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I opened up this thread because I think that finding out what happened here might be rather important. The plateau seems to be at least partially correlated with the socalled "global warming pause", where there may however be discorrelations due to incomplete temperature data, especially after the year 2007 (it should also be pointed out that hte density of methane measuring stations is likewise suboptimal). The correlation between methane and temperature data had been also found for global anomalies and their averaged annual differences (diff12). It is not clear wether methane values precede temperature values or not, or both. If not then a cause could be that higher temperatures may lead to higher methane releases like from melting permafrost. If yes then a reason might be that methane might have a bigger global warming potential than assumed (it is not clear wether a UV absorption line in the methane spectrum was appropriately taken into account). At least partially also both mechanisms or others may take place - the question here is then which influence dominates.
The methane plateau and its correlation with temperatures seems to be a rather important component in establishing this question. It should also be mentioned that global C02 doesn't show this strong plateau. In 2003 (so to say in the middle of the plateau the article"Atmospheric methane levels off: Temporary pause or a new steady state?" was outlining in the abstract:
The globally-averaged atmospheric methane abundance determined from an extensive network of surface air sampling sites was constant at ~1751 ppb from 1999 through 2002. Assuming that the methane lifetime has been constant, this implies that during this 4-year period the global methane budget has been at steady state. We also observed a significant decrease in the difference between northern and southern polar zonal annual averages of CH4 from 1991 to 1992. Using a 3-D transport model, we show that this change is consistent with a decrease in CH4 emissions of ~10 Tg CH4 from north of 50�N in the early-1990s. This decrease in emissions may have accelerated the global methane budget towards steady state. Based on current knowledge of the global methane budget and how it has changed with time, it is not possible to tell if the atmospheric methane burden has peaked, or if we are only observing a persistent, but temporary pause in its increase.