title: Understanding the glacial methane cycle.
abstract: Atmospheric methane (CH4) varied with climate during the Quaternary, rising from a concentration of 375 p.p.b.v. during the last glacial maximum (LGM) 21,000 years ago, to 680 p.p.b.v. at the beginning of the industrial revolution. However, the causes of this increase remain unclear; proposed hypotheses rely on fluctuations in either the magnitude of CH4sources or CH4atmospheric lifetime, or both. Here we use an Earth System model to provide a comprehensive assessment of these competing hypotheses, including estimates of uncertainty. We show that in this model, the global LGM CH4source was reduced by 28-46%, and the lifetime increased by 2-8%, with a best-estimate LGM CH4concentration of 463-480 p.p.b.v. Simulating the observed LGM concentration requires a 46-49% reduction in sources, indicating that we cannot reconcile the observed amplitude. This highlights the need for better understanding of the effects of low CO2and cooler climate on wetlands and other natural CH4sources.
authors.affiliation: Bristol Research Initiative for the Dynamic Global Environment, School of Geographical Sciences and Cabot Institute, University of Bristol, University Road, Bristol BS8 1SS, UK.
authors.forename: Peter O
authors.identifier: 0000-0003-3694-9181
authors.initials: PO
authors.lastname: Hopcroft
authors.affiliation: Bristol Research Initiative for the Dynamic Global Environment, School of Geographical Sciences and Cabot Institute, University of Bristol, University Road, Bristol BS8 1SS, UK.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Valdes
authors.affiliation: Met Office Hadley Centre, FitzRoy Road, Exeter EX1 3PB, UK.
authors.forename: Fiona M
authors.initials: FM
authors.lastname: O'Connor
authors.affiliation: Institute of Earth Surface Dynamics, University of Lausanne, Geopolis Building, CH-1015 Lausanne, Switzerland.
authors.forename: Jed O
authors.identifier: 0000-0001-9919-7613
authors.initials: JO
authors.lastname: Kaplan
authors.affiliation: Department of Animal and Plant Sciences, Alfred Denny Building, University of Sheffield, Sheffield S10 2TN, UK.
authors.forename: David J
authors.initials: DJ
authors.lastname: Beerling