Finding Open Space in a Crowded City

This year’s Climate and Society class is out in the field (or lab or office) completing a summer internship or thesis. They’ll be documenting their experiences one blog post at a time. Read on to see what they’re up to.

Valerie Pietsch, C+S ’16

Open space is “land or water that either remains in its natural state or is used for agriculture, free from intensive development for residential, commercial, industrial, or institutional use” – New York Department of Environmental Conservation

Despite a specific definition at the state level, open space in New York City is not as well defined as you might think. It includes parks, street trees, community gardens, empty grass lots, and green roofs. These areas can be publicly or privately owned, which often affects public accessibility. The Nature Conservancy’s New York City Program is working on putting together a cohesive data set of open space and land ownership in the city in order to assess which entities the Conservancy must work with to promote increased public access to open space.

Source: The Nature Conservancy

Assessing open space in a city this dense is no small task. Satellite and aerial imagery can be used to gauge vegetation, but the wide variety of surface types within a small area requires very high-resolution imagery. A major piece of this project has been mapping out green roofs in New York City.

Green roofs are a type of green infrastructure, consisting of a waterproofing layer covered by a layer of vegetation, typically grass. They provide a variety of environmental and public health benefits, including heat reduction, stormwater mitigation, wildlife habitat, and air quality improvement.

One of the biggest benefits is mitigating the urban heat island effect, a phenomenon where the density of buildings and paved surfaces can cause temperatures to spike about 1.8-5.4°F warmer than surrounding rural areas. Vegetated surfaces can be 20-45°F cooler than paved ones, due to higher albedo, shade from trees, and evapotranspiration of plants. Green roofs, and other vegetated areas, can mitigate the urban heat island effect, as well as provide cool spaces for city residents to access during heat waves. These grassy layers also serve as insulation on buildings, improving energy efficiency and reducing the need for air conditioning, in turn reducing associated heat waste, air pollution, and greenhouse gases.

Source: The Nature Conservancy

Green roofs are also able to absorb stormwater. The plant layer can uptake and evapotranspirate this water in smaller amounts, and the soil can contain water for longer before it reaches the sewer system. In this way, green roofs can reduce the overall flow of stormwater to water treatment plants or at least reduce the peak load of water entering the plants during storms. This is important for preserving water quality of New York City waterways, as water treatment plants cannot handle extremely high flows that sometimes occur during storms. When this happens, the plants will discharge a mix of excess stormwater and undertreated wastewater directly into the city’s waterways.

Another important co-benefit of green roofs is the habitat they create for urban wildlife, especially birds and insects. Green roofs can improve connectivity that allows wildlife to move around the city and they offer a greater spatial distribution of habitats. That’s critical for pollinators such as honeybees and monarch butterflies, which are at risk of extinction due to habitat loss, pesticides, pathogens, and climate change. Green roofs improve their dispersal, increasing genetic diversity and improving chances of species survival.

Finally, green roofs can improve air quality through dry deposition and carbon sequestration, which is significant in cities where pollution is more concentrated and causes health issues. A recent study in Chicago showed that nearly 3,700 lbs. (roughly the weight of an SUV) of air pollutants were removed by just 49 acres (about 37 football fields) of green roofs in a year. The study also projected that if the entire city were covered with green roofs, these roofs could remove 2046.89 metric tons (about the weight of 13 blue whales) of air pollutants annually.

Clearly, green roofs can have a vast environmental impact, but their extent in New York City is not well known. To manually map out every green roof in the city would be highly inefficient, but these roofs can be selected using algorithms that separate vegetation from other surface types.

Still, The Nature Conservancy has run into several issues while working towards mapping green roofs. In the image below, seven green roofs near the Conservancy’s New York City office have been identified using algorithms, yet only two are actually green roofs. This first attempt at green roof mapping has estimated that there are 6,976 green roofs in New York City, though with large error rates, omitting about 24 percent of known green roofs and including many buildings that do not have green roofs.

Source: The Nature Conservancy

Some of these errors occur because green roofs are difficult to distinguish from other vegetated rooftops such as rooftop gardens and parks. Additionally, many buildings in the outer boroughs are smaller, and their surfaces can be overshadowed by trees, leading to false detection of rooftop vegetation. While improving this process, The Nature Conservancy’s New York City team has been emphasizing transferability and replicability.

The Conservancy recognizes that having a reliable process using open source tools and freely available data would allow the process to be repeated by other cities’ programs at the Conservancy or other environmentally oriented groups. They are working towards improving and simplifying their process so it can be passed on to promote further research.