October 04, 2008
Painting Rooftops White Would Delay Global Warming

White paint can cool the Earth.

It turns out that a 1,000 square foot area of rooftop painted white has about the same one-time impact on global warming as cutting 10 tons of carbon dioxide emissions, he and his colleagues write in a new study soon to be published in the journal “Climatic Change.”

As sunlight pours down into Earth’s atmosphere, some of the energy is filtered out or bounces off clouds. About half the energy shines through as visible light and some of that hits the tops of houses. If a roof is white, most sunlight reflects back into space and doesn’t heat the earth. But if a roof is a dark color, the sunlight converts to heat rather than bouncing off as light. That thermal energy then radiates off the roof back toward space, where it is trapped by CO2 in the atmosphere, and then absorbed by this greenhouse gas. As a result, the world’s thermometer reads just a little higher than it did before.

If the estimated 360,000 square miles (less than 1 percent of the world’s land surface) covered by urban rooftops and pavement were a white or light color, enough sunlight would be reflected back into space to delay climate change by about 11 years, the study shows.

Put another way, boosting how much urban rooftops reflect, called albedo (al-BEE-doh) in scientific terms, would be a one-time carbon-offset equivalent to preventing 44 billion tons of CO2 from entering the atmosphere, Akbari says. It’s about the same as taking all the earth’s automobiles off the road for 11 years, the study’s authors say.

So if global warming becomes a problem we've got a cheap low tech thing we can do to delay the effects. The cooler buildings in tropical and temperate zones will cost less to air condition as well.

On the other hand, our clean energy sun is absolutely spotless. If it stays spotless then we'll need to paint everything black instead.

Sept. 30, 2008: Astronomers who count sunspots have announced that 2008 is now the "blankest year" of the Space Age.

As of Sept. 27, 2008, the sun had been blank, i.e., had no visible sunspots, on 200 days of the year. To find a year with more blank suns, you have to go back to 1954, three years before the launch of Sputnik, when the sun was blank 241 times.

"Sunspot counts are at a 50-year low," says solar physicist David Hathaway of the NASA Marshall Space Flight Center. "We're experiencing a deep minimum of the solar cycle."

But if photovoltaics become really cheap in the next 10 years (and I expect they will) then people will put PV on their rooftops and the PV material will absorb more light than white-painted roofing. What will be the net climate effect of massive PV installation? The PV electricity will displace some usage of fossil fuels. But the PV might lower the Earth's albedo (i.e. lower the amount of light reflected into space).

Planting trees also lowers the Earth's albedo because trees are often darker than the surface without trees. But trees gather solar energy and can be burned in place of fossil fuels as well.

Use of whiter materials for laying pavement offers another way to raise the Earth's albedo that won't compete with PV for the Earth's surface.

Share |      Randall Parker, 2008 October 04 11:05 PM  Climate Engineering

K said at October 5, 2008 2:48 AM:

The observation that whiter roads wouldn't compete with PV is a good point.

It might be better to cover large areas of deserts with a reflective material. I don't know much about deserts but the Gobi and Sahara should be good candidates if anyplace is. One drawback is that most desert is light colored already, so the albedo might not rise much.

simon said at October 5, 2008 6:02 AM:

Randall- The global warming story is dead. The scientific research is clearly not supporting the assertion that we are experiencing global warming. I would urge you to stay away from pop science notions such as the one you are now presenting as worthy of attention by intelligent humans. If you are focusing on the anti-science crowd please note in the title.

Russ said at October 5, 2008 8:09 AM:

Simon: so long as there are large-scale arctic melts, the story isn't dead. Second, your comment has got some serious concentrated troll DNA.

Guess what? Me and my buddies have been discussing just this issue, earlier in the week.

Randall Parker said at October 5, 2008 8:45 AM:


I accept that there are large uncertainties regarding the future. One can't scientifically absolutely prove or disprove that global warming is going to happen because scientists can't construct models that accurately simulate all relevant factors.

Consider just a few of the sources of unknown:

1) We do not know when each fossil fuel's production will peak or how rapidly the production will decline. Have we already peaked in oil production? Or can we get oil up higher for a few more years? How much has horizontal drilling made declines sharper once they start happening?

2) What is the sun going to do?

3) How much of the warming we've experienced in recent decades is:

A) part of a longer term warming since the end of the last ice age.

B) Just a fluctuation around a long term average.

C) Due to human influences.

3) How fast will non-fossil fuel energy sources become cheap enough to displace fossil fuel energy sources?

Since I do not know the answers to these and many other relevant questions I think we should consider the possibility that we'll need to cool the Earth.

On the other hand, since I do not know the answers I also think we should consider the possibility that we'll need to warm the Earth.

