The Easy Fix That Isn’t

Touted as a simple way to combat climate change, white roofs may actually increase global warming, according to a new Stanford study. 

NNSA/flickr.

Installing white roofs (or painting them white) has been promoted as a way to help slow global warming. New research shows that white roofs may actually add to global warming.

By Alyson Kenward

If you’re interested in staving off climate change without trying too hard, painting your roof white seems like a complete no-brainer. It’s far cheaper than trading in your SUV for a Prius, and it turns the laws of physics to best advantage. Dark roofs absorb sunlight that heats up your house, office tower, or apartment building. That means you’re bound to crank up the energy-intensive air conditioner to keep pace in the summer months — and since electricity in the U.S. comes largely from fossil fuels, the net result is more heat-trapping greenhouse gas emissions, and more global warming.

But a white roof does just the opposite. It bounces sunlight right back into the sky, just as light clothing helps you stay cool in the summer. Cooler buildings need less air conditioning, which translates to fewer emissions of heat-trapping gases. That’s why Energy Secretary (and Physics Nobel prizewinner) Steven Chu endorsed the idea back in 2009 and why cities like New York and Philadelphia have launched white-roof projects.

Unfortunately, what seems obvious is not always true, and a new study available online and soon coming out in the Journal of Climate reveals some potentially bad news for white roofs. When Stanford University engineer Mark Jacobson, and his grad student John Hoeve modeled the total climate response to white roofs and other urban surfaces, they found the lightening may actually cause more global warming.

Here’s why: the sunlight that bounces off white roofs doesn’t all fly out into space. A lot of it is absorbed by particles of soot and other dark-colored pollutants that float around in the atmosphere (those same particles are already responsible for a good portion of global warming). The particles heat up, just like your house would have, and the net result is a warmer atmosphere. You house might be cooler, but it would be at the expense of heating the planet.

In short, says Jacobson in a press release: “There does not seem to be a benefit from investing in white roofs. The most important thing is to reduce emissions of the pollutants that contribute to global warming.” So much for trying to take the easy way out.

On the other hand, says Jacobson, there is another way to use your roof in the fight against climate change: cover it with solar panels. The panels intercept sunlight before it hits the roof, so your house doesn’t heat up so much. They don’t bounce the light back into the atmosphere where it can heat up soot particles. And they generate at least some electricity without emitting greenhouse gases. It’s not quite as cheap as painting your roof. But unlike that feel-good solution, it’s actually likely to be effective.

This article originally appeared on Climate Central.

  • Mark

    I look forward to reading Dr. Jacobson’s study, but I’m a bit perplexed by the article. I haven’t heard of people “painting” their roofs white, although much construction now uses “cool roofing” materials (which are not always white) to lessen the need to cool the building. Rather than a “feel good” solution, this strategy actually does reduce the need to air condition, and when the electricity to air condition is supplied by fossil-fired power plants (as most still is) the cool roof reduces carbon emissions.

    The question not addressed in the article is how those reductions balance against the warmer atmosphere that Dr. Jacobson claims in his study. Does the article discuss or quantify that?

  • Mark

    That is, does the study (not the article) discuss or quantify that?

  • Alyson

    Mark,

    First off, there are groups/people out there that take the simple step of painting their roofs white to help improve cooling (check out this New York City initiative. But as you point out, there are other ways to install cool roofs too.

    The research article does talk a bit about the how the cool roofs can influence emissions (eg. if less air conditioning is needed) but in this study, he didn’t consider that in his calculations.

    However, Jacobson points out something that people don’t often hear about with cool roofs. Yes, these roofs help keep buildings cool in the summer (so they use less A/C) but they also keep them cool in the winter, so more heating is needed during cold months. Other studies have shown that white roofs increase winter heating more than they decrease summer air conditioning, so in some cases white roofs can actually lead to higher emissions overall.

    • Mark

      Thanks, Alyson. Yes, the location of the building matters a great deal. Obviously, there will be far more benefits in Phoenix than there will be in Seattle.

  • http://www.surviveclimatechange.com Jon Flatley

    I would think another solution wouldn’t have drawbacks and this is planting vegetation on roof tops. Although it could be a hassle keeping the plants watered, it could be maintained like any other public building that has plants – such as in a fancy hotel lobby.

    I’d think it would be win-win since evapotranspiration would lower the percentage of solar radiation that goes into heating and there would be no significant reflection back into the atmosphere. In the winter, the plants (if deciduous) would lose their leaves and allow the welcome winter sunshine through.

  • Surabi Menon

    The recent paper by Stanford’s Mark Jacobson and John Ten Hoeve (2011) on urban heat islands and cool roofs is a useful contribution to the literature. However, their results regarding white roofs are preliminary and uncertain. Along with our own work at the Lawrence Berkeley National Lab, other published papers have addressed the broader benefits of white roofs. In our view, these studies taken together raise important issues that need to be considered from a policy standpoint to fully understand the potential of more reflective (white or cool) surfaces.

    Jacobson and Ten Hoeve note that reflecting light from white roofs may lead to a decrease in cloud cover, thereby increasing, not decreasing, the urban heat effect. But they also note that their findings might change if they used different models. This is an ongoing research area not only for their group, but others, and ours as well. The findings should not be considered settled.

    We have found that white roofs do provide a low-cost solution that can help buildings reduce energy costs, in a wide variety of climates, as well as cool the atmosphere regionally and globally. We have also found disadvantages. The reflective roofs may cause unwanted glare, for example, and may modestly increase heating costs in winter. But answers to these issues are exactly the ones we’re working hard to find.

    Our work has shown that reflective roofs can lead to better air quality, reduce the strain on our electrical grid, improve comfort and decrease emissions from power plants. These are important considerations when evaluating all the available research.

    In our opinion, all of these arguments and studies suggest that selective use of white and other reflective roofs makes sense as part of an integrated strategy for more sustainable human existence on Earth. But the potential benefits offered by cool roofs do not diminish the need for sustained reductions in greenhouse gas emissions to control global climate, or the need for increased use of renewable energy sources.

    It’s important for the public to understand that scientific debate leads to better science. But it’s also important that the public receives—and the news media delivers—a message that properly conveys research news with all its many caveats and cautions. It’s not settled, until it’s settled.

    For more information, go to: http://heatisland.lbl.gov/news/cool-roofs-and-global-cooling-heat-island-group-responds-jacobson-ten-hoeve-2011

    Surabi Menon and Ronnen Levinson
    Berkeley Lab Heat Island Group

  • Alyson

    I appreciate the comments from LBNL — it’s helpful to point out that previous research, as commented above, has found that there are other, non-emissions benefits to cool and white roofs. And in terms of emissions, it also appears that location has an important influence on whether white roofs improve or reduce energy efficiency (when you look over an entire year).

    However, prior to Jacobson’s study there’s been little (mainstream) discussion as to whether white roofs could be anything but beneficial. His contribution helps show the science isn’t yet settled, and I agree with you that is also an important point to make here.