Menus Subscribe Search

Cooling the Asphalt Jungle

• December 11, 2009 • 5:00 AM

As green roofs proliferate to cool and cheer cityscapes, might they also suck up and store some of the carbon urban life pumps out?

The asphalt jungle is due for a makeover as tar beach becomes a sanctuary for native plants, wildflowers and winged pollinators. Like mushrooms after a spring rain, “green roofs” are proliferating on rooftops across the United States and throughout Europe, gaining adherents among sustainable design advocates intent on creating more livable and greener cities.

While rooftop gardens have been a part of city life since the 19th century (if not earlier), their environmental benefits are just beginning to be fully realized. As global temperatures creep upwards, scientists are glancing at the skyline, looking for ways to cool down concrete-bound cities and the planet. One proposal has been to install white roofs, which would reflect solar heat and require less energy to cool urban areas. Another idea is to absorb — or sequester — heat-trapping gases like carbon dioxide by using rooftops as yards.

In a first-of-its-kind study, researchers at Michigan State University have calculated the carbon sequestration benefits extensive green roofs can provide. Findings from horticulturalists Kristen Getter and Brad Rowe in October’s Environmental Science & Technology revealed green roofs’ potential as carbon sinks.

During photosynthesis, plants remove carbon dioxide from the atmosphere and store CO2 in the leaves, soil and root system, converting sunshine into carbon-based compounds such as carbohydrates and sugar. According to Environmental Protection Agency statistics, U.S. forests sequestered 637 million metric tons of the carbon dioxide emitted by made-sources such as coal, fuel and natural gas. Urban forests sequestered on average an additional 74 million metric tons. (All told, the U.S. offsets about an eighth of the carbon it produces, and the vast majority of the offset comes from forests.)

Currently the job of large-scale carbon sequestration is performed in the vast storehouse of the Earth’s ocean and forest ecosystems that play an integral part in regulation of the temperature of the atmosphere.

While there have been studies on how much heat green roofs might fend off, how well a green roof would store carbon had been uncertain up until Getter and Rowe’s study.

Two experiments were run to measure the potential of storing carbon in green roofs. The first involved eight green roofs in Michigan and four in Maryland ranging from 1 to 6 years of age. The second involved planting an extensive green roof of 20 1-square-meter plots at MSU’s campus in East Lansing.

All the green roofs were planted with Sedum, a genus of leafy succulent known for its hardiness and often used as ground cover. “We planted what we knew would grow,” said Getter.

Over a two-year period, the plants on the East Lansing campus were periodically harvested. Leafy parts stored on average 168 grams of carbon per square meter, the roots and the soil respectively stored 160 and 300 grams on average. Combined, each plot had the capacity to store 375 grams of C02 per square meter.

The researchers estimated the city of Detroit has 219 acres of roof space available for conversion. If black tar roofs were retrofitted, 55,000 tons of CO2 could be removed from the air — enough CO2 to offset the carbon emissions of 10,000 mid-sized SUVs or trucks for an entire year, they calculated.

“Implementing a green roof strategy would definitely be one way of managing the sequestration of carbon,” said Getter. And yet … in order to offset man-made carbon emissions it would require a Texas-sized green roof to make a significant contribution to carbon sequestration.

“Green roofs certainly don’t store the kind of carbon that a forest or productive grassland stores, but a traditional roof is essentially a wasteland – no carbon storage whatsoever,” Getter wrote via e-mail.

Jim Mumford, a horticulturalist turned entrepreneur, is dubious about the amount of carbon sequestration a green roof provides. Perceiving it as an added but minor benefit, “it’s debatable about how much of a carbon sink it really is,” he said.

Still, he’s in total agreement there has to be more green and less tar on city rooftops. In 2007, he retrofitted his office with a 478-square-meter green roof, the first of its kind in San Diego, if not the state.

As the founder of GreenScaped Building, he’s completed construction on nine green roofs in San Diego County and has several more projects under proposal throughout California and the United States.

Instead of talking meters and grams, Mumford is faced with the challenge of turning data into a business plan. He believes retrofitting commercial space with green roofs in Southern California is cost-prohibitive, especially in the current business climate. For the time being, “building has ground to a halt,” he said.

Once the market revives, the next challenge he faces will be building large-scale green roofs at a price real estate developers and building owners will pay. Green roofs cost typically twice as much to install as conventional roofs.

Nonetheless, he’s intrigued. “The more I looked at it the more excited I got,” he said. Mumford envisions a day when installing a green roof will be part of a comprehensive plan to create buildings that are both energy efficient and conserve water.

He’s assembled living walls and demonstrated water-caching systems on the grounds of his business complex, transforming his workplace into an “open laboratory” for water and energy conservation strategies, and seeking cost-effective ways to capture rainwater rather than rely on drinking water to irrigate green roofs.

Burnishing a building with a living skin has several environmental advantages. Most notably, it cuts down on storm water runoff, reducing the energy costs associated with heating and cooling buildings, and extends the material lifespan of roofs exposed to the elements.

Inside his office, Mumford has noticed a marked difference. White noise has been reduced. He’s saved 23 percent on his electric bills. And rather than redoing his roof every 10 to 20 years, he believes his green roof can last up to 60 years if maintained properly.

A study by the Berkeley Lawrence Lab found that if 15 percent of the buildings in Los Angeles installed reflective or green roofs, daytime temperatures would be reduced by 3 degrees Celsius — saving Los Angeles half to 1 gigawatt of power during peak-use hours.

“There is a tremendous movement in other cities,” said Mumford as he rattled off the names of cities where interest in green roofs has grown exponentially. That has to be good news for keeping cities cooler and green-thumbed entrepreneurs.

