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

Abstract

To determine in detail how the concentration of tropospheric methane has changed from preindustrial until recent times, an ice core with remarkably fine air-age 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 ( 1u ). 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 .

State of the Antarctic and Southern Ocean climate system

“…Changes in the atmospheric gases CO 2 , CH 4 , and N 2 O and temperature through the past 650,000 years [ Siegenthaler et al , 2005; Spahni et al , 2005] of these glacial/interglacial cycles are recorded with remarkable fidelity [e.g., Etheridge et al , 1992] in deep ice cores recovered from Antarctica. The cores reveal both the responsiveness of the ice sheet to changes in orbitally induced insolation patterns and the close association between atmospheric greenhouse gases and temperature [ EPICA Community Members , 2004].…”

Control of interannual and longer‐term variability of stratospheric water vapor

“…As our focus is on [H 2 O] e rather than on the contribution from methane oxidation ([H 2 O] CH4 ), and its interannual variation due to changes in stratospheric transport and chemistry, we will encapsulate all processes governing methane oxidation in a single parameter γ. That is, where the methane mixing ratio at entry into the stratosphere [CH 4 ] e is set to the tropospheric methane mixing ratio (taken from the data presented by Dlugokencky et al [1998] and Etheridge et al [1992]), and is the mean stratospheric age. The use of the mean stratospheric age instead of fully accounting for the stratospheric age spectrum is justified by the fact that interannual anomalies of tropospheric methane concentrations are much smaller than its average value.…”

“…Figure 8 reproduces the water vapor observations over Washington, D. C., and Boulder, Colorado, and the linear trends (as shown by Oltmans et al [2000, Figures 1 and 3], and the data interpretation of Rosenlof et al [2001] (shaded). Assuming present‐day, constant γ (see ) and historic tropospheric methane concentrations as provided by Dlugokencky et al [1998] and Etheridge et al [1992], the figure further shows an estimate for [H 2 O] e (dark grey shaded).…”

Analytical constraints on layered gas trapping and smoothing of atmospheric variability in ice under low-accumulation conditions

“…Working on ice cores and firn from highaccumulation sites, Etheridge et al (1992), Mitchell et al (2015), and Rhodes et al (2016) have discussed the influence of centimeter-scale physical variability in firn on recorded gas concentrations. They argue that physical heterogeneities can lead to variations in closure depth for juxtaposed ice layers.…”