Atmospheric methane in the recent and ancient atmospheres: Concentrations, trends, and interhemispheric gradient

The concentrations of methane in the old and ancient atmospheres of the earth can be deduced by analyzing the air in bubbles embedded in polar ice cores. Upon analyzing some 80 ice core samples from the polar regions we found that the concentration of methane 250 years ago and earlier was only 700 (±30) ppbv, or about 45% of present levels. A rapid and significant increase of atmospheric methane started about 150 years ago. The rate of increase has escalated since then and is about 1.3%/yr at present. We also found that the concentration of methane in the atmosphere 250 years ago and earlier, when methane was not increasing, was 10% (±4%) higher in the Arctic as compared to the Antarctic. This finding is consistent with the expected ratio of about 1.07–1.11 obtained from a global mass balance model and the primarily land‐based natural sources of methane, estimated to be about 280 Tg/yr, which may have been the only sources several hundred years ago, when human activities did not contribute significantly to the global methane cycle.