title: A 21st-century shift from fossil-fuel to biogenic methane emissions indicated by ¹³CH₄.
abstract: 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.
authors.affiliation: Climate and Atmosphere Center, National Institute of Water and Atmospheric Research (NIWA), 301 Evans Bay Parade, Wellington 6021, New Zealand. hinrich.schaefer@niwa.co.nz.
authors.forename: Hinrich
authors.initials: H
authors.lastname: Schaefer
authors.affiliation: Climate and Atmosphere Center, National Institute of Water and Atmospheric Research (NIWA), 301 Evans Bay Parade, Wellington 6021, New Zealand.
authors.forename: Sara E
authors.initials: SE
authors.lastname: Mikaloff Fletcher
authors.affiliation: Institut für Umweltphysik, Heidelberg University, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany.
authors.forename: Cordelia
authors.initials: C
authors.lastname: Veidt
authors.affiliation: Climate and Atmosphere Center, National Institute of Water and Atmospheric Research (NIWA), 301 Evans Bay Parade, Wellington 6021, New Zealand.
authors.forename: Keith R
authors.initials: KR
authors.lastname: Lassey
authors.affiliation: Climate and Atmosphere Center, National Institute of Water and Atmospheric Research (NIWA), 301 Evans Bay Parade, Wellington 6021, New Zealand.
authors.forename: Gordon W
authors.initials: GW
authors.lastname: Brailsford
authors.affiliation: Climate and Atmosphere Center, National Institute of Water and Atmospheric Research (NIWA), 301 Evans Bay Parade, Wellington 6021, New Zealand.
authors.forename: Tony M
authors.initials: TM
authors.lastname: Bromley
authors.affiliation: National Oceanic and Atmospheric Administration (NOAA), Earth System Research Laboratory (ESRL), Global Monitoring Division, 325 Broadway, Boulder, CO 80305, USA.
authors.forename: Edward J
authors.initials: EJ
authors.lastname: Dlugokencky
authors.affiliation: Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309, USA.
authors.forename: Sylvia E
authors.initials: SE
authors.lastname: Michel
authors.affiliation: National Oceanic and Atmospheric Administration (NOAA), Earth System Research Laboratory (ESRL), Global Monitoring Division, 325 Broadway, Boulder, CO 80305, USA.
authors.forename: John B
authors.initials: JB
authors.lastname: Miller
authors.affiliation: Institut für Umweltphysik, Heidelberg University, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany.
authors.forename: Ingeborg
authors.initials: I
authors.lastname: Levin
authors.affiliation: Climate and Atmosphere Center, National Institute of Water and Atmospheric Research (NIWA), 301 Evans Bay Parade, Wellington 6021, New Zealand.
authors.forename: Dave C
authors.initials: DC
authors.lastname: Lowe
authors.affiliation: Climate and Atmosphere Center, National Institute of Water and Atmospheric Research (NIWA), 301 Evans Bay Parade, Wellington 6021, New Zealand.
authors.forename: Ross J
authors.initials: RJ
authors.lastname: Martin
authors.affiliation: Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309, USA.
authors.forename: Bruce H
authors.initials: BH
authors.lastname: Vaughn
authors.affiliation: Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309, USA.
authors.forename: James W C
authors.initials: JW
authors.lastname: White