Sign up for our free e-newsletter.

Are you on Facebook? Become our fan.

Follow us on Twitter.

Add our news to your site.

Enrique Gili
Enrique Gili lives, writes, and surfs in Ocean Beach, Calif. He covers the intersection of environmental issues and pop culture for regional magazines in the Southland and beyond. Send your thoughts, ideas, or tips to gili92107@gmail.com.

More From Enrique Gili

A weekly roundup of the best of Pacific Standard and PSmag.com, delivered straight to your inbox.

Recent Posts

August 26 • 4:00 PM

Marching in Sync May Increase Aggression

Another danger of militarizing the police: Marching in lock step doesn’t just intimidate opponents. It impacts the mindset of the marchers.


August 26 • 3:03 PM

The Best Reporting on the Federal Push to Militarize Local Police With Riot Gear, Armored Vehicles, and Assault Rifles

A few facts you might have missed about the flow of military equipment and tactics to local law enforcement.


August 26 • 2:00 PM

How the Other 23 Percent Live

Almost one-fourth of all children in the United States are now living in poverty, an increase of three million kids since 2005.


August 26 • 12:00 PM

Why Sports Need Randomness

Noah Davis talks to David Sally, one of the authors of The Numbers Game: Why Everything You Know About Soccer Is Wrong, about how uncertainty affects and enhances the games we watch.


August 26 • 10:00 AM

Honor: The Cause of—and Solution to—All of Society’s Problems

Recent research on honor culture, associated with the American South and characterized by the need to retaliate against any perceived improper conduct, goes way beyond conventional situations involving disputes and aggression.



August 26 • 8:00 AM

The Transformative Effects of Bearing Witness

How witnessing inmate executions affects those who watch, and how having an audience present can also affect capital punishment process and policy.



August 26 • 7:15 AM

Being a Couch Potato: Not So Bad After All?

For those who feel guilty about watching TV, a new study provides redemption.


August 26 • 6:00 AM

Redesigning Birth Control in the Developing World

How single-use injectable contraceptives could change family planning in Africa.


August 26 • 4:15 AM

How Gay Men Feel About Aging

Coming to terms with growing old can be difficult in the gay community. But middle-aged men are inventing new strategies to cope.


August 25 • 4:00 PM

What to Look for In Dueling Autopsies of Michael Brown

The postmortem by Michael Baden is only the beginning as teams of specialists study the body of an 18-year-old African American killed by police in Ferguson, Missouri.


August 25 • 2:00 PM

Thoughts That Can’t Be Thought and Ideas That Can’t Be Formed: The Promise of Smart Drugs

Are we asking the right questions about smart drugs? Marek Kohn looks at what they can do for us—and what they can’t.


August 25 • 12:00 PM

Does Randomness Actually Exist?

Our human minds are incapable of truly answering that question.


August 25 • 10:31 AM

Cesareans Are Still Best for Feet-First Babies

A new study confirms that surgery is the safest way to deliver a breech fetus.


August 25 • 10:00 AM

What Can Hurricanes Teach Us About Socioeconomic Mobility?

Hurricane Katrina wrought havoc on New Orleans but, nine years later, is there a silver lining to be found?


August 25 • 8:00 AM

How Low Voter Turnout Helps Public Employees

To a surprising degree, as voter turnout goes down, public employee compensation goes up.


August 25 • 6:00 AM

Beyoncé Isn’t an Anti-Feminist Terrorist

A new book called Staging the Blues shows she’s embracing a tradition of multi-dimensional stardom, rather than one racist trope.


August 25 • 4:00 AM

A Tale of Two Abortion Wars

While pro-life activists fight to rescue IVF embryos from the freezer, pregnant women in their third trimester with catastrophic fetal anomalies have nowhere to turn.


August 22 • 4:00 PM

The Invention of the Illegal Immigrant

It’s only fairly recently that we started to use the term that’s so popular right now.



August 22 • 2:00 PM

What Can U.S. Health Care Learn From the Ebola Outbreak?

A conversation with Jeanine Thomas, patient advocate, active member of ProPublica’s Patient Harm Facebook Community, and founder and president of the MRSA Survivors Network.


August 22 • 1:22 PM

Two Executions and the Unity of Mourning

The recent deaths of Michael Brown and James Foley, while worlds apart, are both emblematic of the necessity for all of us to fight to uphold the sanctity of human dignity and its enduring story.


August 22 • 10:00 AM

Turbo Paul: Art Thief Turned Art Crime Ombudsman

There’s art theft, there’s law enforcement, and, somewhere in between, there’s Turbo Paul.


August 22 • 8:00 AM

When Climate Change Denial Refutes Itself

The world is warming—and record-cold winters are just another symptom.


Follow us


Subscribe Now

Being a Couch Potato: Not So Bad After All?

For those who feel guilty about watching TV, a new study provides redemption.

How Gay Men Feel About Aging

Coming to terms with growing old can be difficult in the gay community. But middle-aged men are inventing new strategies to cope.

Cesareans Are Still Best for Feet-First Babies

A new study confirms that surgery is the safest way to deliver a breech fetus.

The Impossibility of the Night Shift

Many night workers get “shift-work sleep disorder.” And no one knows how to treat it.

How the Brains of Risk-Taking Teens Work

There's heightened functional connectivity between the brain's emotion regulator and reason center, according to a recent neuroscience paper.

The Big One

One in two full-time American fast-food workers' families are enrolled in public assistance programs, at a cost of $7 billion per year. July/August 2014 fast-food-big-one

Copyright © 2014 by Pacific Standard and The Miller-McCune Center for Research, Media, and Public Policy. All Rights Reserved.