Atmospheric methane (CH₄) is a potent greenhouse gas, and its mole fraction has more than doubled since the preindustrial era¹. Fossil fuel extraction and use are among the largest anthropogenic sources of CH₄ emissions, but the precise magnitude of these contributions is a subject of debate^2,3. Carbon-14 in CH₄ (¹⁴CH₄) can be used to distinguish between fossil (¹⁴C-free) CH₄ emissions and contemporaneous biogenic sources; however, poorly constrained direct ¹⁴CH₄ emissions from nuclear reactors have complicated this approach since the middle of the 20th century^4,5. Moreover, the partitioning of total fossil CH₄ emissions (presently 172 to 195 teragrams CH₄ per year)^2,3 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 CH₄ per year^6,7. Geological emissions were less than 15.4 teragrams CH₄ per year at the end of the Pleistocene, about 11,600 years ago⁸, 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 ¹⁴CH₄ measurements to show that natural geological CH₄ emissions to the atmosphere were about 1.6 teragrams CH₄ per year, with a maximum of 5.4 teragrams CH₄ per year (95 per cent confidence limit)-an order of magnitude lower than the currently used estimates. This result indicates that anthropogenic fossil CH₄ emissions are underestimated by about 38 to 58 teragrams CH₄ 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 CH₄ budget, and will help to inform strategies for targeted emission reductions^9,10.