Towards reconstructing the Arctic atmospheric methane history over the 20th century: measurement and modeling results for the NGRIP firn

Abstract. Systematic measurements of atmospheric methane (CH4) mole fractions at the northern high latitudes only began in the early 1980s, and whilst CH4 measurements from Greenland ice cores covered the period before ~1900, no reliable observational record is available for the intermediate period. In this study, we reconstruct the atmospheric CH4 for that period, when the mole fraction started to increase rapidly. We use a set of trace gas data measured from firn (an intermediate stage between snow and glacial ice formation) air samples collected at the NGRIP (North Greenland Ice Core Project) site in 2001, in combination with a firn air transport model whose performance is validated by using a set of published firn air data at the NEEM (North Greenland Eemian ice Drilling) site. We examine a variety of possible firn diffusivity profiles using a suite of measured trace gases, and reconstruct the CH4 mole fraction by an iterative dating method. We find that, given the currently available firn air data sets from Greenland, reliable reconstruction of the Arctic CH4 mole fraction is possible only back to the mid 1970s. For the earlier period, it is difficult to identify the atmospheric CH4 history that consistently reproduce the depth profiles of CH4 in firn at both NGRIP and NEEM sites. Therefore, the currently proposed Arctic CH4 history should still be considered preliminary and uncertain, and should not be treated as the known history for constraining firn-air transport models.