Rising CH4 = part of cyclical growth rate?

A paper published by Rigby et al. in GRL showed that after almost 10 years of stagnation, methane levels started to increase again in 2007; that increase was simultaneous in BOTH hemispheres. As the transport from one hemisphere to the other takes about one year, it seems not possible that this rising CH4 comes from thawing Siberian permafrost. Looking at the graph of the global growth rates, I wonder:

ch4_growth_rates_2008

Does the (recent) growth rate have a cyclical behaviour, of a period of about 5 years? CH4 levels have an annual cycle, with maximum release during the warmer periods. Could it be that the current state of the atmosphere is more one of relatively stable CH4 levels, slowly oscillating around the 1800 ppb level (nmol/mol)?
It just seems a bit premature to theorize on rising levels ( and to rise the alarm of thawing permafrost releasing huge quantities of a potent GHG) when the phenomen is cyclic. Wait and see…

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