Jake said at October 5, 2008 10:11 AM:

Painting rooftops white will only work in areas where winter time heating is not needed. In the rest of the US, it would require more fuel to heat the buildings during the winter.

Brett Bellmore said at October 5, 2008 10:20 AM:

Actually, the way houses are constructed in areas with serious winters, outside air freely flows between the underside of the roof and the top of the ceiling insulation, so any heat picked up in heating weather by a dark roof only heats outside air. They do it this way so that the interior of the roof won't get so hot in the summer that the shingles melt.

Randall Parker said at October 5, 2008 10:48 AM:


First off, you get a lot more sun on a building in the summer than in the winter due to the length of the days. The net benefit in heating and cooling depends on your winter and summer temperatures, latitude (further north have much shorter winter days), and how cloudy your area is in winter and summer.

Second, a nanomaterial that can have its albedo adjusted would help optimize heating and cooling.

Third, since one typically has a lot of roof insulation the roof is a lousy way to get needed heat in the first place. One needs PV or a water-based solar heat collector to make use of sun heat. Sun heat is most useful in the spring since the days are getting longer. If you are far enough north to have cold springs then maybe a solar heat collector on the roof could make sense. But if you are in Florida then a white roof probably isn't going to raise your heating bills.

Fourth, this analysis is global. There are a lot more people near the equator in Brazil, Indonesia, and India than in the US. White roofs in those places would cost little to nothing in lost heating potential. Also, those places get far more sunlight than the bulk of the US. So they have more light to reflect.

Fifth, I'd really like to see China, India, and other developing nations build with whiter road material.

simon said at October 5, 2008 11:57 AM:

Russ- Look in the mirror or better yet just close your eyes and see your inner troll (Don't mess with DNA testing);)

Randall - The uncertainties you raise are legitimate but do not really address the question of warming or the need for cooling. A quick look at recent temperatures suggests we are experiencing cooling in a magnitude far greater than the "painting solution" you report. Going a step further, we see that uncertainty is more likely to favor cooling than warming. As for trends (geological), the long term trend is cooling.

Randall Parker said at October 5, 2008 1:07 PM:


I think you are whining. In the original post I said (and note the if):

So if global warming becomes a problem

I went on to point out the lack of sunspots. I've previously posted on how the Earth might cool over the next 10 years or so. I think opponents of the global warming theory can go just as far overboard as some proponents do.

I present methods to do climate engineering and have a category archive for these methods. Why? Because if we can do climate engineering then we are better equipped to respond to either natural or man-made causes of climate change. This report shows a cheap way to do climate engineering. That's good news.

TH said at October 5, 2008 4:21 PM:

from the article:

"...Selling the idea

Geoengineers have had similar ideas: covering the Sahara with enormous sheets of white plastic, for instance, or painting the Black Hills of South Dakota white..."

The fact that the author of the article chose to include this leads me to think the issue of global warming has surpassed the ability of its proponents to just plain function in a normal fashion.

Nick G said at October 5, 2008 6:29 PM:


The albedo/solar absorption of PV is unimportant. Remember, the alternative sources of electricity are heat engines, which reject heat equal to 100%-300% of the heat converted to power - that's a lot of heat pollution. Compared to that, the change in albedo is trivial.

I find geoengineering attractive also, but I think it's a mistake to focus on global "warming". Excess CO2 is the main problem, and the consequent ocean acidification is perhaps our biggest threat.

Rogue Warrior said at October 5, 2008 7:32 PM:

Ocean acidification from CO2 is a mental trap that doesn't hold water. Your minds cannot grasp what happens to carbon in the oceans, so you fall back on a simpleton's way of viewing things. That's okay. You really have no choice given what mother nature has given you to work with. But propagating unsupportable myths is no way to live. You spend your time and resources chasing your tails until you're exhausted.

There are some bright ones out there, I've seen some of your comments. Randall I know you are bright enough to understand the issues, if you are not too lazy or too easily swayed by "common knowledge." Can you get beyond what you have always believed?

Nick G said at October 5, 2008 7:40 PM:

"Ocean acidification from CO2 is a mental trap that doesn't hold water"

Would you care to explain that? Provide some detail, and sources?

Meek and Peaceful Friend said at October 6, 2008 1:08 PM:

What the rogue was too arrogant and rude to provide was a link to a scientific approach to ocean pH. Shame on the rogue.

As a meek and peaceful friend, I am willing to provide such a link, with no strings attached: http://seafriends.co.nz/issues/global/acid2.htm

Do not believe the rogue, that your mental abilities are inadequate to comprehend the mystery. You can do so much more than you believe.

Peace, friends.

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