Methane: a worse worst-case scenario

Real Climate has weighed in on the Arctic methane question, and, as always, their contribution is cogent, well-considered, and reasonable:

But the methane worst case does not suddenly spell the extinction of human life on Earth. It does not lead to a runaway greenhouse. The worst-case methane scenario stands comparable to what CO₂ can do. What CO₂ will do, under business-as-usual, not in a wild blow-the-doors-off unpleasant surprise, but just in the absence of any pleasant surprises (like emission controls). At worst comparable to CO₂ except that CO₂ lasts essentially forever.

It's comical (which I suppose is the point) the degree to which Randall Munroe captured this perspective in this 2010 cartoon:

"The worst-case scenario is what's happening now." Indeed it is. And we shouldn't allow ourselves to be distracted from the certain disaster of BAU CO2 emissions by the possible disaster of the rapid release of methyl hydrates.

That said -- and at the risk of sounding like stick-figure Michael Bay -- their worst-case scenario is pretty tame -- they increased Arctic emissions by a factor of a hundred compared to today. While that sounds like a lot, a mere 10% annual increase starting in 2010 would push us past that mark in 2060. That's a fair "nasty surprise" scenario, but I don't see how you can really call it "worst-case."

Helpfully, Real Climate rapidly followed the original post with a second one providing an online methane release model. So we can easily look at a Mississippi-rerouting, flaming alligator scenario. Here goes:

While we have estimated the Arctic methyl hydrates at about 2,000Gt, those estimates have varied by a factor of eight from one study to another. In the worst case, we have underestimated the amount of methane, and there is about 8,000Gt under the Arctic, and 40,000Gt worldwide.

In 2023, the Arctic unexpected flips over to a new high-convection state that is ice-free year-round. The Arctic rapidly warms by 15-20C (as it did the last time CO2 hit 390ppm). In this new regime, Arctic methyl hydrates prove far less stable than we thought, and 80% of them are released over the next 170 years. The warming driven by that plus human CO2 even destabilizes a small portion of the global methyl hydrate deposits, previously thought to be safe: 20% of them degas in the same time period.

So 80% of Arctic methyl hydrates (6,400Gt) plus 20% of the rest (6,400Gt) = 12,800Gt over a hundred and seventy years (2023 -- 2193). Plug that into the model and we get:

We rapidly build to a total change in forcing of +25 W/m^2. Because the atmospheric processes that oxidize methane to CO2 cannot keep up, the methane becomes a long-lived greenhouse gas, the average molecule hanging around for decades instead of years:

 With the radiative forcing of seven doublings of CO2, total warming quickly exceeds +15C (on its way to +20C), rendering most of the earth's surface uninhabitable:

Sherwood (2010). +12C.

The blue and red (and black) on the graph indicate areas likely habitable by humans (absent 24-hour climate-controlled environments) in a +15C world. At +20C, it's Greenland, Iceland, Antarctica or fry.

The Amazon and the boreal forest burns; massive anoxic events spread across the oceans; billions fight over the last scraps of habitable land even as plummeting agricultural yields kill billions by starvation. The living envy the dead.

And that, my friends, is a real worst-case scenario.