Sea-level rise submerges terrestrial permafrost in the Arctic, turning it into subsea permafrost. Subsea permafrost underlies ~ 1.8 million km
of Arctic continental shelf, with thicknesses in places ...exceeding 700 m. Sea-level variations over glacial-interglacial cycles control subsea permafrost distribution and thickness, yet no permafrost model has accounted for glacial isostatic adjustment (GIA), which deviates local sea level from the global mean due to changes in ice and ocean loading. Here we incorporate GIA into a pan-Arctic model of subsea permafrost over the last 400,000 years. Including GIA significantly reduces present-day subsea permafrost thickness, chiefly because of hydro-isostatic effects as well as deformation related to Northern Hemisphere ice sheets. Additionally, we extend the simulation 1000 years into the future for emissions scenarios outlined in the Intergovernmental Panel on Climate Change's sixth assessment report. We find that subsea permafrost is preserved under a low emissions scenario but mostly disappears under a high emissions scenario.
Arctic deltas and their river channels are characterized by three components of the cryosphere: snow, river ice, and permafrost, making them especially sensitive to ongoing climate change. Thinning ...river ice and rising river water temperatures may affect the thermal state of permafrost beneath the riverbed, with consequences for delta hydrology, erosion, and sediment transport. In this study, we use optical and radar remote sensing to map ice frozen to the riverbed (bedfast ice) vs. ice, resting on top of the unfrozen water layer (floating or so-called serpentine ice) within the Arctic’s largest delta, the Lena River Delta. The optical data is used to differentiate elevated floating ice from bedfast ice, which is flooded ice during the spring melt, while radar data is used to differentiate floating from bedfast ice during the winter months. We use numerical modeling and geophysical field surveys to investigate the temperature field and sediment properties beneath the riverbed. Our results show that the serpentine ice identified with both types of remote sensing spatially coincides with the location of thawed riverbed sediment observed with
in situ
geoelectrical measurements and as simulated with the thermal model. Besides insight into sub-river thermal properties, our study shows the potential of remote sensing for identifying river channels with active sub-ice flow during winter vs. channels, presumably disconnected for winter water flow. Furthermore, our results provide viable information for the summer navigation for shallow-draught vessels.
•Sediment thermal properties are highly variable.•Seasonal forcing causes large spatial and temporal sediment temperature variations.•Power cable temperatures strongly depend on thermal properties of ...a given site.•The 2K-criterion is not suitable to detect man-made heat injection.
Analytical and 2D finite-element solutions of the heat transfer equation are used to model the temperature evolution in the upper 15m of marine sediment deposits. These calculations comprise realistic, in-situ measured, thermal properties as well as natural (geothermal and sea bottom temperature) and man-made (e.g. high voltage, subsea power cables) heat sources. A main objective of this article is to focus on the influence of the thermal heterogeneity of sediments in the North and Baltic Seas. In combination with seasonal variations of bottom water temperatures, these result in large temperature differences in the upper 3–5m of the sediments. Also, the impact of additional heat through power cables is considered. The modelling results show maximum cable temperatures as much as 16K difference less compared to a conservative homogeneous scenario previously used from literature values. It can be shown that the 2 K-criterion is not a suitable instrument to monitor the man-made impact to the thermal conditions without knowing the actual thermal settings of a given location.
Submarine permafrost is perennially cryotic earth material that lies offshore. Most submarine permafrost is relict terrestrial permafrost beneath the Arctic shelf seas, was inundated after the last ...glaciation, and has been warming and thawing ever since. As a reservoir and confining layer for gas hydrates, it has the potential to release greenhouse gasses and impact coastal infrastructure, but its distribution and rate of thaw are poorly constrained by observational data. Lengthening summers, reduced sea ice extent and increased solar heating will increase water temperatures and thaw rates. Observations of gas release from the East Siberian shelf and high methane concentrations in the water column and air above it have been attributed to flowpaths created in thawing permafrost. In this context, it is important to understand the distribution and state of submarine permafrost and how they are changing. We assemble recent and historical drilling data on regional submarine permafrost degradation rates and review recent studies that use modelling, geophysical mapping and geomorphology to characterize submarine permafrost. Implications for submarine permafrost thawing are discussed within the context of methane cycling in the Arctic Ocean and global climate change.
The thermal regime in the sediment column below shallow bodies of water in Arctic permafrost controls benthic habitats and permafrost stability. We present a robust, portable device that measures ...detailed temperature–depth profiles of the near-surface sediments in less than 1 h. Test campaigns in the Canadian Arctic and on Svalbard have demonstrated its utility in a range of environments during winter and summer. Measured temperatures were spatially heterogeneous, even within single bodies of water. We observed the broadest temperature range in water less than 1 m deep, a zone that is not captured by single measurements in deeper water.
Most earth system models (ESMs) neglect climate feedbacks arising from carbon release from thawing permafrost, especially from thawing of subsea permafrost (SSPF). To assess the fate of SSPF in the ...next 1000 years, we implemented SSPF into JSBACH, the land component of the Max Planck Institute Earth System Model (MPI-ESM). This is the first implementation of SSPF processes in an ESM component. We investigate three extended scenarios from the 6th phase of the Coupled Model Intercomparison Project (CMIP6). In the 21st century only small differences are found among the scenarios, but in the upper-end emission scenario SSP5–8.5 (shared socio-economic pathway), especially in the 22nd century, SSPF ice melting is more than 15 times faster than in the pre-industrial period. In this scenario about 35 % of total SSPF volume and 34 % of SSPF area are lost by the year 3000 due to climatic changes. In the more moderate scenarios, the melting rate maximally exceeds that of pre-industrial times by a factor of 4, and the climate change induced SSPF loss (volume and area) by the year 3000 does not exceed 14 %. Our results suggest that the rate of melting of SSPF ice is related to the length of the local open-water season and thus that the easily observable sea ice concentration may be used as a proxy for the change in SSPF.
