Changes in tropospheric methane between 1841 and 1978 from a high accumulation-rate Antarctic ice core

To determine in detail how the concentration of tropospheric methane has changed from preindustrial until recent times, an ice core with remarkably fine airage resolution was investigated. The core, called DE08, contains air from as recent as 1978 with an age resolution (80% air-age distribution width) of about 14 years. It was drilled from a region of Law Dome, Antarctica with extremely high snow-accumulation rate (1160 kg m^-2 yr^-1). The ice chronology was determined from the observed chemical and isotopic seasonal variations, verified against a volcanic horizon. The calculated air-age includes the effects of bubble close-off, sealing of the firn and diffusive mixing of air in the firn. The mean air-age was 35 years younger than the host ice except for air in summer ice layers which was 37 years younger than the host ice. The extracted ice-core air was analysed for methane using gas chromatography with flame-ionisation detection. Adjustments of — 6 ppbv and + 0.3% were made to the measured concentrations to allow for the effects of the extraction process and gravitational fractionation, respectively. Methane concentrations in the DE08 record increased from 823 parts per billion by volume (ppbv, in dry air) in 1841 to 1481 ppbv in 1978. The measurement precision was ± 22 ppbv (1σ). The similarity of the methane records from the DE08 ice core and from Cape Grim, Tasmania implies that there was insignificant modification during the enclosure of air in the ice or during its recovery and analysis. Methane concentrations in the period from 1951 to 1978, which were previously estimated from sporadic and inferred data, are particularly well defined in this core. The DE08 record shows that methane growth rates have generally increased since the onset of the industrial revolution to a level of 14 ppbv year^-1 (about 1% per year) by the 1970s. The exception was between about 1920-1945 when the growth rate stabilised at about 5 ppbv year^-1.