Estimating Carbon Sequestration of Green Roofs and Elevated Green Courtyards in Seattle

Abstract

As carbon dioxide (CO2) becomes increasingly concentrated in the atmosphere and rapidly accelerates climate change, efforts should be put forth not just to reduce CO2 emissions but to also remove CO2 from the atmosphere. Vegetation naturally absorbs CO2 from the surrounding atmosphere through the process of photosynthesis. Thus, increasing the amount of vegetation is a method to remove more CO2 from the air. Meanwhile, as cities continue developing while often seeking more greenspace for human and urban ecological functions, roofs of urban buildings take up vast amounts of space and go largely unused. Adding greenspace on roofs and other elevated portions of buildings can provide a plethora of functions including absorbing, or sequestering, CO2. Research on the CO2 sequestration of green roofs is a relatively new but expanding field which provides promise for significant amounts of CO2 sequestration if green roofs are widespread. General interest in Seattle’s green roofs has also grown over the past 10 - 15 years, and policy has been put forth to encourage their implementation. Efforts have also been put forth to map, quantify, and categorize existing green roofs. However, these efforts are far from complete. Seattle, which has been experiencing rapid building and development over many years, with likely continuation into the future, has potential for more green roofs and additional CO2 sequestration. The question driving this research is: How much carbon is annually sequestered by Seattle’s green roofs and elevated green courtyards? This thesis provides an up-to-date mapping of much of Seattle’s rooftop vegetation. On the basis of this data, available green roof carbon sequestration experiments are analyzed, and their sequestration rates collected and categorized into green roofs and elevated green courtyards of differing vegetation size, or intensity. The city is explored with aerial/satellite/building-level/street-level imagery, and building vegetation is recorded on a map. Areas not mapped are estimated for vegetative content. Carbon sequestration amount is estimated based on amount of green roof and elevated courtyard vegetation with a basis of previous research on rooftop sequestration rates. In total, 2,669,267 sq. ft. of green roofs and elevated green courtyards, or 0.096 sq. miles, are measured and estimated across Seattle. The estimated annual sequestration of these green roofs and elevated green courtyards is 2,151,079 kg CO2 per year, or 2,151 metric tons of CO2 per year. These results are discussed, limitations are described, and suggestions are made for future research. Ultimately, more direct carbon sequestration and mapping measurements in Seattle could ensure a more locally-accurate set of sequestration rates and green roof/elevated green courtyard quantifications. In creating an up-to-date map for much of the city and estimations for the rest, and creating Seattle’s first carbon sequestration estimation for these vegetated urban features, it is hoped that a strong foundation has been built to continue refining sequestration estimates in Seattle and to elucidate the potential for additional carbon sequestration in Seattle and beyond.