The core issue about methane is that it is 20 to 25 times more dangerous as a GHG than carbon dioxide. A key component in disregarding methane has been it's relatively short estimated 'half life' of 8 years before it breaks down into the less dangerous carbon dioxide and vapor.

I posted before about the danger of increasing the concentration of methane in the upper atmosphere when combined with an increase in that 'half life'. As hydroxyl radicals are used up in the troposphere, the sink for methane becomes depleted, increasing the effective half life, or persistence of methane in the upper atmosphere.

Every sign is, we are on that very path, and no this chart does not illustrate that, necessarily.

We've pumped a whole bunch of methane into the upper atmosphere now for some 200 years.

Please read the whole diary, check the comments and look at the links, rather than just comment on one graph. Thanks. To get an understanding of the seriousness of the problem, this graph below might make that more clear. The rise in CH4 levels since the 1800s is directly related to population, increased population of ruminants, increased agricultural use of methanogenic plants like rice and industrial pollution.

However, the impact of AGW [climate change] on methane production potentially dwarfs human caused increases due to non AGW reasons since the 1800s, as large areas of permafrost are exposed for the first time in tens of thousands or perhaps hundreds of thousands of years. Bacteria that have remained frozen dormant will quickly add to the release of methane.

Methane bubbles, miles wide

Late in April 2009, astronauts aboard the International Space Station observed a strange circular area of thinned ice in the southern end of Lake Baikal in southern Siberia. The ice ring had a diameter of 2.7 miles (4.4 km). Credit: NASA

Methane clathrates [ocean bound deposits of methane] hold yet even more potential danger. Most scientists say any substantial release of clathrates would be very unfortunate for most life forms on this planet. [Gross Understatement.]

link Large-Scale Controls of Methanogenesis Inferred from Methane and Gravity Spaceborne Data

A. Anthony Bloom,1 Paul I. Palmer,1,* Annemarie Fraser,1 David S. Reay,1 Christian Frankenberg2 Wetlands are the largest individual source of methane (CH4), but the magnitude and distribution of this source are poorly understood on continental scales. We isolated the wetland and rice paddy contributions to spaceborne CH4 measurements over 2003–2005 using satellite observations of gravity anomalies, a proxy for water-table depth {Gamma}, and surface temperature analyses TS. We find that tropical and higher-latitude CH4 variations are largely described by {Gamma} and TS variations, respectively. Our work suggests that tropical wetlands contribute 52 to 58% of global emissions, with the remainder coming from the extra-tropics, 2% of which is from Arctic latitudes. We estimate a 7% rise in wetland CH4 emissions over 2003–2007, due to warming of mid-latitude and Arctic wetland regions, which we find is consistent with recent changes in atmospheric CH4.

Here's a very nice posting about the very subject.

http://knol.google.com/...

I replied at the bottom of that posting with my concerns.

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updated, now that I found some spare time .. and shortly off to get some sleep!