Valentina V. Malakhova

Estimates for time horizons of permafrost degradation at the Arctic shelf and stability of methane hydrates under antropogenically forced climate changes are obtained. In this, a model for thermophysical processes in the shelf sediments, which is driven by the output of the IAP RAS Earth system model of intermediate complexity for the next 15 kyr. It is obtained that the subsea permafrost at the outer shelf is completely degraded to the late 21st or in the early 22nd century. However, in the regions of the contemporary shallow and intermediate shelve, sediments may be kept perennially frozen for 3 to 8 kyr from the present day, depending on intensity of geothermal heat flux. Anthropogenically-induced warming increases the rate of permafrost degradation several-fold relative to the preindustrial climate. In contrast to the subsea permafrost, methane hydrates stability zone may persist for several centuries after permafrost degradation provided that geothermal heat flux is not larger than 60 mW/m². If this flux is larger (e.g., 75 mW/m²), methane hydrates become unstable even before the permafrost disappearance.

Using a model of thermophysical processes in shelf sediments, the state of the subsea permafrost and the methane hydrate stability zone on the Arctic shelf is analysed for 10 kyr into the future. In this, anthropogenic climate change prescribed according to the simulations with the CLIMBER-2 Earth system model of intermediate complexity under scenario SSP5-8.5 (Shared Socio-economic Pathways, 5-8.5) of CO₂ emissions into the atmosphere. The change in bottom temperature during climate warming was estimated using a statistical model. It has been found that the permafrost at the outer shelf disappears either at the early stage of anthropogenic CO₂ emissions (before year 2100), or in several centuries after their onset. In contrast, at the shallow and middle shelf, frozen sediments persist for at least 5 kiloyears after the CO₂ emissions onset. In these parts of the shelf, the zone of stability of methane hydrates begins to disappear no earlier than 3 kiloyears after the start of anthropogenic forcing. This forcing affects only the rate of permafrost degradation from top and can increase this rate by an order of magnitude. In contrast, the dynamics of the methane hydrate stability zone occurs mainly at the lower boundary. The heat flux from the Earth interior is the major factor in this.

An Erratum to this paper has been published: https://doi.org/10.1134/S1024856025030017

The thickness and spatial distribution of a methane hydrate stability zone (MHSZ) associated with submarine permafrost is estimated based on numerical simulation. Using CMIP6 ensemble model calculations with a scenario of high anthropogenic greenhouse gas emissions (SSP5-8.5), a weak dependence of MHSZ shrinkage on ongoing warming is found, and mainly on the side of its base. This process is, first of all, a consequence of the Holocene marine transgression and depends on geothermal flux intensity. The spatial distribution of methane fluxes from bottom sediments caused by degradation of gas hydrates under the violation of their existence conditions is derived. The intensity of methane emission from seafloor to water is estimated at 15 Tg/yr in the modern period and 16–17 Tg/yr to 2300 (similar estimates of the intensity of methane emission from water to the atmosphere are not made in this work). Significant changes in the intensity of methane emissions from seafloor to water are hardly probable for at least several thousand years. The resulting fields of methane fluxes from bottom sediments can be used in numerical ocean models for assessing methane emissions to the atmosphere.

To study the impact of future climate change on the methane emissions to the atmosphere as a result of the Arctic submarine permafrost degradation, scenario experiments were carried up to 2100 out using the SibCIOM ocean and sea ice model. For the atmospheric forcing, we used the results of six climate model simulations from the RCP8.5 scenario of the Coupled Model Inter-comparison Project Phase 5 archive. Based on the numerical results, it is found that the total annual methane fluxes will increase by a factor of 2–4 depending on the atmospheric forcing by 2100 compared to current levels. The flux intensity calculated in the numerical experiments does not exceed 8 Tg/yr. High future methane emissions are mainly due to reduced ice extent and increased ice-free periods. This resulted in a shift in the timing of maximal methane emissions from summer to fall and winter.

There is considerable uncertainty about the methane emission from Arctic shelf seas. Methane fluxes in this region can be underestimated and play a significant role due to the large volume of gas contained in bottom sediments in the permafrost and gas hydrates. We analyzed the model sensitivity to the parameterization of gas exchange processes on the sea surface based on the numerical modeling of the transport of dissolved methane in Arctic seas. A dissolved methane transport model is included into the basic model of the ocean and sea ice SibCIOM developed at Institute of Computational Mathematics and Mathematical Geophysics, Siberian Branch, Russian Academy of Sciences. Methane emissions into the atmosphere were estimated with various parameterization of the gas exchange process in the “water–atmosphere” and “water–ice–atmosphere” systems with NCEP/NCAR reanalysis data. The uncertainty of the estimate of annual methane emission amounted to 6–12% when considering different dependencies of gas exchange on wind. The scheme which considers the ice cover has a more pronounced influence on methane flux calculations: the uncertainty increased to 50–130%. Parameterization of the relation between ice cover and gas exchange can have a great effect on the calculated methane fluxes and lead to underestimation of its emission from Arctic shelf seas.

—This paper presents the results of numerical modeling of the permafrost thermal regime and thermobaric conditions of methane hydrates in the north of Western Siberia over the past 70 thousand years. The area of hydrate formation was determined and the rate of accumulation of hydrates was estimated in connection with the migration of fluid from the underlying gas-saturated layers under the conditions of cover glaciation. The estimates obtained for the change in hydrate saturation as a result of fluid migration during the 10 thousand-year glaciation period, depending on the permeability of the soil, are from 6 to 40% in the upper 350 m. Based on quantitative characteristics of the equilibrium and metastable states of methane hydrates, the conditions for the preservation of relict methane hydrates in permafrost under the paleoclimatic scenario were determined, taking into account periods of ice cover and transgression. It is shown that due to the effect of self-preservation at temperatures below –4°C, it is possible to preserve relict methane hydrates in the upper 200 m of soil under non-equilibrium conditions. The effect of lowering the temperature while the hydrates dissociate prevents the complete decomposition of the deposit and leads to an increase in the thickness of the frozen soil.

The current ideas about the permafrost thickness on the New Siberia Island are very ambiguous both in terms of forming factors and in its magnitude (from 240 to 370 m), and in general are based on the assumption of the Pliocene beginning of rock freezing. In the last 20 years, new data have been obtained on the history of the region’s development in the Neopleistocene. They include the long existence of the New Siberia Island in marine conditions, which excluded the rocks freezing at that time, as well as the development of glaciation in the Middle Neopleistocene. These data indicate a significantly later onset of freezing of the island, which correlates with the nonstationarity of the temperature field of the frozen strata. The authenticity of the data is debated. The article is devoted to assessing the thickness of permafrost in accordance with three possible scenarios, allowing to take into account different points of view on the geological development of the region. The assessment is made using numerical modeling of the formation of frozen strata over the past 200 thousand years. One scenario includes the existence of glaciation during the Middle Neopleistocene, while in the other two it is absent. The last two scenarios differ in the stratigraphic schemes used. The reliability of the scenarios is assessed based on a comparison of the modeling results for each of them with geothermal data in a well on the island of New Siberia, obtained in the 1970s. The comparison results show that only the version with the existence of a glacier is consistent with the geothermal data. According to the data obtained, freezing of the island began under the glacier at the end of the Middle Neopleistocene, which existed in coastal-marine conditions. The most active increase in the frozen strata thickness dates back to the late Neopleistocene due to the drainage of the island. The obtained result is in agreement with new data on the age of the main geological bodies of the region, as well as with the idea of the glaciotectonic nature of dislocations in the Cretaceous - Middle Neopleistocene rocks.