A doomer commentary on methane has been doing the rounds on social media. Pictures of methane bubbles certainly look scary, but the overall atmospheric concentration of methane has been showing only a mild rise. From the Advanced Global Atmospheric Gases Experiment (AGAGE) data series:
Moreover, the current climb is far slower than that seen in the 1980s. From a paper by Kirschke et al (click for larger image).
Although methane is a very powerful greenhouse gas (about 20 times as powerful as CO2), it presently makes up a little under 2 parts per million (ppm) of the atmosphere compared to around 400 ppm for CO2.
Critically, methane’s atmospheric life is short, about 12 years, after which it converts into CO2 (and thus becomes 20 times less potent). For this reason, it doesn’t accumulate easily. Keeping this in mind, a post by David Archer on the Real Climate blog looked at chronic versus catastrophic methane releases (click for larger image).
So, in order to recreate a disaster movie scenario, we either need to see a massive and sustained release of methane or a ginormous spike in methane emissions. Where would this come from? The candidates are generally given as methane hydrates or other sources of trapped methane at high northern latitudes. But to see how realistic such places are as a source, we need to see where the methane is coming from at present (source here; click for larger image).
As you can see, anthrogenic sources such as wet-field rice cultivation, fossil fuel extraction and animal-rearing over-shadow other sources such as hydrates. Indeed, to get hydrates to become the principal driver of atmospheric methane concentrations we would need to see a 10 to 100-fold rise, and this would then need to be sustained for a long period of time.
According to scientists such as David Archer and Gavin Schmidt, such emission scenarios don’t look plausible (for more detail see here). In short, they see little evidence of a methane bomb ready to explode.
Simplistically, the difference between methane and CO2 is that the latter stays up in the atmosphere once put there while the former doesn’t. In sum, CO2 provides plenty of disaster movie material; we don’t have to look further afield to scare ourselves senseless.