Abstract
Cities around the world have united to form coalitions, like the C40 network, in pursuit of ambitious climate goals. These efforts often include reducing methane emissions. However, sources and magnitudes of urban methane emissions are not well known, and there is not currently a method to evaluate implementation of mitigation measures. Here, we fill this observational gap with a tracer-tracer approach using space-based observations of methane and carbon monoxide from the TROPOspheric Monitoring Instrument satellite instrument. We measure methane emissions of 92 global cities, including their broader metropolitan area, and find aggregate emissions of 31.2 Tg CH(4)/y (95%CI: 22.3, 40.4 Tg CH(4)/y) in 2023, equivalent to [Formula: see text]10% of the global anthropogenic methane budget. We track emissions for 72 of these cities (51 C40 cities and 21 non-C40 cities) from 2019-2023. Methane emissions from these cities weakly declined in 2020 followed by steady growth, with a 2.3 Tg aggregate increase over 4 y. This growth contributes minimally to the recent atmospheric methane surge. While C40 cities have largely pledged 34% reductions by 2030, we observe significant growth from 2020 to 2023 (10%, 95%CI: 2%, 17%), similar to growth observed in non-C40 cities (12%, 95%CI: [Formula: see text]1.5%, 25%). Inventories fail to capture observed growth, suggesting urban emissions are not well characterized, and mitigation approaches may not be optimally designed. For the C40 network to achieve its methane target (34% by 2030), steep, rapid reductions will be needed. Emission reductions of this magnitude would be detectable with the space-based approach used in this work.