Methane Update

As we have noted previously, the second most important anthropogenic greenhouse gas (after carbon dioxide, or CO₂), is methane (CH₄), and although its concentration (only about 1.8 ppm) is far less than that of CO₂ (at 390 ppm), it’s a stronger greenhouse gas on a per-molecule basis, and it ends up transformed into CO₂ by atmospheric chemistry processes anyway. Also, more than half the atmospheric CH₄ load is due to changes wrought by mankind. Atmospheric methane concentration had stabilized from about 1999 to 2007, but recently began rising again, as reported in Rigby et al (2008).

Data collected since their report has confirmed that CH₄ began rising in 2007. But has the rise continued? I retrieved data from the World Data Centre for Greenhouse Gases, and selected those stations which included at least some data since mid-2010. There were 10 such stations, and if we plot their CH₄ data on a single graph, it’s too cluttered to be very informative:

There’s a clear offset between different stations. Also, some have noisier data than others, they have different annual cycles (some larger, some smaller), so the bare “spaghetti graph” isn’t very enlightening, especially about the last few years.

We can combine data from different stations, but we’ll have to compensate for the fact that different locations have values which are offset from each other. Basically, we need to combine the data records by aligning them to a common average value over their period of overlap. This is the same process done with temperature data to align different records. Incidentally, I’ve recently abandoned my own method of aligning data records in favor of a modified form of the Berkeley method.

When we align the 10 station records we get this:

We can then average the aligned values to get a composite record:

There’s clear indication that the increase, which began about 2007, is still under way.

Of course that conclusion is somewhat clouded by the annual cycle of the data, and especially by the fact that each of the individual records shows a different annual cycle. Therefore, let’s take each raw data record and remove the annual cycle, transforming to CH₄ anomaly:

The annual cycles are gone, but we still have offsets between stations because I’ve defined anomaly using a different baseline for each station — it’s own complete record. We can overcome this simply by aligning the anomaly data (just as we did with the raw data):

Now there’s even clearer indication that the methane increase has continued. Again, we can average the aligned anomalies to form a composite of methane anomaly values:

This leaves little doubt that methane concentration, averaged over these 10 stations, has continued to increase since its rise began in 2007. We can even take the post-2007 data and fit a linear regression line to estimate the rate of increase:

It appears that atmospheric CH₄ concentration is increasing at a rate of 5.9 +/- 1.6 ppb/yr.

UPDATE:

Per request, I retrieved data for d13CH4. There are only 13 stations with sufficient data, which cover the time range from 1998 through 2009. Here are the individual aligned records of d13CH4 anomaly:

Here’s the combined data:

What does it mean? I don’t know.

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