A recent paper by Nisbet et al. notes that methane (CH4) in the atmosphere isn’t just increasing, it’s accelerating; the increase has gotten faster recently.
There was a time — from about 1999 until 2007 — when atmospheric methane wasn’t increasing at all. It had been, before 2000, but it remained steady for that 8-year period. But in 2007 it started rising again, as is plain to see in the following graph. What is not plain to see is that around 2014 it started rising even faster; the years 2014 through 2018 saw very rapid increase in CH4 levels.
Nisbet et al. identify the increased rate of rise in those four years, and consider the implication for the Paris climate agreement. The stated goal is to keep global temperature rise “well below 2°C.” So far, all our plans, our computer models, our strategies that have a decent chance of accomplishing that goal have relied on no increase in CH4, some even rely on decreasing CH4 in the air. The fact that it’s going the wrong way, at increasing speed, is a genuine threat to our chances of success.
I took the CH4 data from Cape Grim (shown above) and first removed the annual cycle, yielding de-seasonalized data. Then I fit two models. One is a lowess smooth, shown here in red (deseasonalized data in black):
The other is a piecewise-linear model in blue (deseasonalized data in black, the thin red lines show when the slope changes occur):
The slope change times were chosen to give the best fit. According to this model, the most recent slope change happened right around 2014. Especially useful is the fact that both models enable me to estimate how fast CH4 is changing. In fact the piecewise-linear model estimates the average rate of increase during each linear “piece.”
What do these models say about the rate of increase? This:
Red shows the estimated growth rate of CH4 from the lowess-smooth model, blue the estimate from the piecewise-linear model. Both make clear the increase in the growth rate that happened about 2014.
What’s the source of the increase? One clue is the 13C/12C isotopic ratio, which has been decreasing lately. Fossil-fuel CH4 is usually “heavier” (i.e. higher in 13C than other sources) while microbial CH4 (from biological sources) is lighter. But we don’t have enough geographical coverage of 13C/12C isotopic ratio to nail down the source of that change. Nonetheless, Nisbet et al. speculate on several possible causes of the acceleration of atmospheric methane.
One possibility is an increase in microbial CH4, from wetlands and cattle, which would account for the atmosphere’s decreasing 13C/12C ratio. Another possibility is a change of the fossil-fuel mix toward natural gas, which has a lighter 13C/12C ratio than coal. Yet another possibility is a decrease in the atmosphere’s ability to break down CH4, which would likewise account for both the increased amount, and the decreasing 13C/12C ratio. The most worrisome possibility — although not yet a likelihood thank goodness — is a dramatic increase in microbial CH4 from permafrost melt or other feedback sources.
And of course we cannot rule out the possibility of “all of the above.”
It’s imperative that we cease the increase of atmospheric methane, if at all possible.
This blog is made possible by readers like you; join others by donating at My Wee Dragon.