Atmospheric methane variability: Centennial‐scale signals in the Last Glacial Period

Abstract

In order to understand atmospheric methane (CH 4 ) biogeochemistry now and in the future, we must apprehend its natural variability, without anthropogenic influence. Samples of ancient air trapped within ice cores provide the means to do this. Here we analyze the ultrahigh-resolution CH 4 record of the West Antarctic Ice Sheet Divide ice core 67.2-9.8 ka and find novel, atmospheric CH 4 variability at centennial time scales throughout the record. This signal is characterized by recurrence intervals within a broad 80-500 year range, but we find that age-scale uncertainties complicate the possible isolation of any periodic frequency. Lower signal amplitudes in the Last Glacial relative to the Holocene may be related to incongruent effects of firn-based signal smoothing processes. Within interstadial and stadial periods, the peak-to-peak signal amplitudes vary in proportion to the underlying millennial-scale oscillations in CH 4 concentration-the relative amplitude change is constant. We propose that the centennial CH 4 signal is related to tropical climate variability that influences predominantly low-latitude wetland CH 4 emissions.Plain Language Summary Using a new method to measure methane concentrations of ancient air trapped in ice cores, we have detected variability in atmospheric methane concentration on centennial time scales in the Last Glacial Period for the first time. We know these signals represent past changes in atmospheric methane because they appear in several ice core records. We propose that changes in methane emissions from tropical wetlands are responsible. How this new variability might be related to similar signals found in the late Holocene ice core records and the instrumental record of atmospheric methane is an open question.

Is the Noble Gas‐Based Rate of Ocean Warming During the Younger Dryas Overestimated?

“…The degree of smoothing depends on site conditions; cold, low accumulation sites tend to have the widest gas age distributions and thus experience the most smoothing (Spahni et al, ). Because of its high accumulation and relatively moderate temperature, WD has an exceptionally narrow gas age distribution width (20 to 60 years; Rhodes et al, ); the noble gas MOT record should be virtually unaltered by smoothing at this site. In contrast, the Taylor Glacier deposition site has lower accumulation and colder temperatures, so a wider gas age distribution is expected.…”

Very Strong Atmospheric Methane Growth in the 4 Years 2014–2017: Implications for the Paris Agreement

“…If the processes driving growth are not directly anthropogenic, for example, tropical rainfall or OH, Cl, and soil sink changes, then the ~75 ppb scale of the recent rise so far is unprecedented in the Holocene record. For comparison, for fluctuations in the late Holocene Rhodes et al () find variabilities of 10–40 ppb. As noted above, large uncertainties remain in the global methane budget, and top‐down and bottom‐up estimates have not yet been reconciled.…”

An 83 000 year old ice core from Roosevelt Island, Ross Sea, Antarctica

“…Centennial-scale variability of methane in the late pre-industrial Holocene and Preboreal Holocene has been described from other records (Etheridge et al, 1998;Mischler et al, 2009;Sapart et al, 2012;Mitchell et al, 2013;Rhodes et al, 2017 Curiously, over the past 2,000 years, centennial-scale variability of methane only weakly correlates with temperature and precipitation of key methane producing regions (Mitchell et al, 2011), the main climatic factors controlling biogenic methane production (Fung et al, 1991). The apparent lack of coherence with climatic forcing and changes in the stable isotopic com-5 position of methane during this time period has led several studies to propose that this mode of variability may be linked to human population and early-anthropogenic land-use practices (Ferretti et al, 2005;Houweling et al, 2008;Mischler et al, 2009;Mitchell et al, 2011Mitchell et al, , 2013Sapart et al, 2012).…”

“…3), before the rise of agriculture, when estimates of human populations were much lower. Recently, centennial-scale variability has also been documented in much older ice (Rhodes et al, 2017), dating from the last glacial period. This evidence implies that centennial-scale variability can occur solely from natural forcing, but it is not currently clear what that forcing is.…”