A 21st-century shift from fossil-fuel to biogenic methane emissions indicated by 13 CH 4

Between 1999 and 2006, a plateau interrupted the otherwise continuous increase of atmospheric methane concentration CH4 since preindustrial times. Causes could be sink variability or a temporary reduction in industrial or climate-sensitive sources. We reconstructed the global history of CH4 and its stable carbon isotopes from ice cores, archived air, and a global network of monitoring stations. A box-model analysis suggests that diminishing thermogenic emissions, probably from the fossil-fuel industry, and/or variations in the hydroxyl CH4 sink caused the CH4 plateau. Thermogenic emissions did not resume to cause the renewed CH4 rise after 2006, which contradicts emission inventories. Post-2006 source increases are predominantly biogenic, outside the Arctic, and arguably more consistent with agriculture than wetlands. If so, mitigating CH4 emissions must be balanced with the need for food production.