Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 1On the time scale of the coming century, it islikely that most of the marine hydrate reservoirwill be insulated from anthropogenic climatechange. The exception is in shallow oceansediments where methane gas is focused bysubsurface migration. These deposits will verylikely respond to anthropogenic climate changewith an increased background rate of sustainedmethane release, rather than an abrupt release.7.2. Destabilization of PermafrostHydratesHydrate deposits at depth in permafrost soilsare known to exist, and although their extentis uncertain, the total amount of methane inpermafrost hydrates appears to be much smallerthan in marine sediments. Surface warmingeventually would increase melting rates ofpermafrost hydrates. Inundation of some depositsby warmer seawater and lateral invasionof the coastline are also concerns and may bemechanisms for more rapid change.Figure 1.9. A piece of methane clathrate displays itspotential as an energy source. As the compound melts,released gas feeds the flame and the ice framework dripsoff as liquid water. Inlay shows the clathrate structure.Source: U.S. Geological Survey.7.1. Destabilization of MarineMethane HydratesThis issue is probably the most well knowndue to extensive research on the occurrence ofmethane hydrates in marine sediments, and thelarge quantities of methane apparently presentin this solid phase in primarily continentalmargin marine sediments. Destabilizationof this solid phase requires mechanisms forwarming the deposits and/or reducing pressureon the appropriate time scale, transport of freemethane gas to the sediment-water interface,and transport through the water column to theatmosphere (Archer, 2007). Warming of bottomwaters, slope failure, and their interaction arethe most commonly discussed mechanisms forabrupt release. However, bacteria are efficient atconsuming methane in oxygen-rich sedimentsand the ocean water column, and there are anumber of physical impediments to abruptrelease from marine sediments.Destabilization of hydrates in permafrostby global warming is unlikely over the nextfew centuries (Harvey and Huang, 1995). Nomechanisms have been proposed for the abruptrelease of significant quantities of methanefrom terrestrial hydrates (Archer, 2007). Slowand perhaps sustained release from permafrostregions may occur over decades to centuriesfrom mining extraction of methane from terrestrialhydrates in the Arctic (Boswell, 2007),over decades to centuries from continued erosionof coastal permafrost in Eurasia (Shakovaet al., 2005), and over centuries to millenniafrom the propagation of any warming 100 to1,000 meters down into permafrost hydrates(Harvey and Huang, 1995).7.3. Changes in Wetland Extentand Methane ProductivityAlthough a destabilization of either the marineor terrestrial methane hydrate reservoirs is themost likely pathway for an abrupt increasein atmospheric methane concentration, thepotential exists for a more gradual, but substantial,increase in natural methane emissionsin association with projected changes inclimate. The most likely region to experience adramatic change in natural methane emission26