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Atmospheric Methane on the Rise Again?
Reference
Rigby, M., Prinn, R.G., Fraser, P.J., Simmonds, P.G., Langenfelds, R.L., Huang, J., Cunnold, D.M., Steele, L.P., Krummel, P.B., Weiss, R.F., O'Doherty, S., Salameh, P.K., Wang, H.J., Harth, C.M., Muhle, J. and Porter, L.W. 2008. Renewed growth of atmospheric methane. Geophysical Research Letters 35: 10.1029/2008GL036037.

Background
The authors begin the report of their study by noting that "previous measurements of atmospheric CH4 [methane] showed a persistent increase in the global burden throughout the latter part of the twentieth century, followed by a period of little change since 1999."

What was done
Rigby et al. analyzed data obtained from the Advanced Global Atmospheric Gases Experiment (AGAGE) -- a network of five stations located in coastal regions at latitudes from 53N to 41S -- and the Australian Commonwealth Scientific and Industrial Research Organization (CSIRO) -- a network of eight locations around the globe that "provides an independent and complementary data set, and a wider latitudinal site distribution."

What was learned
The sixteen scientists report that their methane measurements, which run from January 1997 to April 2008, "show renewed growth from the end of 2006 or beginning of 2007 until the most recent measurements," with the record-long range of methane growth rates mostly hovering about zero, but sometimes dropping five parts per billion (ppb) per year into the negative range, while rising near the end of the record to mean positive values of 8 and 12 ppb per year for the two measurement networks.

What it means
Although some people might be alarmed by these findings, as well as by the US, UK and Australian researchers' concluding statement that the methane growth rate during 2007 "was significantly elevated at all AGAGE and CSIRO sites simultaneously for the first time in almost a decade," there is also reassurance in the recent findings. We note, for example, that near the end of 1998 and the beginning of 1999, both networks measured even larger methane growth rate increases of approximately 13 ppb per year, before dropping back to zero at the beginning of the new millennium. And we note that the most current displayed data from the two networks indicate the beginning of what could well be another downward trend.

Additional reassurance comes from the work of Simpson et al. (2002), the findings of whom we reproduce in the figure below. As can be seen there, even greater methane growth rates than those observed by Rigby et al. occurred in still earlier years. Hence, these periodic one-year-long upward spikes in methane growth rate must be the result of some normal phenomenon, the identity of which, however, has yet to be determined.


Figure 1. Global tropospheric methane (CH4) growth rate vs. time. Adapted from Simpson et al. (2002).

So what does the future hold? Our "best bet" would be that the 2007 methane growth rate spike will taper off over the next few months and probably dip back into negative territory, possibly remaining there for a couple of years until the next spike comes along. Whether we are right or wrong, we will be reporting the data that provide the answer when they are published by the scientists making the measurements. Stay tuned.

Refernece
Simpson, I.J., Blake, D.R. and Rowland, F.S. 2002. Implications of the recent fluctuations in the growth rate of tropospheric methane. Geophysical Research Letters 29: 10.1029/2001GL014521.

Reviewed 3 December 2008