Efficacy of cool roofs varies from city to city

July 27, 2011 by Julie Chao
This map shows afternoon temperature changes in the three summer months resulting from increasing the surface reflectivity in urban areas. As expected, urban areas in California and the northeast experienced cooling. Some rural areas in parts of the southeast saw temperatures go up due to surface feedback.

(PhysOrg.com) -- While cool roofs and pavements have been found to cool the planet by preventing energy from being radiated back into the atmosphere, previous studies have not accounted for atmospheric feedbacks that may result from changing the surface reflectivity of urban areas. A new study from Lawrence Berkeley National Laboratory (Berkeley Lab) breaks new ground by using a high-resolution model of the continental United States that incorporates land-surface feedback to probe the effects of deploying light-colored roads and rooftops.

Berkeley Lab researchers Dev Millstein and Surabi Menon found that atmospheric feedback—such as changes in cloud cover or precipitation—does have an important effect, resulting in different amounts of cooling in different cities, but that cool roofs and are still beneficial for combating global warming. Their results were published in the journal Environmental Research Letters.

“Although further studies based on varying assumptions are required to validate our results, our modeling indicated cool roofs are not necessarily as effective in a city like Dallas as in a city like Los Angeles,” said Millstein, a postdoctoral fellow in the Atmospheric Sciences Department. “In places near Dallas and parts of the southeast, the absence of summer cooling is associated with less rainfall and more sun reaching the surface—fewer clouds and more sun, basically. Still, no major urban area saw any significant warming due to feedback effects.”

This study used the same assumptions as that of a previous Berkeley Lab study, in which the average albedo (solar reflectance) of all roofs was increased by 0.25 and of pavements by 0.15. However, the model used in this study had a higher degree of complexity than that used in previous studies of cool roofs thanks to continental scale, fine spatial resolution, feedback effects and more years of data. The researchers used the Weather Research and Forecasting model, with a domain that spans the continental United States and has a resolution of 25 square kilometers, allowing it to calculate changes in individual cities. It was run over a 12-year period using weather data from 1998 to 2009.

They then used the model to investigate an opposite scenario, darkening the albedo in southern California to simulate the installation of 1 terawatt of photovoltaic arrays in the Mojave Desert, enough capacity to power the entire country at noontime. Although such a deployment is several orders of magnitude larger than current solar developments in the United States, Millstein noted that a project of similar scale has been considered in the Sahara Desert to power Europe.

Again, the researchers found significant and consistent feedback effects to the solar arrays, including changes in wind patterns several hundred kilometers away. However any changes to climate at the continental scale were obscured by year-to-year variability. “Some years it decreased the amount of radiation reflected back to space and some years it increased, and that’s because we had the feedback effects,” Millstein said. “Without the feedback, you’d always see a penalty, or heating. That doesn’t mean it’s not there. We could see the benefits of cool roofs, but it’s not easy to see the penalties of desert-based photovoltaics.”

More reflective surfaces, such as cool roofs and pavements, reflect radiation back into the atmosphere and into space and thus help cool the planet in two ways. At the scale of individual cities, they can combat the urban heat island effect, and at a continental scale, they can combat global warming. Of course, in air-conditioned buildings, cool roofs can also help lower energy bills by decreasing the need for air conditioning.

On the local scale, this study validated previous studies finding California and the greater northeast of the U.S. as good candidates for cool roofs. Cities such as Los Angeles, Detroit and New York saw summer temperatures drop by 0.30 to 0.53 degrees Celsius. “Half a degree Celsius makes a big difference in terms of air quality,” Millstein said.

As for the southeast, some rural areas in Oklahoma, northern Texas and parts of Louisiana and Florida saw increases in temperature whereas cities either stayed the same or cooled slightly. But because temperature affects the chemistry of the , causing higher ozone levels and more smog, cool roofs can still play an important role in improving air quality. “Even when you take feedback into account, cool roofs are still beneficial for most places,” Millstein said. “With the exceptions, there may be more study needed. The southeast is certainly not ruled out as a candidate for cool roofs.”

