The natural sources

The estimates vary for natural sources

Due to the discrete and variable nature of natural methane emissions estimates of the total natural methane budget vary.

"Our ability to quantify the global methane budget is poor"  (Dlugokencky et al., 2011)

Unsurprisingly we hence see discrepancies between publications as to what natural (and anthropogenic) methane emission levels are. What we also see though is quite striking differences for the methane budget when working from a bottom up compared to a top down approach. Kirschke et al's peer assessed paper Three decades of global methane sources and sinks published in Nature in 2013 provides a good example of this:

For their 2000-2009 emissions calculations the two figures varied by a hardly negligible 60%:

218Tg/year by top down calculation compared to 347Tg/year by bottom up. I have chosen to reference this paper in particular as it is widely seen as drawing on findings from the most independent studies and was consequentially the one employed in the IPCC AR5 report. 

I have created the following diagram highlighting the relevant importances of the natural sources:

Source: Authors own

The significance of wetlands' is clearly huge, despite losses to wetland areas over the last century. Something to consider over the coming century is the potential for wetland areas to increase globally, as sea levels rise. Another interesting point to note is the current relative insignificance of both hydrates and permafrost. To see where this natural methane is coming from geographically the following digram of surface methane, produced by the NOAA is highly useful:

Source: NOAA 2010, available here

Natural breakdown of emissions

On the note of natural sources a note on the breakdown of emissions is fitting. We've heard in previous posts as to the significance of the OH radical in breaking down methane - this figure makes reference to the other, less significant but still important sinks. This paper suggests OH is responsible for breaking down 528Tg/year (83% - less than the figure of about 90% most papers suggest). Other sinks are stratospheric loss (cited at 8%), tropospheric Cl breakdown (4%) and breakdown by soils (at 4.5%).

In summary of this week's post:

1. Natural methane emissions are about 250Tg/year
2. Wetlands, producing 62% of natural methane emissions, are by far the most significant natural contributor
3. The role of OH in breaking down methane is perhaps slightly lower than the often cited 90%, according to Kirschke et al's 2013 data it is 83%, despite Kirschke et al citing the popular 90% figure in the introduction of the same paper.

References:

Dlugokencky, E., Nisbet, E., Fisher, R. and Lowry, D. (2011). Global atmospheric methane: budget, changes and dangers. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 369(1943), pp.2058-2072.

Kirschke, S., Bousquet, P., Ciais, P., Saunois, M., Canadell, J., Dlugokencky, E., Bergamaschi, P., Bergmann, D., Blake, D., Bruhwiler, L., Cameron-Smith, P., Castaldi, S., Chevallier, F., Feng, L., Fraser, A., Heimann, M., Hodson, E., Houweling, S., Josse, B., Fraser, P., Krummel, P., Lamarque, J., Langenfelds, R., Le Quéré, C., Naik, V., O'Doherty, S., Palmer, P., Pison, I., Plummer, D., Poulter, B., Prinn, R., Rigby, M., Ringeval, B., Santini, M., Schmidt, M., Shindell, D., Simpson, I., Spahni, R., Steele, L., Strode, S., Sudo, K., Szopa, S., van der Werf, G., Voulgarakis, A., van Weele, M., Weiss, R., Williams, J. and Zeng, G. (2013). Three decades of global methane sources and sinks. Nature Geoscience, 6(10), pp.813-823.

Ciais, P., C. Sabine, G. Bala, L. Bopp, V. Brovkin, J. Canadell, A. Chhabra, R. DeFries, J. Galloway, M. Heimann, C. Jones, C. Le Quéré, R.B. Myneni, S. Piao and P. Thornton, 2013: Carbon and Other Biogeochemical Cycles. In: Cli- mate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.