Description and Evaluation of an Emission-Driven and Fully Coupled Methane Cycle in UKESM1

Methane (CH4) is one of the most important trace gases in the atmosphere owing to its role as an exceedingly effective greenhouse gas and atmospheric pollutant. Better understanding of the global methane cycle and its interactions with the Earth system is therefore necessary for robust future projections of anthropogenic climate change and assessments of multi-gas mitigation strategies. Here we present a newly developed methane emission-driven Earth system model to simulate the global methane cycle fully interactively. We provide an evaluation of methane sources and sinks and a full-cycle methane budget and its change over the historic period. We further evaluate the methane atmospheric abundance and lifetime against available observations. The new methane emission-driven model simulates all the components of the methane cycle within observational uncertainty. We calculate a total present-day (2000–2009 decadal average) methane source of 591 Tg(CH4) yr−1 with 197 Tg(CH4) yr−1 coming from wetlands. These sources are nearly balanced by the global methane sinks amounting to 580 Tg (CH4) yr−1; reaction of methane with the hydroxyl radical in the troposphere alone removes 525 Tg(CH4) yr−1. The imbalance between sources and sinks of 11 Tg(CH4) yr−1 represents the atmospheric methane growth rate and is in fairly good agreement with current best estimates of 5.8 Tg(CH4) yr−1 with a range of 4.9–6.6 Tg(CH4) yr−1. At present-day the model shows a maximum systematic negative-bias of approximately 200 ppb in the methane surface mole fraction.