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Or, trying to “correct” for the different lifetimes of the gases using Global Warming Potentials, over a 100-year time horizon (which still way under-represents the lifetime of the CO2), you get that the methane would be equivalent to increasing CO2 to about 500 ppm, lower than 750 because the CO2 forcing lasts longer than the methane, which the GWP calculation tries in its own myopic way to account for.

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.

The “runaway greenhouse effect” that planetary scientists and climatologists usually call by that name involves water vapor. A runaway greenhouse effect involving methane release (such as invoked here) is conceptually possible, but to get a spike of methane concentration in the air it would have to released more quickly than the 10-year lifetime of methane in the atmosphere. Otherwise what you’re talking about is elevated methane concentrations, reflecting the increased source, plus the radiative forcing of that accumulating CO2. It wouldn’t be a methane runaway greenhouse effect, it would be more akin to any other carbon release as CO2 to the atmosphere. This sounds like semantics, but it puts the methane system into the context of the CO2 system, where it belongs and where we can scale it.

So maybe by the end of the century in some reasonable scenario, perhaps 2000 Gton C could be released by human activity under some sort of business-as-usual scenario, and another 1000 Gton C could come from soil and methane hydrate release, as a worst case. We set up a model of the methane runaway greenhouse effect scenario, in which the methane hydrate inventory in the ocean responds to changing ocean temperature on some time scale, and the temperature responds to greenhouse gas concentrations in the air with another time scale (of about a millennium) (Archer and Buffett, 2005). If the hydrates released too much carbon, say two carbons from hydrates for every one carbon from fossil fuels, on a time scale that was too fast (say 1000 years instead of 10,000 years), the system could run away in the CO2 greenhouse mode described above. It wouldn’t matter too much if the carbon reached the atmosphere as methane or if it just oxidized to CO2 in the ocean and then partially degassed into the atmosphere a few centuries later.

The fact that the ice core records do not seem full of methane spikes due to high-latitude sources makes it seem like the real world is not as sensitive as we were able to set the model up to be. This is where my guess about a worst-case 1000 Gton from hydrates after 2000 Gton C from fossil fuels in the last paragraph comes from.

On the other hand, the deep ocean could ultimately (after a thousand years or so) warm up by several degrees in a business-as-usual scenario, which would make it warmer than it has been in millions of years. Since it takes millions of years to grow the hydrates, they have had time to grow in response to Earth’s relative cold of the past 10 million years or so. Also, the climate forcing from CO2 release is stronger now than it was millions of years ago when CO2 levels were higher, because of the band saturation effect of CO2 as a greenhouse gas. In short, if there was ever a good time to provoke a hydrate meltdown it would be now. But “now” in a geological sense, over thousands of years in the future, not really “now” in a human sense. The methane hydrates in the ocean, in cahoots with permafrost peats (which never get enough respect), could be a significant multiplier of the long tail of the CO2, but will probably not be a huge player in climate change in the coming century.

This is not the first time a methane burp has been blamed for an extinction. Though the Cretaceous asteroid cleared the stage, mammals did not really get going until 10m years later, in the Eocene epoch. The preceding Palaeocene epoch was also brought to an end, the rocks suggest, by a sudden release of methane.

The burp could, of course, have been provoked by the eruptions, so the volcanoes are not off the hook completely. But, for those of a nervous disposition, the tying of an ancient greenhouse warming to an ancient mass extinction might suggest lessons for the future.

The ocean methane hydrate reservoir can be considered a very slow but irreversible tipping point in the Earth’s carbon cycle. The warming effect potentially caused by methane release from this source is likely to be very gradual and of moderate magnitude, but it would last for millennia.

Archer et al. simulate the spatial distribution of ocean methane hydrates and assess their sensitivity to changing climate. They estimate the current total inventory of methane in ocean hydrates is between 1600-2000 Pg of C. When the sediment column warms and hydrates melt, methane bubbles are produced. If the volume of these methane bubbles exceeds a critical value, the sediment column releases methane. A critical question for the future is how much methane from melting hydrates will potentially escape from the seafloor to reach the ocean or the atmosphere. The authors find that, in response to a 3°C ocean warming and assuming a 10% critical bubble fraction for gas escape, only 2% of the methane inventory (about 30 Pg of C) would potentially escape. However, if the critical bubble fraction is 2.5%, about 50% of the methane (940 Pg of C) could escape. When the hydrate model was embedded in a global climate model forced by two fossil fuel CO2 emission scenarios (1000 Pg of C and 5000 Pg of C) they found that methane is released over a time period of several thousand years. Even for high methane emission responses, the atmosphere only warms by about 0.4-0.5°C. Thus, the potential warming effect of melting hydrates on the atmosphere would be quite moderate and would take millennia to manifest. However, the induced warming was shown to persist for at least 10-kyr.

Summary courtesy of Environment Canada

As the specific humidity continues to climb, the limiting infrared cooling that can occur for a planet comes at a threshold known as the Simpson-Kombayasi-Ingersoll (SKI) limit. If the incoming absorbed solar radiation exceeds this radiation threshold then the surface temperature must rise until the oceans are either depleted or the body becomes hot enough to radiate in significantly shorter wavelengths to which the air is rather transparent. The SKI limit therefore sets the inner edge of the habitable zone (where a planet can evolve with liquid water). This standard scenario is very important for understanding the evolution of other climates (in particular Venus, or exoplanets close to their host star). The runaway greenhouse scenario is likely the harsh fate Earth will encounter in the geologically distant future, as the sun gradually brightens over time.