the answer, or part of it, lies with methane clathrate [0]
> a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice. Originally thought to occur only in the outer regions of the Solar System, where temperatures are low and water ice is common, significant deposits of methane clathrate have been found under sediments on the ocean floors of the Earth.
deposits of solid methane ice can exist trapped on the seafloor [1]. this is possible only at the extreme pressures of the seafloor [2].
so, on its own, a little methane in the atmosphere is not a big deal, because of the short half-life.
except, you have methane ice deposits in the ocean that can melt if the oceans get a little bit warmer.
this can enter into a fun little runaway feedback loop pretty quickly. little bit of methane melts, causing a little bit more ocean warming, causing more methane to melt, etc.
if this feedback loop were to occur, the evidence you would expect to see would be spontaneous release of methane emissions from the ocean, and you would expect it to be happening in places where the warming ocean put methane clathrates right at their melting point for that depth & pressure of seawater. which...is exactly what's happening [3]
> The study, to appear in the journal Geochemistry, Geophysics, Geosystems, a journal of the American Geophysical Union, shows that of 168 bubble plumes observed within the past decade, a disproportionate number were seen at a critical depth for the stability of methane hydrates.
> "We see an unusually high number of bubble plumes at the depth where methane hydrate would decompose if seawater has warmed," said lead author H. Paul Johnson, a UW professor of oceanography. "So it is not likely to be just emitted from the sediments; this appears to be coming from the decomposition of methane that has been frozen for thousands of years."
a similar feedback loop is possible in the Arctic tundra [4], where permafrost can melt and release trapped bubbles of gases that include methane, which in turn will warm the planet a bit and cause more permafrost to melt.
and of course the two feedback loops are able to reinforce each other, because a bit of methane released from the arctic also helps heat the oceans, and a bit of methane released by the oceans also helps melt the permafrost.
> a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice. Originally thought to occur only in the outer regions of the Solar System, where temperatures are low and water ice is common, significant deposits of methane clathrate have been found under sediments on the ocean floors of the Earth.
deposits of solid methane ice can exist trapped on the seafloor [1]. this is possible only at the extreme pressures of the seafloor [2].
so, on its own, a little methane in the atmosphere is not a big deal, because of the short half-life.
except, you have methane ice deposits in the ocean that can melt if the oceans get a little bit warmer.
this can enter into a fun little runaway feedback loop pretty quickly. little bit of methane melts, causing a little bit more ocean warming, causing more methane to melt, etc.
if this feedback loop were to occur, the evidence you would expect to see would be spontaneous release of methane emissions from the ocean, and you would expect it to be happening in places where the warming ocean put methane clathrates right at their melting point for that depth & pressure of seawater. which...is exactly what's happening [3]
> The study, to appear in the journal Geochemistry, Geophysics, Geosystems, a journal of the American Geophysical Union, shows that of 168 bubble plumes observed within the past decade, a disproportionate number were seen at a critical depth for the stability of methane hydrates.
> "We see an unusually high number of bubble plumes at the depth where methane hydrate would decompose if seawater has warmed," said lead author H. Paul Johnson, a UW professor of oceanography. "So it is not likely to be just emitted from the sediments; this appears to be coming from the decomposition of methane that has been frozen for thousands of years."
a similar feedback loop is possible in the Arctic tundra [4], where permafrost can melt and release trapped bubbles of gases that include methane, which in turn will warm the planet a bit and cause more permafrost to melt.
and of course the two feedback loops are able to reinforce each other, because a bit of methane released from the arctic also helps heat the oceans, and a bit of methane released by the oceans also helps melt the permafrost.
0: https://en.wikipedia.org/wiki/Methane_clathrate
1: https://en.wikipedia.org/wiki/Methane_clathrate#/media/File:...
2: https://en.wikipedia.org/wiki/File:Methane_Hydrate_phase_dia...
3: https://www.ocean.washington.edu/story/Bubble_plumes_suggest...
4: https://en.wikipedia.org/wiki/Arctic_methane_emissions