Stuart Gaffin is a climate researcher at Columbia University and a regular contributor with his blog “Exhausted Earth”. Thomson Reuters is not responsible for the content - the views are the author’s alone.

It seems that every few months I appreciate a new environmental benefit to green roofs. Two that have recently impressed me are: (i) the realization that green roofs are ideal new locations for urban weather stations (as opposed to traditional asphalt roofs with their extreme temperature biases); and (ii) a burgeoning urban beekeeping movement may be a new synergy to tap into with green roofs.

Siting weather stations in urban areas has always been a tricky endeavor. Issues like security and extreme localized heat sources (e.g. asphalt, vehicles, heating and air conditioning sources) are primary concerns. For these reasons, the National Weather Service stations are usually sited in urban parks or airports. Nevertheless many urban weather stations are still located on rooftops, but they can be suspect because of the temperature biases of dark roof membranes which can easily reach 176 degrees F (80 C). Green roofs completely remove the temperature biases of rooftops as they are essentially meadows in the sky! You can look at some of the comparative temperature data at my station ‘dashboards’ (see research stations on right-hand side).

If the number of such green roof weather station locations grow, this will improve data on true micro-climate variations within cities. A recent publication of mine about this is at this link. 

And then there is the growing interest in urban beekeeping that is perhaps part of the urban agriculture movement here and here. A number of city-dwellers love the idea of keeping hives and harvesting honey, etc.

The worrisome bee colony collapse syndrome, makes this interest in urban beekeeping even more important. I’m pretty sure that any urban beekeepers and their bees would prefer to have a green roof near their hive. The ones we have installed in New York City look to me like pretty strong oases of bee activity.

The recent photo (right) shows one of our native grassland green roofs that was just brimming with bees recently. Top left is a wider shot of the native roof planting.

The symbolic color associated with environmentalism is obviously “green.” 

From ‘green movement,’ ‘Green Party,’ ‘green collar jobs,’ to ‘Greenpeace,’ the color reference is to plants, chlorophyll, the green pigment central to photosynthesis, which is the basis of all life. Quite often, however, the chief environmental goal being advocated has little to do with plants, but rather promoting low-impact technologies, practices and lifestyles.

This is the case with “green building design” which is receiving growing attention because of the under-appreciated magnitude of building emissions worldwide. Recently, New York City audited the source of all its CO2 emissions and found that nearly 80 percent is from building energy consumption. Worldwide the estimate is closer to 45 percent, making “buildings the biggest single contributor to anthropogenic climate change - a worse offender than all the world’s cars and trucks put together.”

The vast bulk of green building design focuses on efficient heating, cooling, lighting,  insulation and window technologies. All of these are great things of course, but what’s not mentioned in the Nature article is a truly ‘green’ building technology - living green roofs and living walls. These are technologies that introduce plants into building facades, especially rooftops.

Typical dark rooftop temperatures in summer sunlight can reach extraordinary levels of 150 degrees F (~65 degrees C) or more. It makes little sense not to address such an extreme building heat source in green building design. Moreover, in cities, rooftops, among other dark, impervious surfaces like streets and parking areas are a chief contributor to the “urban heat island” effect which elevates the temperature and climate in cities well above surrounding suburban and rural areas. This extra heat can be deadly during heatwaves, especially if air-conditioning is not available or fails during blackouts. Resident proximity to high-floors and rooftops was a risk factor in both the deadly 1995 Chicago and 2003 European heat waves.

Green roofs harness the unrivaled power of plants to remain cool when exposed to sunlight during the summer, as I showed in my last post. There is an ever-increasing palette of plant choices available, but the most common and reliable are “sedums.”  They are members of the succulent family of plants and are very tough in surviving all sorts of weather conditions, including extreme heat and droughts.  The plants grow in a porous medium that is usually a few inches deep, depending on the building structural support.

Green roofs give back to the urban environment in multiple ways however. Along with urban heat source reduction they are great at reducing storm-water runoff from buildings as well. This runoff leads to a ubiquitous problem in cities known as “combined sewer overflow” where combined sanitary and runoff water is released to nearby waterways because of limited water treatment capacity. Green roofs can also create new ecological habitats that are generally limited in urban areas. It’s estimated that New York City may have 10 or more times the area of Central Park available for green roof adoption. 

Green living walls, including ivies and vines, are a more challenging technology and will take longer to develop into a common building practice. But already fascinating potential properties of living walls are being noted, such as how in summer they can provide shade to cool buildings down, while in the winter they will allow light to penetrate and warm the building.

If these green building facade technologies catch on, along with traditional street trees, plants will partially re-conquer the urban spaces they originally occupied, and make cities much more liveable for us in the process.