Assessment of the response of subaqueous methane hydrate deposits to possible climate change in the twenty-first century

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Large reserves of methane are trapped in oceanic hydrate deposits. Increase in temperature of the oceans makes a contribution to the dissociation of oceanic hydrate accumulations and release of poten� tially large amounts of methane into the atmosphere. Such releases into the atmosphere lead to an increase in the greenhouse effect and, accordingly, serious climate changes and to accelerate the gas hydrate dissociation. In this work we evaluated the sta� bility of the existing reserves of subaqueous (underwa� ter) gas hydrates and possible methane release with the dissociation of methane hydrates in the twentyfirst century (1). Methane hydrates are compounds in which meth� ane molecules are in cells formed by water molecules. They are widely distributed in the permafrost zones and the oceanic bottom sediments along the continental slopes, where they are stable at the current PTvalues. Methane hydrates are a potentially large source of energy in comparison with other known sources of hydrocarbons. The total carbon in hydrates is esti� mated as 10 4 Gton C (2), which is a significant amount in comparison with the carbon content (3.8 ×10 4 Gton C) dissolved in the oceanic water and in soil and plants (2 × 10 3 Gton C) and the atmosphere (7.3 ×10 2 Gton C) (3). The total fossil fuel reserves, including coal, are about 5 ×10 3 Gton C (4); i.e., they are consistent with gas hydrate reserves. Methane is the third (after water vapor and carbon dioxide) greenhouse gas, which has a significant effect on the radiation balance of Earth's climate system and can be released into the atmosphere as a result of min� ing and use of hydrates as an energy source. Sudden releases of methane into the atmosphere may occur due to massive underwater shifts of the Earth's crust and an increase in temperature in the oceanic bottom sediments. According to the model estimates, in case of an increase in the oceanic water temperature by a few degrees, methane hydrate reserves should be much smaller (5). Releases of methane in decomposition of methane hydrates could have been a cause of abrupt climate change in the past (6, 7). The Paleocene- Eocene temperature maximum is a wellknown exam� ple of a period of abrupt climate change that was likely associated with massive release of methane from hydrates 55 Ma ago. In some areas (including the Car� ibbean Sea, North Atlantic, the Weddell Sea, and tropical Pacific Ocean), a shift of 2.5- δ 13 С in biogenic carbonate and organic matter was noted. This may be associated with the release of 1500-2000 Gton of methane over several thousand years (6). Such a large release of methane could influence cli