Methane Update: Less Worried, Still Very Worried

I just saw an interview by Skeptical Science with Ms. Sharapova, a Russian scientist, on the Russian findings that sparked my recent methane worries. Her responses clarified the answers to the main questions I had about Arctic sea methane clathrates. Her key information, imho, was the following:

1. The Russians recently discovered that methane clathrates could form not just in the 200-1000 m depth range, but also in the 20-200 m range.
2. They also found that clathrates there did not just melt from the top down; they also melted in pockets below the surface melt.
3. The 2011 survey, for the first time, looked at the 20-200 m Siberian coastal shelf, rather than the 200-2000 m deeper waters.

Let’s look at the implications for my methane analysis. In the first place, this explains why methane was able to bubble to the surface, instead of popping or being eaten by methane-munching bacteria: it was too close to the surface, and especially if, as appears to have been the case, it was being released in larger chunks/bubbles.

In the second place, this appears to indicate that the ramp-up in methane emissions at any particular point is less than I feared. There are several possible reasons to anticipate that at greater depths, methane release from the sediment would ramp up more slowly than a 100-fold increase in one year. Likewise, there are several possible reasons to anticipate that initial methane releases from the shallow continental shelves would be greater than that from deeper areas, if there were methane clathrates there in the first place.

However, in the third place, this newly discovered source of methane clathrates appears to be a much bigger source of emissions, both in terms of melting more rapidly and of having more methane stored to begin with. Because sea shelves slope more rapidly the deeper they get (to a point beyond 1000 m), the sea-surface area of 20-200 m deep shelves is comparable to the sea-surface area of 200-1000 m ones.

Under the surface, the methane clathrates can be stored much deeper before earth heating and pressure melt them. Take these two things together, and the amount of methane in Arctic clathrates may be 2-4 times the amount previously estimated. Meanwhile, this 20-200 m range lies almost entirely in the “shallow ocean” range where warming currents from the south plus warming of newly exposed surface waters by the summer sun create hotter water next to the sediment – and thus melt things faster. These points are confirmed by the observations of rapid bubble generation and much larger funnels from which methane flows in the 20-200 m range.

In the fourth place, the ability of 20-200 m methane bubbles to rise to the surface means that we probably grossly underestimate the percentage of emitted methane that will rise into the atmosphere as methane rather than carbon dioxide. Frankly, this is probably good news, since that means less of it will eventually stay in the atmosphere as carbon dioxide – but there’s still a chance it may stay as methane for a long time – and be far worse for global warming. This would happen if there’s too much methane up there and the OH in the atmosphere that removes a lot of that methane runs out, a possibility some scientists have raised.

In the fifth place, the existence of pockets indicates that methane emissions may be bursty, as surface melt “burns through” to those pockets that are themselves melted, but are still trapped by frozen clathrates above. Those bursts should be frequent enough to keep methane emissions at a higher yearly rate.

Implications

Overall, this makes me a little more hopeful about overall methane emissions and their effect on global warming. While there is perhaps 2-4 times the amount of methane clathrates to emit than I thought, there may be 30-70% lower emission rates than I anticipated, and that’s the key to methane’s overall effect in the next 160 years, when it matters. To put it another way, the net methane emission rates per year should be lower than I expected, and the amount in the atmosphere as methane in the next 160 years should be lower than my worst-case all-methane scenario (assuming there’s enough OH). Hopefully, the amount of carbon dioxide should be lower as well, because of the decreased yearly emissions amount and increased percentage arriving as methane (only half of that turns into carbon dioxide). However, this isn’t sure, because if the OH runs out, the effect of yet more “steady state” methane over, say, 600 years on global warming will be worse than I had anticipated.

All in all, I would now tend to put the likely overall new natural-source methane emission effects (also including permafrost and wetlands) in the 3-6 degree C range over the next over the next 200 years, and in the 2-5 degree Celsius range in the 400 years after that – overall, perhaps a 25% boost to global warming rather than a 50% one, protracted over more years. High water may not be delayed, but hell may be a little less hellish in temperature, and the end of life on earth ever so slightly less likely, than I feared.

Unless, of course, the OH in the atmosphere runs out … the worries never end, do they?