Mitigation of methane emissions in cities: How new measurements and partnerships can contribute to emissions reduction strategies

Cities generate 70% of anthropogenic greenhouse gas emissions, a fraction that is growing with global urbanization. While cities play an important role in climate change mitigation, there has been little focus on reducing urban methane emissions. Here we develop a conceptual framework for methane mitigation in cities by describing emission processes, the role of measurements, and a need for new institutional partnerships. Urban methane emissions are likely to grow with expanding use of natural gas and organic waste disposal systems in growing population centers; however, we currently lack the ability quantify this increase. We also lack systematic knowledge of the relative contribution of these distinct source sectors on emissions. We present new observations from 4 North American cities to demonstrate that methane emissions vary in magnitude and sector from city to city, and hence require different mitigation strategies. Detections of fugitive emissions from these systems suggest that current mitigation approaches are absent or ineffective. These findings illustrate that tackling urban methane emissions will require research efforts to identify mitigation targets, develop and implement new mitigation strategies, and monitor atmospheric methane levels to ensure the success of mitigation efforts. This research will require a variety of techniques to achieve these objectives, and should be deployed in cities globally. We suggest that metropolitan-scale partnerships may effectively coordinate systematic measurements and actions focused on emission reduction goals.