The relative importance of methane sources and sinks over the Last Interglacial period and into the last glaciation

A fairly limited number of studies have investigated this cycle at the millenium time scale with most of them examining the doubling in CH4 from the Last Glacial Maximum (LGM) to the PIH. Though it is still a matter of debate, a general consensus suggests a predominant role to the change in methane emissions from wetlands and only a limited change in the oxidising capacity of the atmosphere. In the present study we provide an estimate of the relative importance of sources and sinks during the LIG period, using a complex climate-chemistry model to quantify the sinks, and a methane emissions model included in a global land surface model, for the sources. We are not aware of any previous studies that have explicitly tackled sources and sinks of methane in the previous interglacial. Our results suggest that both emissions and sinks of methane were higher during the LIG period, relative to the PIH, resulting in similar atmospheric concentrations of methane. Our simulated change in methane lifetime is primarily driven by climate (i.e. air temperature and humidity). However, a significant part of the reduced methane lifetime is also attributable to the impact of changes in NOx emissions from lightning. An increase in biogenic emissions of non-methane volatile organic compounds during the LIG seems unlikely to have compensated for the impact of temperature and humidity. Surface methane emissions from wetlands were higher in northern latitudes due to an increase of summer temperature, whilst the change in the tropics is less certain. Simulated methane emissions are strongly sensitive to the atmospheric forcing, with most of this sensitivity related to changes in wetland extent.