RealClimate: Arctic & American Methane In Context
EDIT
Methane hydrate seems menacing as a source of gas that can spring aggressively from the solid phase like pop rocks (carbonated candies). But hydrate doesn’t just explode as soon as it crosses a temperature boundary. It takes heat to convert hydrate into fluid + gas, what is called latent heat, just like regular water ice. There could be a lot of hydrate in Arctic sediments (it’s not real well known how much there is), but there is also lot of carbon as organic matter frozen in the permafrost. Their time scales for mobilization are not really all that different, so I personally don’t see hydrates as scarier than frozen organic matter. I think it just seems scarier.
The other thing about hydrate is that at any given temperature, a minimum pressure is required for hydrate to be stable. If there is pure gas phase present, the dissolved methane concentration in the pore water, from Henry’s law, scales with pressure. At 0 degrees C, you need a pressure equivalent to ~250 meters of water depth to get enough dissolved methane for hydrate to form.
The scariest parts of the Siberian margin are the shallow parts, because this is where methane bubbles from the sea floor might reach the surface, and this is where the warming trend is observed most strongly. But methane hydrate can only form hundreds of meters below the sea floor in that setting, so thermodynamically, hydrate is not expected to be found at or near the sea floor. (Methane hydrate can be found close to the sediment surface in deeper water depth settings, as for example in the Gulf of Mexico or the Nankai trough). The implication is that it will take centuries or longer before heat diffusion through that sediment column can reach and destabilize methane hydrates.
Is there any way nature might evade this thermodynamic imperative?
If hydrate exists in near-surface sediments of the Siberian margin, it would be called “metastable”. Metastability in nature is common when forming a new phase for which a “seed” or starting crystal is needed, like cloud droplets freezing in the upper atmosphere. But for decomposition to form water and gas one would not generally expect a barrier to just melting when energy is available. Chuvilin et al (2011) monitored melting hydrate in the laboratory and observed some quirkiness.
EDIT
http://www.realclimate.org/index.php/archives/2013/11/arctic-and-american-methane-in-context/#more-16393
