Flaring is widely used by the fossil fuel industry to dispose of natural gas. Industry and governments generally assume that flares remain lit and destroy methane, the predominant component of natural gas, with 98% efficiency. Neither assumption, however, is based on real-world observations. We calculate flare efficiency using airborne sampling across three basins responsible for >80% of US flaring and combine these observations with unlit flare prevalence surveys. We find that both unlit flares and inefficient combustion contribute comparably to ineffective methane destruction, with flares effectively destroying only 91.1% (90.2, 91.8; 95% confidence interval) of methane. This represents a fivefold increase in methane emissions above present assumptions and constitutes 4 to 10% of total US oil and gas methane emissions, highlighting a previously underappreciated methane source and mitigation opportunity. Fueling global warming Flaring, the process of burning natural gas escaping from oil and gas wells, is primarily intended to combust the powerful greenhouse gas methane to minimize its emission. But is flaring as effective as is claimed? Plant et al . used airborne sampling to measure flare efficiency in three major gas production regions in the United States and found that methane emissions are five times higher than currently thought (see the Perspective by Duren and Gordon). Therefore, flaring is often not as efficient as presumed—or methane plumes simply are not combusted at all. —HJS Natural gas flaring destroys much less methane than thought.