Methane variations on orbital timescales: a transient modelling experiment

Abstract

Methane (CH4) variations on orbital timescales are often associated with variations in wetland coverage, most notably in the summer monsoon areas of the Northern Hemisphere. Here we test this assumption by simulating orbitally forced variations in global wetland emissions, using a simple wetland distribution and CH4 emissions model that was coupled off-line to a climate model containing atmosphere, ocean and vegetation components. The transient climate modeling simulation extends over the last 650 000 yrs and includes variations in land-ice distribution and greenhouse gases. Tropical temperature and global vegetation are found to be the dominant controls for global CH4 emissions and thus atmospheric concentrations. The relative importance of wetland coverage, vegetation coverage, and emission temperatures depends on the specific climatic zone (boreal, tropics and Indian/Asian monsoon area) and timescale (precession, obliquity and glacial-interglacial timescales). Simulated variations in emissions agree well with those in measured concentrations, both in their time series and spectra. The simulated lags with respect to the orbital forcing also show close agreement with those found in measured data, both on the precession and obliquity timescale. We only find covariance between monsoon precipitation and CH4 concentrations, however we find causal links between atmospheric concentrations and tropical temperatures and global vegetation. The primary importance of these two factors explains the lags found in the CH4 record from ice cores.