Methane hydrates and global warming

There is an enormous amount of methane (CH4) on earth frozen into a type of ice called methane hydrate. Hydrates can form with almost any gas and consist of a ‘cage’ of water molecules surrounding the gas. (The term ‘clathrate’ more generally describes solids consisting of gases are trapped within any kind of cage while hydrate is the specific term for when the cage is made of water molecules). There are CO2 hydrates on Mars, while on Earth most of the hydrates are filled with methane. Most of these are in sediments of the ocean, but some are associated with permafrost soils.

Methane hydrates would seem intuitively to be the most precarious of things. Methane hydrate melts if it gets too warm, and it floats in water. Methane is a powerful greenhouse gas, and it degrades to CO2, another greenhouse gas which accumulates in the atmosphere just as fossil fuel CO2 does. And there is a lot of it, possibly more than the traditional fossil fuel deposits. Conceivably, climate changes could affect these deposits. So what do we know of the disaster-movie potential of the methane hydrates?

Ocean hydrates. Most of the methane hydrate is in sediments of the ocean. Of that, most is what can be called the stratigraphic-type deposits. Organic carbon from plankton is buried over millions of years. Hundreds of meters below the sea floor, bacteria produce methane from the dead plankton. If methane is produced quickly enough, some of it will freeze into methane hydrates. This type of deposit holds thousands of gigatons of carbon as methane [Buffett and Archer, 2004; Milkov, 2004]. For comparison, the most abundant type of traditional fossil fuel is coal, which is typically credited with about 5000 Gton C [Rogner, 1997].