Blog: An Ice Record of Methane Sources?

Portrait_MN_4

By James Levine. 29th June 2012.

Methane is a key atmospheric constituent, being both a potent greenhouse gas and an influence on the tropospheric oxidising capacity – the ability of the atmosphere to rid itself of pollutants.   Future changes in the concentration of methane ( [CH4] ) thus have implications for both our climate and air quality, and an understanding of past changes in [CH4] is a prerequisite for meaningful future predictions. The study of past changes also offers valuable insights into interactions between composition and climate on timescales of anything up to 800,000 years – the length of time for which Antarctic ice provides a record of climate and the air bubbles trapped within it provide a record of [CH4].

The most striking natural features of this record are: the differences in [CH4] between glacial and interglacial periods, for instance almost doubling between the Last Glacial Maximum (LGM; 21,000 years ago) and the pre-industrial era (PI; 200 years ago); and rapid rises in [CH4], of up to half the glacial-interglacial difference in less than 100 years, accompanying northern-hemisphere warmings at the beginning of Dansgaard-Oeschger (D-O) events during the last glacial period (21,000-110,000 years ago). Whilst there has been much debate regarding the relative roles of changes in methane sources and sinks between the LGM and the PI (the dominant ones being wetlands and oxidation by the hydroxyl radical, OH, respectively), the rises in [CH4] at the beginning of D-O events have received less attention. Yet these could offer insights into the likelihood of future rapid rises in [CH4] in response to continued climate change.

Two controls on the oxidising capacity have been identified as having been influential between the LGM and the PI: emissions of non-methane volatile organic compounds (NMVOCs) from vegetation, with which methane competes for reaction with OH; and air temperatures, which affect the production of OH and the reactivity that OH shows towards methane (in addition to the kinetics of other chemical reactions). In a previous computer-modelling study, we found that the net effect of these two controls on the oxidising capacity was negligible between the LGM and the PI; their separate effects, though substantial, were roughly equal and opposite, implying the near-doubling of [CH4] during that period was essentially entirely source-driven. This conclusion, though subject to significant uncertainties, is consistent with the most recent estimates of the changes in methane sources between the LGM and the PI.

Here, we report a recent extension of that study (Levine et al., 2012) to a modelled D-O event featuring a characteristically rapid rise in [CH4] in response to a northern-hemisphere warming driven by a change in ocean circulation. We find that the influences of changes in NMVOC emissions and air temperatures across the rapid rise in [CH4] continue to offset each other, and the net effect on the oxidising capacity is again negligible. This suggests the rapid rises in [CH4] at the beginning of D-O events may have also been almost entirely source-driven, and the most striking natural features of the ice record of [CH4], over the last 110,000 years, may almost purely reflect changes in methane sources.

Levine, J. G.1, E. W. Wolff1, P. O. Hopcroft2, and P. J. Valdes2 (2012), Controls on the tropospheric oxidizing capacity during an idealized Dansgaard-Oeschger event, and their implications for the rapid rises in atmospheric methane during the last glacial period , Geophys. Res. Lett., 39, L12805, doi:10.1029/2012GL051866.

1British Antarctic Survey, High Cross, Madingley Road, Cambridge, UK
2Bristol Research Initiative for the Dynamic Global Environment, University of Bristol, Bristol, UK

Advertisements

Clathrate Gun Shot Down?

Greenland Ice Core_0

23rd July 2010.

Arctic and Antarctic ice cores provide a rich source of evidence that both temperature and atmospheric concentrations of methane and carbon dioxide have fluctuated over the past 800,000 years. These studies have also shown that the fluctuations are cyclic, and that higher temperatures are generally associated with higher levels of methane and carbon dioxide in the atmosphere. The source(s) of the periodic increases in methane concentrations are less clear, and have been the subject of much debate. Two competing hypotheses have been proposed: (1) that the extra methane comes from increased wetlands emissions (the ‘wetland hypothesis’) and (2) that methane hydrate disintegration is responsible for the increase in methane which in turn can trigger a dramatic temperature rise (the ‘clathrate gun hypothesis’).

A new study (Bock et al., 2010) of the hydrogen isotope ratios in methane from North Greenland ice-cores dated from 33,700 – 41,000 years ago, has now tipped the balance in favour of the wetland hypothesis for the series of climate fluctuations known as the Dansgaard-Oeschger events.

The study uses the fact that the hydrogen isotope composition of methane emitted from wetlands falls in the range -300 to -400‰ δD(CH4), distinctly more depleted in deuterium than that in hydrate-sourced methane, which averages ~ -190 δD(CH4). A number of other factors (e.g. precipitation, temperature-related Rayleigh distillation, kinetic fractionation associated with the OH sink) also impact on the hydrogen isotope ratio, but these are secondary effects which only alter the δD(CH4) by a few per mill

Bock et al. observed that over the 7,300 years represented by their ice cores, the methane from the interstadial (warmer temperature) periods was ~ 10 ‰ more depleted in deuterium than that from the cooler periods (stadials), and that this change in δD(CH4) could only be explained by the wetland hypothesis. Modelling indicated the hydrogen isotope measurements in the interstadials were consistent with a six fold increase in high latitude wetland emissions, from ~5 to ~ 32 Tg CH4 year-1, an increase of ~84 to ~118 Tg CH4 year-1 from tropical wetlands, and a constant rate of emission from marine hydrates of ~ 25 Tg CH4 year-1.

Hydrates expert Professor Mark Maslin, Director of the UCL Environment Institute, agrees that the paper “provides good evidence that gas hydrates were not involved in the initial methane rise…” during the Dansgaard-Oeschger events, but believes that there is “still considerable debate about whether tropical wetland, boreal wetlands or flooded shelf due to millennial scale sea level rise are the true source of the methane rise….” The authors of the paper concede that the methane cycle in this period remains underdetermined. However, the ruling out of one important potential methane source represents a significant advance in our understanding.

References

Michael Bock, Jochen Schmitt, Lars Möller, Renato Spahni, Thomas Blunier and Hubertus Fischer (2010), Hydrogen isotopes preclude marine hydrate CH4 emissions at the onset of Dansgaard-Oeschger events, Science, v.328, 1686-1689.

Mark Maslin, Matthew Owen, Richard Betts, Simon Day, Tom Dinkley-Jones and Andrew Ridgwell (2010), Gas hydrates: past and future geohazard? Phil. Trans. R. Soc. A, v.368, 2369-2393