Cooling the Asphalt Jungle (green rooftops)

[SoCalDem]

http://www.miller-mccune.com/science_environment/cooling-the-asphalt-jungle-1671

Cooling the Asphalt Jungle



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

By: Enrique Gili | December 11, 2009 | 05:00 AM (PST)

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.


snip

[NJmaverick]

1. I would love to see every roof covered in grass and solar panels

[Xithras]

11. The biggest problem tends to be weight.

Most truss roof systems on homes are only designed to hold about 20lbs per square foot on top of their regular roofing weight. My house actually has a label in the attic warning that the maximum permanent load on the truss tops is 18 lbs per square foot, and the stringers can only hold 12 lbs per square foot. And my home was built 15 years ago when construction standards were higher. New houses in new subdivisions have even narrower margins of error.

Have you weighed a square foot of wet earth lately? Covering your average home with 8-10 inches of the stuff, planting it with grass, and then soaking it through (as would happen in the rain) would simply cause most homes built in the past 50 years to collapse.

If you don't believe me, talk to a structural engineer who does residential work. They'll confirm it.

[caraher]

2. Doesn't their being on buildings limit the "sink" they provide?

What happens when you tear a building down? The rooftop garden would most likely end up as compost, right?

A green roof is probably much better than tar for a number of reasons, but I don't think you provide much long-term carbon storage the way a forest might. The cooling effect is going to be far more important. From the article linked in the OP:

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.

But then there's this:

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.

[SoCalDem]

3. true, but it's very unlikely that there will be a forest anytime soon in downtown Detroit

and once a building comes down, even the vacant lot is probably more "green" than the building... but while it's there, the building can help a little by having a green roof..:)

[Laughing Mirror]

5. But there's already red foxes there

The red fox is carving out a place of its own deep into downtown, joining the ranks of raccoons, skunks, opossum, white-tailed deer and red-tailed hawks finding homes in untended lots, houses and buildings in the rusting one-time car capital. ....

...
Many neighborhoods have so few remaining houses that adjoining lots resemble small prairies and woodlands, and Detroit's extensive freeway system and old railroad connections linking the inner city to less populated areas are now serving as routes for wildlife.

http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=389x6097099

[SoCalDem]

6. Isn't it amazing how resourceful the wild critters can be?

All they need is for humans to leave them alone:)

[madokie]

4. Green roofs huh

I worked at a foundry once and a foundry uses lots of sand and this one transported a lot of that sand across the roof in pipes. Sand being abrasive as it is and all there were a lot of leaks and piles of sand here and there so I dropped a few seeds in one of them and the plants got 6 ft tall before anyone noticed that that tree, we had willows grow up there, was not a willow after all. So they jerked my little attempt to green the roof up by the roots. :rofl:
just had to tell the little story

[SoCalDem]

7. you rascal..

It always amazes me how plants can spring up in odd places.. I have tried several times to "rescue" a palm tree trying to grow in a crack in concrete, but when I transplant it & coddle it, it always dies:( ungrateful bastards:grr:

[kestrel91316]

8. Palm trees (Mexican and American fan palms) are ACCURSED WEEDS

in this area. I was glad when a tree company cut one down in my neighborhood - those fronds are lethal when they come down in a strong wind - serrated knifelike edges 6 ft long!

[SoCalDem]

9. Our neighbor sold one he had @ $350.00 a ft.. It was VERY tall

After selling the tree, he & his wife moved to a new house:( The damned tree couldn't have been on OUR side of the fence??? Would that have been too much to ask :rofl:

There are actually "palm wranglers" who drive around looking for fantastic trees to buy...so they can be "installed" at "nicer locations" aka. rich people's places :)

[eppur_se_muova]

12. And there are palm rustlers, who do the relocation without paying.

Unfortunately, this happens to endangered plants, like Saguaro cactus, as well.

[bridgit]

10. I like it! And systems are coming online to maximize space available

http://www.inhabitat.com/2009/02/09/synthe-green-roof-by-alexis-rochas/