Atmospheric methane peaks from sampling stations at Barrow northern Alaska and Alert northern Canada (off N Greenland) are centred on January of each annual cycle (Figures 1 and 4). This makes it highly unlikely that the annual cycle in methane concentrations is caused by the freeze thaw cycle of Arctic tundra.
Rather, the rise of methane concentrations 1983 to 1990 and subsequent flattening of the curve, combined with annual cycles with mid-winter peaks (Figure 2), matches Russian (or N American) natural gas production.
Figure 1 Atmospheric methane concentration data from Alert station, Canada. The annual peaks are centred on January each year when the Alert station is surrounded by frozen ground and frozen seas for more than 500 miles on all sides.
Methane emission from the Arctic shelf has a maximum in September -October. [when sea ice minimum occurs]
I was already conditioned to believe that the annual cycle present in Arctic atmospheric methane data was due to the natural freeze thaw cycle of the circum Arctic tundra. And I was therefore already conditioned to believe that the cycle maxima present in the NOAA data presented by Dr Box were centred on late summer. My intention was to write a quick and cheerful post placing the NOAA data into appropriate context by converting to ppm (parts per million) from ppb (parts per billion) and using a more appropriate y-axis scale (Figure 3). I then checked the data on an expanded x-axis (time axis) scale and was quite astonished to find that the methane peaks were centred on January (Figures 1 and 4).
There are two Arctic stations with extensive data series. One at Alert, Canada, that sits on the edge of the Arctic Ocean off the N coast of Greenland and the other at Barrow on the N coast of Alaska. The Barrow data are more noisy (Figures 2 and 4) but show the same trend as the Alert data with peaks centred on January (compare Figures 1 and 4).
Figure 2 The Barrow Alaska sampling station has the longest record of Arctic atmospheric methane data starting in 1983. The annual cycle defined by the majority of data points is plain to see but there are a large number of points that scatter above and below the main trend. Those below are clearly data errors. Those above were described by Dr Box as Dragon’s breath alluding to degassing of the tundra or submarine clathrates. That story will have to wait to another day. The annual peaks of the Barrow methane data are also centred on January (Figure 4).
Figure 3 The Barrow Alaska data, 1983 to 2013, converted to ppm and plotted on a 0 to 100 ppm y-axis scale. This is the same data as plotted in Figure 2. There is hardly any methane in the atmosphere and plotted at this scale it is impossible to discern any annual or decadal variation. 10,000 ppm = 1%, thus 100 ppm = 0.001%. Methane in the arctic atmosphere has a concentration of 2 ppm = 0.0002% of the atmosphere. The rise in methane since 1983 is about 0.2 ppm or 0.00002% of the atmosphere. NOAA assert that increasing methane from pre-indutrial levels accounts for 20% of anthropogenic climate forcing. Source of chart image is IGBP.
Figure 4 The Barrow Alaska data from 2004 to present also have methane peaks centred on January each year.
Figure 5 If the Arctic methane concentration data are not controlled by the annual freeze thaw cycle of the tundra then another explanation is required. The rise in methane 1983 to 1990 matches the rise in Soviet Union gas production but not the subsequent fall nor the dip associated with the 2008 financial crash.
In mid winter it is very difficult to explain the methane peaks by a natural cyclic thawing process since both sampling sites are surrounded by hundreds of miles of frozen ground, snow and sea ice at this time. So what is the origin of the annual cycle in the data? I have no axe to grind here and can but observe that Russian natural gas production rose until 1990 and thereafter plateaued and that Russian (indeed N hemisphere) natural gas production and consumption goes up in winter normally reaching a peak in January and February (Figure 5). If anyone else has a better idea then please offer it in the comments.
I have Googled around a bit looking for articles on Arctic methane and there are of course an abundance of them describing methane bubbles, plumes, melting permafrost, tipping points, ice breakers crunching their way through the Arctic sea ice trying to avert global melt down. But none of those I read in my quick search include reference to the beautiful NOAA data sets. I wonder why not?
Note added 10 September:
Commenters Peter Shaw and Greg Goodman suggested that the annual cycle with summer troughs may be due to optical destruction of methane. In other words sunlight accelerates the oxidation of methane to CO2. Roger Andrews posted the chart below that shows the methane cycle is reversed in the southern hemisphere. Optical destruction of methane in summer is a leading contender to explain the annual cycles.