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Green Roof Tops Cool our Cities Down by Paul Rivers for ClimateYou

New York City, like other urban areas worldwide, struggle with the urban heat island effect (UHI). This phenomenon takes advantage of the energy absorption by building materials and results in a warmer environment in city areas compared to surrounding rural districts. This problem fundamentally lies in the physical characteristics of city building materials which are made to hold in heat and makes the outside areas of buildings warmer.  Hotter conditions makes for more  heat related health risks. However, these deleterious health conditions can be avoided by covering rooftops with living plants and vegetation, lessening the effects of UHI significantly. These green roofs, as they are commonly known, have proven to cool down cities while providing significant reduction in air pollution, and offer evaporative cooling, and even economic benefits.


The US Postal Service has a huge green roof on top of their Morgan mail processing facility in NYC  http://wirednewyork.com/forum/showthread.php?t=6963

First of all, what are green roofs and how are they made?

Hanson & Schmidt, authors affiliated with the Brooklyn Botanical Garden, found that greenroofs were most commonly layered vegetative systems, with clearly outlined designs and types. Two of the basic systems they identified are either extensive or intensive: extensive is a system in which soil reaches a depth of 2-6 inches, weighs 10-45 pounds / square foot, and costs roughly $20-$25 / square foot. Plants grown on extensive systems are generally limited to Sedum varieties, which are drought tolerant, ground covering plants. Extensive roofing systems are shallower than intensive ones and only support small size plant varieties, but provide thermal benefits at the cheapest price. Intensive systems are deeper at up to two feet in depth, heavier at 45-200 pounds / square foot, require high-load bearing roof capacities, and are more expensive at $30> / square foot. While more expensive, these systems can support larger plants, and a wider variety of vegetation. With the specifications these authors provide, it should be easier for residential, commercial, or governmental customers to pick the green roof that best suits their needs.

Green roofs can mitigate air pollution in urban environments directly though their plant layer, and indirectly by reducing the need for electric utilities. According to Vijayaraghavan, a researcher at the Indian Institute of Technology Madras, plants can directly consume gaseous pollutants through their stomata. Green roofs also reduce the underlying issue of power plant emissions. By reducing the need for building cooling systems like A/C, green roofs lower energy needs. Besides lowering electricity bills for consumers, green roofs lessen the power needs from utilities, and corresponding emissions for electricity production plants. This can yield economic benefits in air pollution and health costs. Researchers affiliated with the University of Michigan provide an economic measure of these benefits, specifically looking at pollutants like nitrous oxides. They reference a study by the U.S Environmental Protection Agency (EPA) and Office of Air and Radiation, which used a metric of premature deaths from chronic bronchitis in the Eastern United States to quantify reductions in air pollution. They found that fewer cases of chronic bronchitis translated into an economic benefit between $1680 and $6380 adjusted for 2006 dollars. As well, the researchers calculated that green roofs provide an annual NO­­X (nitrous oxide) uptake benefit of $895 – $3392 for a 2000 square foot vegetated area. Quantifying air pollution benefits in this way allows the interested consumer to easily connect investment information and potential health benefits.

By investigating the environmental, health, and economic benefits of greenroofs citizens concerned about rising temperatures can begin to address the impacts of UHI and request informed changes from local governance.

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