If you were an alien arriving on earth for a visit, what would you see? If you came in over one of the mega-cities that increasingly cover the earth’s surface, roughly a third of your field of view would be rooftops. And beneath them would be a lot of concrete, asphalt, glass, and metal.
Now I am a big fan of all these materials. They allow me to live in a level of comfort and ease that would be unimaginable to our forbears of even a few centuries ago. But as the planet becomes increasingly dominated by human influence and built environments, a pressing challenge is how to construct those environments in ways that are not only comfortable and convenient for us, but also maintain as much harmony as possible with the rest of the global ecosystem on which we depend, in terms of energy use, waste production, and suitability for other life forms. An especially critical frontier is achieving this harmony in urban environments, which are likely the way of the future for humanity. Although the human imprint is most intense in cities, they actually create a lower per capita ecological footprint than sprawled-out suburbia because of the concentration and efficiency of services, reduced per-capita resource use on travel, etc.
So back to the visiting aliens. One of the interesting recent trends toward greening urban habitat has been the proliferation of “green roofs”, i.e., roofs planted with live vegetation (check out the gallery of varied and very cool green roofs at Eco-geek). The basic idea is far from new — roof gardens were popular among rulers of ancient civilizations. Indeed the hanging gardens of Semiramis, in modern-day Syria, were considered one of the seven wonders of the world. These people were only a handful of human lifetimes away from a life of hunting and gathering, and perhaps were the first humans to experience the vague discord of separation anxiety from nature.
But why focus on roofs now? One reason is that, because roofs make up such a large fraction of the horizontal surface area in cities, they are important mediators of the heat and water budgets of urban environments. As summarized in a recent paper by Oberndorfer and colleagues, such roofs provide a variety of ecosystem services in cities, including more effective management of storm-water runoff, regulation of building temperatures, reduced urban heat-island effects, reduced vulnerability to fire, sound insulation, and better habitat for plants, insects, and animals that might otherwise be excluded from highly urbanized areas. If built with sufficient load capacity, they can also increase available living and recreation space in cities (see photo at top).
For example, the graph at right shows how retention of storm-water runoff was enhanced in a green-roof test plot in Canada. The green roof had 15 centimeters of growing medium (soil, approximately) and was planted with plain old lawn grasses (I wonder if it needs mowing). In every month other than June, the green roof retained a large fraction of the water that would otherwise run off, as compared with an adjacent conventional roof of the same size.
Green roofs also provide insulation that buffers buildings from temperature fluctuations. The graphs at left show heat flux across roof membranes for different roofing systems in Canada (positive and negative values are flux in and out of the house respectively). The graphs show results for a summer (top) and winter (bottom) day. The green roof strongly reduced temperature fluctuations, especially in summer.
Germany led the way in modern green roof use, beginning in the early 1900s for the specific purpose of reducing exposure damage to roofing materials. Today, the German government provides incentives for green roof construction and it’s estimated that about 14% of new roofs under construction there will be vegetated. There are course many challenges to growing such gardens in thin soil atop buildings where they are exposed to harsh conditions of desiccation and wind. Ideal candidate species are native stress-tolerant plants that are already adapted to local conditions. One of the most ambitious green-roof projects in the US is the recently built roof on the California Academy of Sciences building, which mimics the natural hilly contours of the San Francisco Bay area and is planted with native vegetation (see a cool video of the roof being built here).
But green roofs are more than engineering projects. Oberndorfer and colleagues look at them as mini-ecosystems and emphasize how understanding the ecological interactions among the soil, microorganisms, and vegetation (and even goats?!) influence the ecosystem’s functioning and the benefits provided to us, one of which is that they help maintain habitat for other species:
“Green-roof habitats show promise for contributing to local habitat conservation. Studies have documented invertebrate and avian communities on a variety of living-roof types in several countries (Coffman and Davis 2005, Brenneisen 2006, Kadas 2006). Green roofs are commonly inhabited by various insects, including beetles, ants, bugs, flies, bees, spiders, and leafhoppers (Coffman and Davis 2005). Rare and uncommon species of beetles and spiders have also been recorded on green roofs (Brenneisen 2006, Grant 2006). Species richness in spider and beetle populations on green roofs is positively correlated with plant species richness and topographic variability (Gedge and Kadas 2004). Green roofs have also been used by nesting birds and native avian communities (Baumann 2006). Rare plants and lichens often establish spontaneously on older roofs as well (Brenneisen 2006, Köhler 2006). These findings have mobilized local and national conservation organizations to promote green-roof habitat, particularly in Switzerland and the United Kingdom. Further-more, these results have encouraged discussion of green-roof design strategies to maximize biodiversity (Brenneisen 2006).” (References to cited literature are available in the original paper, see below)
Like many “environmentally friendly” technologies, green roofs tend to have higher up-front costs of installation than conventional roofs. On the other hand, they can be cheaper over the long run because their roof membranes generally last longer, not to mention the potential savings in heating and AC costs. And besides, which would you rather look at as you enjoy a martini on the balcony in Manhattan?