Guest post by Indur M. Goklany
In the earlier post reporting on the recent greening of the Arctic, some commentators — Crispin in Waterloo, BillD, Jimbo — have alluded to the notion that Arctic thawing could lead to positive feedback by adding to methane emissions to the atmosphere.
This global warming bogeyman is founded on the plausible notion — plausible, at least at first blush — that warming might release methane from methane clathrates (or hydrates) stored in the Arctic permafrost which would increase its concentration in the atmosphere.
But methane has a “global warming potential” averaged over 100 years of 25, that is, methane, ton-for-ton, is 25 times more powerful a greenhouse gas than carbon dioxide (AR4WG1 Technical Summary: 33). Thus, such releases of methane would constitute a positive feedback for global warming.
The initial concerns about methane stemmed from the fact that by the 1990s the atmospheric concentration of methane, which had been growing rapidly, had exceeded 1,730 parts per billion (ppb), almost twice the maximum amount measured over the past 650,000 years in ice cores (AR4WG1: 3).
Concern of runaway methane feedback was also stoked by a number of modeling studies which suggested rapid disintegration of the permafrost with global warming (e.g., Lawrence and Slater 2005, Zimov et al. 2006). However, in a modeling study which took into consideration the thermal profile of the permafrost, and the fact that the melting effect of warm air surface temperatures on the upper layers of permafrost would be countered by cooling due to colder deeper layers of permafrost, Delisle (2007) showed that “massive releases of methane in the near future are questionable.”
Even more compelling is that the growth in atmospheric concentrations has slowed substantially. As noted by the IPCC AR4WG1 (p. 796):
Recent measurements show that CH4 growth rates have declined and were negative for several years in the early 21st century … The observed rate of increase of 0.8 ppb yr–1 for the period 1999 to 2004 is considerably less than the rate of 6 ppb yr–1 assumed in all the [IPCC] SRES scenarios for the period 1990 to 2000.”
The latest observations indicate that the rate of change is not increasing, and that they “are not consistent with sustained changes … yet” (Dlugokencky et al. 2009: 4). [Dlugokencky’s “yet” seems gratuitous — no matter, I’ll give it a pass.] They also indicate that the geographical pattern and the isotopic signature of methane increases suggests that the major sources are wetlands — probably tropical wetlands —rather than Arctic permafrost.
Petrenko et al. (2009) examined the source of isotopic methane in a glacial ice core from West Greenland to determine the probable source of the large increase in methane during the abrupt warming of +10±4°C that occurred during the transition from the Younger Dryas to the Preboreal (~11,600 years ago) (Grachev and Severinghaus 2005). They concluded that “wetlands were the likely main driver of the [methane] increase and that clathrates did not play a large role,” a finding they noted “is in agreement with findings from previous ice core CH4 isotopic studies” (Petrenko et al. 2009: 508). This study essentially reiterated the results of another paper by many of the same researchers that appeared in Nature the previous year (Fischer et al. 2008). Notably the Petrenko et al. study’s publication was accompanied by an announcement titled, “Ancient Greenland methane study good news for planet, says CU-Boulder scientist” (Eureka Alert 2009).
So it seems that while methane emissions might increase if there is warming, there is no evidence of catastrophic releases from clathrates.
1. The above is, for the most part, extracted from:
Goklany, Indur M. (2009). Trapped Between the Falling Sky and the Rising Seas: The Imagined Terrors of the Impacts of Climate Change. Prepared for University of Pennsylvania Workshop on Markets & the Environment, draft, 13 December 2009.
2. Specific references follow:
AR4WG1 ≡ IPCC’s Fourth Assessment Report for Work Group 1 ≡ IPCC (2007). Climate Change 2007: The Physical Science Basis. Cambridge: Cambridge University Press.
Delisle, G. (2007), Near-surface permafrost degradation: How severe during the 21st century?, Geophys. Res. Lett., 34, L09503, doi:10.1029/2007GL029323.
Dlugokencky, E. J., et al. (2009). Observational constraints on recent increases in the atmospheric CH4 burden. Geophysical Research Letters, 36, L18803, doi:10.1029/2009GL039780.
Eureka Alert. 2009. Ancient Greenland methane study good news for planet, says CU-Boulder scientist. PR announcement, 23 April 2009. Available at http://www.eurekalert.org/pub_releases/2009-04/uoca-agm042109.php.
Fischer, H., Melanie Behrens, Michael Bock, Ulrike Richter, Jochen Schmitt, Laetitia Loulergue, Jerome Chappellaz, Renato Spahni, Thomas Blunier, Markus Leuenberger & Thomas F. Stocker (2008). Changing boreal methane sources and constant biomass burning during the last termination. Nature 452: 864 -865.
Grachev, Alexi M. and Jeffrey P. Severinghaus (2005). A revised +10±4 °C magnitude of the abrupt change in Greenland temperature at the Younger Dryas termination using published GISP2 gas isotope data and air thermal diffusion constants. Quaternary Science Reviews 24 ( 5-6): 513-519.
Lawrence, D. M., and A. G. Slater (2005). A projection of severe nearsurface permafrost degradation during the 21st century, Geophys. Res. Lett., 32, L24401, doi:10.1029/2005GL025080.
Petrenko, Vasilii V.; Andrew M. Smith, Edward J. Brook, Dave Lowe, Katja Riedel, Gordon Brailsford, Quan Hua, Hinrich Schaefer, Niels Reeh, Ray F. Weiss, David Etheridge, and Jeffrey P. Severinghaus. 14CH4 Measurements in Greenland Ice: Investigating Last Glacial Termination CH4 Sources. Science 324: 506-508.
Zimov, S. A., E. A. G. Schuur, and F. S. Chapin III (2006). Permafrost and the global carbon budget, Science, 313, 1612–1613.