Cities are major aggregated sources of methane (CH4) emissions and can therefore play a role in mitigating climate warming. However, diverse, spatially distributed sources make characterizing urban CH4 emissions challenging. A limited synthesis of existing research has hindered understanding of source characteristics and contributions, implicating research priorities, policies, and mitigation. This review consolidates findings from 106 peer-reviewed articles on CH4 emissions in U.S. and Canadian cities, identifying key insights, gaps, and opportunities. We found that top-down (TD) estimates of city-scale CH4 emissions from 34 studies exceeded, on average, bottom-up (BU) estimates by a factor of 3.9 (±6.7). Urban CH4 footprints were dominated by sources from natural gas distribution and end-use and landfills. Across 11 U.S. studies, the estimated mean CH4 loss rate from delivered natural gas corrected for CH4 content in cities was 2.3% (±0.9%). TD estimates of CH4 emissions from six U.S. landfills were, on average, 2.4 (±1.7) times greater than self-reported BU estimates. Preferred methods for reporting may miss large fugitive point sources, systematically underestimating landfill CH4 emissions. The studies indicated that wastewater systems emit less CH4 than landfills and natural gas sources, but the research remains limited, and many wastewater sources are poorly characterized. Mitigation effectiveness varied by source, with scalability a challenge for small, distributed sources such as sewers, and the confirmation of reductions sensitive to measurement scale. Overall, results highlight challenges in quantifying, attributing, and mitigating CH4 emissions in urban settings. Key research priorities are: (i) expanding CH4 measurements from urban natural gas (distribution and end-use) and wastewater sources, and granular investigations to pinpoint and understand the causes of emissions; (ii) new emissions data to improve BU models and integrate into BU estimates; (iii) improving measurement-model coupling for landfill CH4 quantification; and (iv) evaluating mitigation strategies for urban CH4 sources.

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