Preindustrial CH indicates greater anthropogenic fossil CH emissions.

Atmospheric methane (CH) is a potent greenhouse gas, and its mole fraction has more than doubled since the preindustrial era. Fossil fuel extraction and use are among the largest anthropogenic sources of CH emissions, but the precise magnitude of these contributions is a subject of debate. Carbon-14 in CH (CH) can be used to distinguish between fossil (C-free) CH emissions and contemporaneous biogenic sources; however, poorly constrained direct CH emissions from nuclear reactors have complicated this approach since the middle of the 20th century. Moreover, the partitioning of total fossil CH emissions (presently 172 to 195 teragrams CH per year) between anthropogenic and natural geological sources (such as seeps and mud volcanoes) is under debate; emission inventories suggest that the latter account for about 40 to 60 teragrams CH per year. Geological emissions were less than 15.4 teragrams CH per year at the end of the Pleistocene, about 11,600 years ago, but that period is an imperfect analogue for present-day emissions owing to the large terrestrial ice sheet cover, lower sea level and extensive permafrost. Here we use preindustrial-era ice core CH measurements to show that natural geological CH emissions to the atmosphere were about 1.6 teragrams CH per year, with a maximum of 5.4 teragrams CH per year (95 per cent confidence limit)-an order of magnitude lower than the currently used estimates. This result indicates that anthropogenic fossil CH emissions are underestimated by about 38 to 58 teragrams CH per year, or about 25 to 40 per cent of recent estimates. Our record highlights the human impact on the atmosphere and climate, provides a firm target for inventories of the global CH budget, and will help to inform strategies for targeted emission reductions.