Natural methane emissions – from the glacial to the present

Methane (CH₄) is a greenhouse gas about 28 times more potent than carbon dioxide. Its atmospheric concentration, reconstructed from Antarctic ice cores, was always higher for warmer than for glacial periods. The concentration doubled between the LGM from ca. 350 parts per billion (ppb) to about 700 ppb in the PI state, and more than doubled again from the industrial revolution to the present level of 1860 ppb. The latter increase is due to human activities – mainly the use of fossil fuels, but also agriculture and waste facilities. The former increase, however, occurred when humans did not yet affect the Earth System in significant ways. What drove this natural increase in the atmospheric CH₄ concentration?

Kleinen and colleagues used a methane-enabled version of the Max Planck Institute Earth System Model (MPI-ESM) to simulate six climate states from 20000 years ago to the present. The new model considers most natural sources of methane: wetlands, wildfires and termites. Wetlands are the most important natural source of methane, emitting about 90% of the total net emissions, with tropical wetlands supplying more than twice as much methane to the atmosphere, as high latitude wetlands. Dr Thomas Kleinen: ”We carefully validated the model against the most recent methane budget assessments, and the spatial distribution of emissions in the present-day climate, as well as the emission totals, compare very well indeed.”

The researchers found that climate changes led to roughly a doubling of methane emissions between LGM and PI. Prof Victor Brovkin: ”This is an encouraging advance, as our results are consistent with the ice core data. Previous studies generally underestimated the change in emissions and thus needed to invoke changes in the atmospheric lifetime of CH₄, which are not supported by recent research in atmospheric chemistry.” For the methane change between LGM and PI, climate change is the ultimate cause, but the changes in temperature and ice sheet size are not directly responsible for much of the change in methane emissions. Instead, Kleinen et al. could show that the direct cause of the lower CH₄ emissions at LGM are the lower soil carbon stocks and atmospheric CO₂ concentrations, each responsible for about 40% of the emission reduction, while only 20% can be attributed directly to temperature and ice sheet change. Dr Kleinen: “As our model reproduces essential features of the methane cycle realistically, we believe our factor separation to be robust. Neglected factors likely won’t change this result significantly.”

The research is part of the BMBF-funded PalMod project, where researchers from several German research institutes aim to model the entire last glacial cycle employing state-of-the-art Earth System Models.