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The Greening Literally of a Columbia Rooftop
Cynthia Rosenzweig

One of the participants at the recent Columbia 250 symposium "Earth's Future: Taming the Climate" was Cynthia Rosenzweig, a research scientist with the Goddard Institute for Space Studies. A few years ago, Rosenzweig carried out an assessment of the impact of climate change on the 31-county New York metropolitan area, the first of its kind to be done on an urban area. Her report, co-written with William D. Solecki, explored the potential local effects of global warming. Rosenzweig identified a number of "heat islands" -- industrial areas such as Camden , N.J. , and Long Island City that are far hotter than other areas. Lately, she has been exploring one possible way of mitigating the heat island effect: by literally turning rooftops green. -- Peter Kobel

PK: How does the green-rooftop concept work? Is this a new idea?

Rosenzweig: It's a new idea for New York , but it's not a new idea in many cities around the world. There are a lot of green roofs in Germany . They've had some of them there for 40 years. There are also green roofs in Canada and in Japan . In this country, there are some green roofs in Portland , Seattle and Chicago . And two places that are doing research on it are Penn State and Michigan State .

The beauty of green roofs is that they solve multiple environmental problems in cities. Let's take the energy balance. Instead of having a black tar roof that absorbs radiation and reradiates it at night, the green roof absorbs the radiation, and evaporates the water and uses some of that energy to evaporate water. And evaporating water is a cooling process.

PK: Are you talking about a kind of grass that wouldn't need very much water?

Rosenzweig: It is actually not grass. The current plant of choice is sedums, which are adapted to semi-arid environments. They are succulents, and they are very sturdy. They are dependent on natural rainfall. And here is another thing: the vegetation is not grass, and the soil is not soil. It is a soil-like material that is very light. The technology has evolved to have very light, almost like lava rocks known as "tuff." So the German companies have helped with the technologies, and there are companies in the States that have created the technology for the waterproof membranes, the layers of the soil and the plantings so that it doesn't leak through. So it is basically a vegetative roofing material.

PK: And you want to test this concept here at Columbia?

Rosenzweig: What we want to do is not just to have a green roof, but to use monitoring instruments. So we know how much it is cooling, how it is changing the temperature in the room underneath, and how it is changing the energy use of the building.

Because if it is cooler on the roof, then you don't have to pay so much in the summer for air-conditioning. Therefore you are lowering your energy cost and you are lowering your greenhouse gas emissions, because you are using less fossil fuel. So you see how it helps with the urban heat island effect, it helps with the cooling, it helps with the costs, and it helps with the greenhouse gases.

PK: Where and when might this happen?

Rosenzweig: Pace University has said it would put a green roof on its building right near the Brooklyn Bridge . So there is much excitement about getting a green roof going. But what we want to do at Columbia is demonstrate the functions so it is not only a matter of having a green roof at Columbia , but also of doing the research. We are talking with Facilities about buildings in the Manhattanville area. We'd like to start this summer. Wouldn't that be great, to have one at Columbia ? It would be so fantastic!

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Published: June 07, 2004
Last modified: Jan 10, 2005

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