Description and Evaluation of an Emission-Driven and Fully Coupled Methane Cycle in UKESM -- Revised Paper

Methane is a very important greenhouse gas. The global methane cycle needs to be
understood fully to accurately model the way methane affects climate change. We describe a
new version of the UKESM1 Earth system model, UKESM1-ems, that uses emissions of
methane to drive the atmospheric chemistry. In case of emissions from global wetlands, such
as bogs, swamps and tundra, the methane emissions are calculated by the model during
runtime. Methane emissions react directly to changes in the modelled climate. UKESM1-ems
simulates the global cycle of methane from emissions via oxidation in the atmosphere to
uptake at the surface more realistically. We also test the model against measurements from
satellites and ground-based stations to ensure the relevant processes in the model behave
accurately. The comparison with observations shows that UKESM1-ems performs well and
represents an improvement in simulating important processes in climate and the Earth
system. However, we also found that the methane concentration in the model is too low
compared to observations for the period of the 20th and early 21st century during which
human activity, especially the use of fossil fuel, is dominating the methane cycle.