On the global or continental scale, the findings also confirmed the benefit of brightening and pavements. “Even with the feedbacks from decreasing clouds in certain locations, we still had more reflection overall,” Millstein said.

For each square meter of cool roof surface deployed, the increased reflectivity is equivalent to offsetting 175 kilograms of carbon dioxide. For the continental U.S., it would achieve a one-time offset of 3.3 gigatons of CO2, or about half of total U.S. emissions in 2009.

The researchers’ next step will be to study the role of air pollution in weather patterns and investigate how photovoltaics and other forms of renewable energy may be used to reduce air pollution.

Explore further: Global model confirms: Cool roofs can offset carbon dioxide emissions and mitigate global warming

More information: “Regional climate consequences of large-scale cool roof and photovoltaic array deployment,” Environmental Research Letters iopscience.iop.org/1748-9326/6/3/034001/

Related Stories

Hyperlocal Effects From A Changing Climate

August 6, 2010

Cities are made chiefly of concrete and asphalt, which soak up more sunlight during the day than soil and have a harder time radiating the heat away during the night. Add to that all the energy -- natural gas, electricity, ...

Recommended for you

Rush hour pollution may be more dangerous than you think

July 21, 2017

The first in-car measurements of exposure to pollutants that cause oxidative stress during rush hour commutes has turned up potentially alarming results. The levels of some forms of harmful particulate matter inside car cabins ...

Mountain glaciers recharge vital aquifers

July 21, 2017

Small mountain glaciers play a big role in recharging vital aquifers and in keeping rivers flowing during the winter, according to a new study published in Geophysical Research Letters, a journal of the American Geophysical ...

A super-algae to save our seas

July 20, 2017

Coral reefs are our most diverse marine habitat. They provide over US$30 billion to the world economy every year and directly support over 500 million people. However, they are vulnerable with climate change impact models ...


Adjust slider to filter visible comments by rank

Display comments: newest first

1 / 5 (5) Jul 27, 2011
Although further studies based on varying assumptions are required to validate our results, . . .

Are solar models, assumptions, and manipulation of experimental data and observations at the base of the global warming scandal and current economic collapse?


1. Eisenhower warning on 17 Jan 1961



2. News report on 21-28 Feb 1972


3. What serendipity has revealed on recent history (1945-2011):



With kind regards,
Oliver K. Manuel
Former NASA Principal
Investigator for NASA
1 / 5 (4) Jul 27, 2011
3. What serendipity has revealed on recent history (1945-2011):

3 / 5 (1) Jul 27, 2011
High albedo "Cool" roof and road surfaces are generally not thought of as a way to combat global warming, because it would be so much work for so little effect. However, it is great for the 'heat island' affect in southern cities, like phoenix, where it is hot already, and gets even hotter due to all the asphalt and dark roofs.

By preventing energy from being radiated back into the atmosphere

Of course the energy is being radiated back into the atmosphere. Think about what you're saying. We want to radiate it back at a wavelength that it is not ABSORBED by the atmosphere, and pass harmlessly back into space. I know what they are saying, but it should definitely be worded for clarity.
1 / 5 (1) Jul 28, 2011
High albedo "Cool" roof and road surfaces are generally not thought of as a way to combat global warming, because it would be so much work for so little effect.
... he says, without offering any proof. Did you read this http://www.physor...649.html ?
not rated yet Aug 02, 2011
It the 100 largest cities in the world...

You do realize that the work it would take to have this affect would be decades of work for several industries, and once you have the effect, that's it. It's a single notch on the indicator. Once it's in, there's little you can do to increase the effect, while CO2 going into the atmospher is a cumulative process.

I think you underestimate the enourmity of the work.

And I just mentioned every other part of that study and supported using this process.

You're an idiot barakn

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.