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

Abstract

Cities are major aggregated sources of methane (CH _4 ) emissions and can therefore play a role in mitigating climate warming. However, diverse, spatially distributed sources make characterizing urban CH _4 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 CH _4 emissions in U.S. and Canadian cities, identifying key insights, gaps, and opportunities. We found that top-down (TD) estimates of city-scale CH _4 emissions from 34 studies exceeded, on average, bottom-up (BU) estimates by a factor of 3.9 (±6.7). Urban CH _4 footprints were dominated by sources from natural gas distribution and end-use and landfills. Across 11 U.S. studies, the estimated mean CH _4 loss rate from delivered natural gas corrected for CH _4 content in cities was 2.3% (±0.9%). TD estimates of CH _4 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 CH _4 emissions. The studies indicated that wastewater systems emit less CH _4 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 CH _4 emissions in urban settings. Key research priorities are: (i) expanding CH _4 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 CH _4 quantification; and (iv) evaluating mitigation strategies for urban CH _4 sources.

Keywords

• methane emissions
• cities
• urban
• natural gas
• landfills
• waste