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

Cities are major aggregate sources of methane (CH₄) emissions and play a critical role in mitigating near-term global temperature rise. However, characterizing urban CH₄ emissions remains challenging due to the diversity and spatial distribution of sources. Furthermore, limited synthesis and integration of the literature has led to a poor understanding of the characteristics and contributions of different sources, with implications for policies and mitigation. This review consolidates findings from 103 peer-reviewed articles on CH₄ emissions from cities in the U.S. and Canada, highlighting key research priorities. We find that top-down (TD) estimates of total city-level CH₄ emissions exceed bottom-up (BU) inventory estimates by a factor of 0.7 to 4.9 in 34 studies. In city-level studies that disambiguated emissions by source, natural gas distribution and use, and landfills, dominated urban CH₄ footprints. The mean natural gas loss rate in cities of 1.8% ± 0.9% suggests a broader natural gas supply chain loss rate of 4.0% ± 0.9%. Notably, TD estimates of CH₄ emissions from six select U.S. landfills were, on average, 2.6 (± 1.8) times greater than self-reported estimates, suggesting that preferred calculation methods for reporting may systematically underestimate emissions and miss fugitive point sources. A limited number of studies examined mitigation but indicate that measurement is essential to identify mitigation opportunities and verify reductions. We raise questions and highlight challenges around existing BU inventories, urban natural gas loss rates, combustion slip, landfill emissions estimation techniques, and mitigation effectiveness. We conclude with recommendations on research priorities to address key knowledge gaps: (i) new source-level measurement datasets and modeling approaches for BU 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; (iv) mitigation-focused studies.