The synthesis of top-down and bottom-up approaches enables us to assess the fluxes over three decades, characterized by a stabilization in the 1990s, followed by a resumption of the rise in CH4 levels in the 2000s. The most likely explanation is a transient decrease in CH4 emissions linked to a relative decline in the extraction and use of fossil fuels.
Air bubbles occluded in the polar ice caps allow us to go back beyond the recent industrial period, during which atmospheric CH4 content almost tripled. By comparison, variations in the Holocene period (last 11,500 years BP) are much weaker, though very real and consistent between the two hemispheres. There was a decrease of around 100 ppb from 11,000 to 6,000 years BP, followed by an increase since around 4,000 years BP. The CH4 content of air bubbles in Greenland is slightly higher than that measured for Antarctica, indicating fluxes from the continental biosphere.
To better understand Holocene variability, it is useful to consider much longer-term CH4 variations. Over the last million years, atmospheric concentrations show a cyclicity of around 20,000 years. CH4 maxima correspond to episodes of high insolation in the subtropical latitudes of the Northern Hemisphere. The periodicity of these phases is linked to the cycles of the precession of the equinoxes.
A comparison of CH4 levels during the various interglacial phases reveals a particular feature of the Holocene. This is the only interglacial period during which CH4 levels diverge significantly from an astronomical forcing of decreasing insolation. Assuming that CH4 levels should follow insolation, it is possible to estimate the start of the divergence at around 5,000 years BP and the amplitude of the anomaly at over 200 ppb before the recent disturbance of the industrial era.
