The challenge is that New York City is an "urban heat island"--that is, it traps heat in, creating a higher temperature than in the surrounding suburbs due to all that pavement, which captures and throws off too much heat. And since impervious surfaces retain absolutely no moisture, there's no way to cool the city down. Also consider that on hot summer nights, those surfaces retain heat after the sun fades away, so they're already hot in the morning when it comes back up, thereby compounding the problem.

Green roofs act as an antidote to this problem by keeping the city cooler. Rooftops provide large empty spaces--perfect for plantings--but as the biggest heat-retaining culprits, they also need to be singled out for change. Implementing sky gardens, roofs can reflect urban heat, tranpire, and create shade. They can keep indoor areas cooler in sumer (thus requiring less energy for air conditioning) and can even help insulate the same spaces in winter. In fact, with enough green gardens, we could significantly reduce stress on the energy grid, which could easily become overtaxed due to global warming. (Remember those novelty blackouts in California, kids? Yeah, well, it's not that far away.) Green roofs also retain water, which
is excellent news, since runoff not only collects pollutants on its way to the ground, it can also overwhelm the sewer sytem, causing it to overflow into the city's waterways. (And you don't even want to know that that happens about one out of every two times it rains. Ew. Gross.) Furthermore, green roofs provide air filtration and also beget yet more space for growing a greater number of plants.

It should be noted that these aren't your regular old, I'm planting my garden in a container becasue I move every two years type of gardens. These gardens are bona fide, though they can be installed on a regular rooftop. Lightweight engineered roofing systems make it possible to allow expansive vegetation to thrive while still protecting the integrity of the underlying roof. The greenery is then free to do its job.

In some ways, New York can already be considered an enviro-minded city (think about it: we take public transport, walk, and use far less gas than the average American), though Chicago
and Tokyo have definitely already beat us to the rooftop garden punch. But with Gotham Greening on New York's side, we figure there's still time for us to dominate. After all, it'll always be the Big Apple, only now, it'll be green. ::Greening Gotham [by MO]