chu, I'll make a leap of faith. If nothing else, it would probably be significant to the heat island effect.

    One concern that occurred to me wrt using concrete instead of asphalt is the amount of CO2 produced. I believe that concrete processing accounts for a significant portion of our CO2 output. But I don't know how much CO2 is produced by the processes related to asphalt.
     
  5. Jul 28, 2009 #4

    Ivan Seeking

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Last edited by a moderator: May 4, 2017
  6. Jul 28, 2009 #5

    russ_watters

    User Avatar

    Staff: Mentor

    It's certainly a useful idea, and it is near becoming a standard on commercial buildings, but I'm curious about this statement:
    It is very curious to talk about time in that context - if you're going to talk about 11 years of cars off the road, you also need to talk about the time the roofs and roads are exposed to the sun. Or is that supposed to say if they make roads and roofs white, they'd save in 1 year what taking all cars off the road for 11 years would save? Strange that Chu would make such an obvious error in his key point and not correct it.

    I'd be interested in seeing the analysis of that, in any case.

    ...link is dead, by the way....
     
    Last edited: Jul 28, 2009
  7. Jul 28, 2009 #6

    russ_watters

    User Avatar

    Staff: Mentor

    I'm not capable of judging albedo from a photo like that! Too many different colors!
     
  8. Jul 28, 2009 #7

    Pengwuino

    User Avatar
    Gold Member

    Maybe he got tripped up and just said it incorrectly. Any calculation would obviously be done on a cars/year kinda figure and he'd be smart enough to not make a crazy calculation like that (which wouldn't even make sense to calculate)
     
  9. Jul 28, 2009 #8

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    Concrete freeways have been a requirement in LA for >10years because of this, a friend did a PhD on the effect at Caltech
     
  10. Jul 28, 2009 #9

    LeonhardEuler

    User Avatar
    Gold Member

    Actually, I've heard him repeat this claim in almost exactly the same words on the Daily Show with Jon Stewart, and I had the same confusion as russ at the time:

    http://www.thedailyshow.com/watch/tue-july-21-2009/steven-chu
     
    Last edited: Jul 28, 2009
  11. Jul 28, 2009 #10

    russ_watters

    User Avatar

    Staff: Mentor

    So I suppose we can assume that he meant we can leave the cars off the roads for 11 years and it'll save as much energy as if we painted all the roads and roofs white and left them that way for the next 4 billion years, until the sun burns out? :uhh:

    Clarification needed!
     
  12. Jul 28, 2009 #11

    Pengwuino

    User Avatar
    Gold Member

    Wow really? That's weird... pretty big mistake to make repeatedly if its accidental.
     
  13. Jul 29, 2009 #12
    The biggest carbon offset from painting your roof white is a reduction in energy expended for cooling. A lot of energy is used to heat and cool buildings. Living roofs are even better, they provide insulation, evaporative cooling, particulate scrubbing, and an environment for butterflys and other native fauna.
     
  14. Jul 29, 2009 #13

    sylas

    User Avatar
    Science Advisor

    It is sensible to compare a number of years without cars with a number of white roofs. A number of years without cars is a certain amount less CO2, which is a forcing. So too, a certain number of white roofs is a rise in albedo: also a forcing. Here's a guesstimate.

    The idea is this. I'll calculate a forcing, in W, from painting a roof white.

    Then I'll calculate how roughly how much CO2 we get from cars, in eleven years, and make that a forcing. This should give a rough idea of how many roofs. Each step will involves back of the envelope estimates, so this should only give a very crude idea if the magnitude of the comparison is sensible. I'm doing this on the fly; I don't know what answer I'll get yet.

    1. Effect of painting a roof white

    Using some albedo numbers from the book Boundary layer climates, found online, I'll say that the albedo of a tile roof is 0.2; and that it can be made 0.9 by painting white.

    I'll go with a roof size of around 150 m2; a medium sort of house, I think.

    I'll assume that the solar incoming flux at the surface is 184 W/m2, using the energy flow diagrams from other threads here, based on Trenberth and Kiehl.

    Thus the change in energy flux for a white roof is 184*(0.9-0.2)*150 = about 20000 W; rounding to one figure.

    Effect of removing cars

    A quick google suggests to me that roughly 20% of CO2 emissions are from the transport sector, and about half that is from cars. So about 10% of the rise in CO2 levels might be from cars.

    CO2 levels are rising at a bit over 2ppm per year, taking eleven years without any cars means roughly 2ppm CO2. The forcing for that, using a conventional formula I've described in these threads a number of times, is 3.7*log2(387/385) = 0.028 W/m2.

    How many roofs?

    The surface area of the Earth is about 5.15*1014 m2. Hence the forcing in Watts for the cars is about 1.4*1013 W.

    Divide by the impact of a white roof... and I get 7*108 roofs... 700 million.

    Now that is pretty crude, but it indicates an order of magnitude. Chu spoke of roads and pavements as well as roofs. I would think that home roofs in the USA only would be more like 70 million; but if all other buildings and pavement is included, the comparison seems roughly credible. I don't know the basis of Chu's estimate.

    Cheers -- sylas

    PS. I think skyhunter is probably correct though; the most important factor is probably efficiency and domestic energy use, rather than albedo.
     
  15. Jul 29, 2009 #14

    LeonhardEuler

    User Avatar
    Gold Member

    I see what you're saying, sylas, that makes a lot more sense now.
     
  16. Jul 29, 2009 #15

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    The effect is even larger if you consider the electrical power needed to cool a building with AC that is recieving an extra 20KW of heat input.
     
  17. Jul 31, 2009 #16
    Its true that manufacture of concrete releases large amounts of CO2 when CaCO2 is calcined into lime (CaO). When asphalt is produced from the bottom of the oil barrel in refining it contains the largest percentage of carbon in the product. Therefore, asphalt reduces CO2 that would otherwise be released if all of the oil was burned.

    I agree with others regarding the CO2 reductions due to a significant energy savings from reduced cooling loads using lighter color roofs.
     
  18. Jul 31, 2009 #17

    turbo

    User Avatar
    Gold Member

    When I bought this place, it had a dark asphalt-shingle roof and it was a bear to cool even with a big 220V AC unit. I covered the shingles with 1" thick Styrofoam insulation and shiny silver galvalume standing-seam roofing. Now we can keep the place very cool with just a couple of small portable 120V AC units that vent through windows. Maine is a temperate climate, but if you applied that same type of insulation and roofing to houses all across the deep south from coast to coast, the savings in electricity for AC could be tremendous. It would be a great idea in SoCal, especially in areas susceptible to wildfires. BTW, the same material comes pre-painted if you want, and the roofing contractor can certainly get it in white or other light colors if you don't want a shiny roof.
     
    Last edited: Jul 31, 2009
  19. Jul 31, 2009 #18
    I've thought about this for years now, wondering why it wasn't a standard. I just assumed that black had some beneficial factor during the winter months that I wasn't aware of.
     
  20. Jul 31, 2009 #19

    Ivan Seeking

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

  21. Jul 31, 2009 #20

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    And, of course, white roofs never grow black mold and algae or get dirty from dust and pollution. We're supposed to whitewash our roofs how often? How's that going to look with my http://www.vchouseplans.com/" house?

    Great idea, Chu! Brilliant!:rolleyes: I spent a fortune on that roof......
     
    Last edited by a moderator: Apr 24, 2017
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Paint your roof white to reduce global warming
  1. Global Warming (Replies: 35)

  2. Global warming (Replies: 1)

  3. Global Warming (Replies: 2)

  4. Global warming (Replies: 5)

  5. Global Warming (Replies: 2)

Loading...