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.
Highlights
Methane (CH4) is an important atmospheric greenhouse gas emitted from both natural and anthropogenic sources
We use a set of trace gas data measured from firn 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 20 NEEM (North Greenland Eemian ice Drilling) site
80 In this study, we present a set of mole fractions of CH4 and other trace gases in firn air samples collected at the North Greenland Ice Core Project (NGRIP) site
Summary
Methane (CH4) is an important atmospheric greenhouse gas emitted from both natural and anthropogenic sources. The comprehensive 65 Law Dome ice core and firn data set (Etheridge et al, 1998; MacFarling Meure et al, 2006) almost continuously covers the last 200 years and are well connected to the direct measurements at South Pole (SPO, data provided by NOAA/ESRL/GML) and Cape Grim (CGO) (Etheridge et al, 1998; MacFarling Meure et al, 2006) Such consistency and continuity among the datasets suggests that the Antarctic CH4 data can serve as a good reference to represent the global atmospheric CH4 trend over the past centuries. The Arctic CH4 scenario by Buizert et al (2012) is consistent 75 with the direct measurements that started in the 1980s, and ice core reconstructions before ~1900 (Figure 1) It is considered as the most likely synthetic atmospheric CH4 trend for the northern high latitudes, and is treated as a “known” history, by which the diffusivity profiles in firn are tuned in firn-air transport models (Witrant et al, 2012; Trudinger et al, 2013). Uncertainty of the Arctic atmospheric CH4 history for use in firn air modeling is discussed
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