City-level CH4 emissions are the sum of all potential natural and anthropogenic sources within cities such as wetlands, geologic seeps, solid waste, wastewater collection and treatment, manufacturing and processing, and natural gas distribution and end-use, among others. Cities are thus large sources of CH4 emissions in aggregate, and some sources and sectors might contribute more to total city-level emissions than others. In general, the peer-reviewed research literature suggests that cities are larger aggregate sources of CH4 emissions than previously estimated. Total city-level CH4 emissions can be estimated using top-down (TD) or bottom-up (BU) approaches. The studies estimated city-level emissions rates by pairing a TD measurement platform such as a stationary sensor network, aircraft, or satellite with a quantification technique like tracer scaling, atmospheric inversion, mass balance, or other emissions estimation technique. The methods reported in each study are listed in S2. In contrast, the BU approach -which underpins government and other emissions inventories -derives an emissions rate from the product of emissions factors (emissions per unit or activity) and activity factors (counts of units or activities). In the context of cities, a key difference between these two approaches is that BU estimates are constrained to known sources of CH4 emissions, whereas TD estimates include emissions that may be unaccounted for, or underestimated in BU estimates.