A review of methane emissions source types and characteristics, rates, and mitigation across U.S. and Canadian cities

As major sources of methane (CH4) emissions, cities have an important role in mitigating near-term global temperature rise. However, cities are challenging environments for characterizing CH4 emissions due to the diversity and spatial extent of sources. Furthermore, the characteristics and contributions of different sources are poorly understood due to a lack of synthesis and integration of the literature, with knock-on implications for policies and mitigation. Here, we review peer-reviewed journal articles on CH4 emissions from cities in the U.S. and Canada to consolidate the current state of knowledge and highlight key research priorities. From 32 of 94 studies reviewed, we find that estimates of total city-level CH4 emissions derived from top-down measurements are on average 5.6 (± 7.8) times larger than bottom-up inventory estimates. Emissions from natural gas distribution and end use, and landfills, dominate city-level CH4 footprints. The average urban natural gas loss rate of 1.8% ± 0.9% from 12 studies increases the overall natural gas supply chain loss rate estimate to 4.0% ± 0.9%. Top-down estimates of CH4 emissions from landfills were on average 10.6 times greater than Greenhouse Gas Reporting Program estimates. Landfill studies indicate that better accounting of spatial and temporal phenomena such as fugitives, hotspots, and variations in weather and soil conditions is central to improving emissions rate estimates. A handful of studies examined mitigation and highlighted the role of measurement to identify specific mitigation opportunities and verify CH4 emissions reductions. The review findings raise questions and highlight challenges around existing bottom-up inventory approaches, urban natural gas loss rates and slip, landfill emissions estimation techniques, and mitigation effectiveness. The review concludes with recommendations on research priorities to address key knowledge gaps: (i) new source-level measurement datasets and modeling approaches for bottom-up emissions estimation, (ii) more granular investigations to understand the specific sources and causes of CH4 emissions from urban natural gas infrastructure and end use, (iii) a better coupling between measurement and modeling of landfill CH4 emissions, and (iv) mitigation-focused studies.