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):
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.”
[Read more here]
If this rise in methane is a sign that positive feedbacks are developing, then we are in serious trouble. Methane is 86 times as potent a greenhouse gas as carbon dioxide. Fortunately it rapidly (within 12 years) decays into CO₂. So if emissions stabilised, within 12 years the atmospheric level would also stabilise. CO₂, in contrast, takes millennia to work its way out of the atmosphere. And if CH₄ emissions started to fall, then the level of CH₄ in the atmosphere would also soon start declining. However, if the methane is from permafrost melt, then we are helpless to cut methane emissions, and the beginning of runaway global warming will have begun.
It really is a climate emergency. We must act to cut emissions of carbon dioxide and of the methane sources we control, now, before it's too late.
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