ISSN 1028-334X, Doklady Earth Sciences, 2011, Vol. 441, Part 2, pp. 1706–1709. © Pleiades Publishing, Ltd., 2011. Original Russian Text © S.N. Denisov, M.M. Arzhanov, A.V. Eliseev, I.I. Mokhov, 2011, published in Doklady Akademii Nauk, 2011, Vol. 441, No. 5, pp. 685–688. 1706 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 twenty-first 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 PT-values. 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 well-known 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...–3‰ 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- matic conditions strongly. Measurements of the oxy- gen isotopic composition and the Mg/Ca ratio in spe- cies of foraminifera indicate that the ocean surface temperature increased sharply by 1–8°C (depending on the region) [4]. In addition, the significant meth- ane release influenced the chemical composition of the ocean. The dissolution of carbonates from bottom sediments occurred during the Paleocene–Eocene thermal maximum in all the oceans. Large temperature jumps during the last glacial period can also be associated with sudden methane releases from methane hydrates [4]. Methane releases can be associated with changes in the sea level and methane hydrate instability on continental slopes. The future climate warming expected could lead to destabilization of hydrates, rapid methane release, and an overall warming increase in the case of the forma- tion of the corresponding positive feedback. Estimates of possible methane releases from meth- ane hydrate deposits due to global warming and their potential impact on climate change are becoming more relevant. The thermodynamic stability of hydrates in bottom sediments and, consequently, the zone of their stability depend on temperature, pressure, and water salinity. In the areas of occurrence of methane hydrates, the hydrostatic pressure at the bottom exceeds the pres- sure required for hydrate stability at the near-bottom water temperature. The upper boundary of the zone of Assessment of the Response of Subaqueous Methane Hydrate Deposits to Possible Climate Change in the Twenty-First Century S. N. Denisov, M. M. Arzhanov, A. V. Eliseev, and Corresponding Member of the RAS I. I. Mokhov Received September 12, 2011 DOI: 10.1134/S1028334X11120129 Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Pyzhevskii per. 3, Moscow, 119017 Russia e-mail: denisof@ifaran.ru GEOPHYSICS