Danny Harvey

University of Toronto, Department of Geography

Methane clathrate is an ice-like compound in which methane molecules are caged in cavities formed by water molecules. It looks like ice and it forms in the oceans in continental slope sediments. It is stable under certain temperature and depth combinations, and is not found shallower than 250 meters deep. Most of the methane clathrate is of biological origin and is concentrated at the base of the stable zone. In principle, there may be an enormous reserve of methane (CH₄) in clathrate but there is a several orders of magnitude uncertainty about how much is actually present. With warming, it could move from a stable to an unstable condition, resulting in the release of the enclosed CH₄, leading to a significant positive climate feedback (surprise).

The potential marine release of methane is much more significant than that of terrestrial permafrost regions because the release from land would be greatly delayed whereas oceanic continental shelves might produce a quick, continuous release followed by pulse releases, or a delayed but sudden release.

In short, the destabilized methane could:

• migrate into the stable zone, release latent heat in sudden release
• accumulate beneath the stable zone with pressure build up and fracture release
• accumulate beneath the stable zone, release shear strength, thereby causing sediment slumping if on a slope

• some bubble dissolution into ocean water
• some oxidation of dissolved methane before it reaches the atmosphere - variation by 7 orders of magnitude - so how much will reach atmosphere is a big question.

The model presented by Harvey represents an attempt to construct a plausible, reasonable worst-case scenario driven by anthropogenic emissions of CO₂ and methane. Assumptions are biased toward worst case; they neglect possible impact of lower sea level, the possible retention of methane in residual clathrate structure, etc. For these assumptions, climate-clathrate feedback enhances global warming by 6 to 7% for low climate sensitivity, and by 20% for high climate sensitivity. Harvey estimates that there is a 20 to 30% probability of a 20% feedback.