Abrupt Climate Change Scenarios

It has been suggested that a sea-level drop of nearly 120 m during the last glacial maximum reduced the hydrostatic pressure sufficiently to raise the lower limit of gas-hydrate stability by about 20 m in the low latitudes (Dillon and Paull, 1983). When a hydrate dissociates, its consistency changes from a solid to a mixture of sediment, water and free gas. Experiments on the mechanical strength behavior of hydrates has shown that the hydrated sediment is markedly stronger than water ice (10 times stronger than ice at 260° K) (Zhang et al., 1999). Thus, a change in the physical properties of the sediment at the base of the hydrate stability field could produce a zone of weakness where sedimentary failure could take place, encouraging low-angle faulting and mass wasting along the continental slope. The common occurrence of Pleistocene slumps on the seafloor have been ascribed to such a catastrophic mechanism and major slumps have been identified in sediments of this age in widely separated margins of the world (Kvenvolden, 1998).

Whether mass wasting at the hydrated-sediment depths is due to hydrate dissociation or other causes, such as, earthquakes or gravity failure, it can potentially release significant quantity of methane trapped below the level of the slump, in addition to the gas emitted from the dissociated hydrate itself. During the glacial periods, these emissions are envisaged to increase together with the incidence of slumping and the easing of hydrostatic pressure on the as the sea level falls. This may eventually trigger a negative response to advancing glaciation and encourage a reversal and termination of the glacial cycle. Thus, there may be a built-in terminator to glaciation, via the hydrate connection (Paull etal., 1991).

Negative response to glaciation in this scenario can initially function efficiently only in the lower latitudes. At higher latitudes glacially-induced freezing could delay the reversal, but once deglaciation begins, even relatively small increases in the mean temperature of the higher latitudes could set off additional emissions of methane from near-surface sources and enhanced greenhouse warming. One model suggests that a small triggering event and liberation of one or more Arctic gas pools could initiate massive release of methane frozen in the permafrost, leading to accelerated warming (Nisbet, 1990). The abrupt termination of the Younger Dryas glaciation (now dated at ca. 11.6 k.y. ago) has been ascribed to such a mechanism (Nisbet, 1990).

To test this idea, Thorpe et al. (1998) modeled the effect of an abrupt release of a "realistic" amount of methane into the atmosphere at the end of a glacial cycle, as constrained by the ice core records. It was concluded that the direct radiative effects of such a pulse of methane emission alone would be too small to account for the reversal of the glacial cycle. However, with certain combinations of methane, carbon dioxide and heat transport inputs, it was possible to simulate changes of the same magnitude as those indicated by empirical data.

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