Atmospheric methane underestimated in future climate projections

Methane (CH₄) is the second most important naturally occurring greenhouse gas (GHG) after carbon dioxide (Myhre G et al 2013 Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge: Cambridge University Press) pp 659–740). For both GHGs, the present-day budget is dominated by anthropogenic emissions (Friedlingstein P et al 2019 Earth Syst. Sci. Data 11 1783–838; Saunois M et al 2020 Earth Syst. Sci. Data 12 1561–623). For CO₂ it is well established that the projected future rise in atmospheric concentration is near exclusively determined by anthropogenic emissions (Ciais P et al 2013 Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Inter-governmental Panel on Climate Change (Cambridge: Cambridge University Press) pp 465–570). For methane, this appears to be the common assumption, too, but whether this assumption is true has never been shown conclusively. Here, we investigate the evolution of atmospheric methane until 3000 CE under five Shared Socioeconomic Pathway (SSP) scenarios, for the first time using a methane-enabled state-of-the-art Earth System Model (ESM). We find that natural methane emissions, i.e. methane emissions from the biosphere, rise strongly as a reaction to climate warming, thus leading to atmospheric methane concentrations substantially higher than assumed in the scenarios used for CMIP6. We also find that the natural emissions become larger than the anthropogenic ones in most scenarios, showing that natural emissions cannot be neglected. (letter)