Reconstructing historical climate change from deep ground temperature measurements in cold regions is often complicated by the presence of permafrost. Existing methods are typically unable to account ...for latent heat effects due to the freezing and thawing of the active layer. In this work, we propose a novel method for reconstructing historical ground surface temperature (GST) from borehole temperature measurements that accounts for seasonal thawing and refreezing of the active layer. Our method couples a recently developed fast numerical modeling scheme for two‐phase heat transport in permafrost soils with an ensemble‐based method for approximate Bayesian inference. We evaluate our method on two synthetic test cases covering both cold and warm permafrost conditions as well as using real data from a 100 m deep borehole on Sardakh Island in northeastern Siberia. Our analysis of the Sardakh Island borehole data confirms previous findings that GST in the region have likely risen by 5–9°C between the pre‐industrial period of 1750–1855 and 2012. We also show that latent heat effects due to seasonal freeze‐thaw have a substantial impact on the resulting reconstructed surface temperatures. We find that neglecting the thermal dynamics of the active layer can result in biases of roughly −1°C in cold conditions (i.e., mean annual ground temperature below −5°C) and as much as −2.6°C in warmer conditions where substantial active layer thickening (>200 cm) has occurred. Our results highlight the importance of considering seasonal freeze‐thaw in GST reconstructions from permafrost boreholes.
Plain Language Summary
Long‐term changes in the temperature of the atmosphere are recorded in the solid Earth due to the insulating properties of soil and rock. As a result, it is possible to estimate past changes in temperature at the interface between the ground and the atmosphere by measuring ground temperatures deep below Earth's surface. In cold regions, the presence of permafrost, that is, ground that remains frozen throughout the year, complicates such analyses due to the effects of water freezing and thawing in the soil. In this work, we present a new method for reconstructing past changes in ground surface temperature from boreholes situated in permafrost using a computational model of heat flow that accounts for these effects. We evaluate our method on both synthetic test cases as well as real data from a 100 m deep borehole in northeastern Siberia. Our results demonstrate that annual freezing and thawing of water near the surface has a substantial impact on the reconstructed ground surface temperature (GST), especially in regions where permafrost is thawing. The proposed method is the first to be widely applicable to ground temperatures measured in permafrost and thus constitutes a valuable new tool for understanding past and present climate change in cold regions.
Key Points
We propose a new method to estimate historical ground surface temperatures from boreholes in permafrost using Bayesian inverse modeling
We evaluate our method on both synthetic test cases for cold and warm conditions as well as real data from a 100 m deep borehole in Siberia
We find that seasonal freezing and thawing of the active layer has a significant impact on the reconstructed ground surface temperatures
Offshore permafrost plays a role in the global climate system, but observations of permafrost thickness, state, and composition are limited to specific regions. The current global permafrost map ...shows potential offshore permafrost distribution based on bathymetry and global sea level rise. As a first‐order estimate, we employ a heat transfer model to calculate the subsurface temperature field. Our model uses dynamic upper boundary conditions that synthesize Earth System Model air temperature, ice mass distribution and thickness, and global sea level reconstruction and applies globally distributed geothermal heat flux as a lower boundary condition. Sea level reconstruction accounts for differences between marine and terrestrial sedimentation history. Sediment composition and pore water salinity are integrated in the model. Model runs for 450 ka for cross‐shelf transects were used to initialize the model for circumarctic modeling for the past 50 ka. Preindustrial submarine permafrost (i.e., cryotic sediment), modeled at 12.5‐km spatial resolution, lies beneath almost 2.5 ×106km2 of the Arctic shelf. Our simple modeling approach results in estimates of distribution of cryotic sediment that are similar to the current global map and recent seismically delineated permafrost distributions for the Beaufort and Kara seas, suggesting that sea level is a first‐order determinant for submarine permafrost distribution. Ice content and sediment thermal conductivity are also important for determining rates of permafrost thickness change. The model provides a consistent circumarctic approach to map submarine permafrost and to estimate the dynamics of permafrost in the past.
Key Points
Submarine permafrost is modeled as 1‐D transient heat flux over multiple glacial‐interglacial cycles on the circumarctic shelf
Modeled permafrost ice content closely matches available geophysical observations from the Beaufort and Kara Seas
Almost all modeled preindustrial submarine permafrost in the Arctic is warming, thawing, and thinning
The CryoGrid community model is a flexible toolbox for simulating the ground thermal regime and the ice-water balance for permafrost and glaciers, extending a well-established suite of permafrost ...models (CryoGrid 1, 2, and 3). The CryoGrid community model can accommodate a wide variety of application scenarios, which is achieved by fully modular structures through object-oriented programming. Different model components, characterized by their process representations and parameterizations, are realized as classes (i.e., objects) in CryoGrid. Standardized communication protocols between these classes ensure that they can be stacked vertically. For example, the CryoGrid community model features several classes with different complexity for the seasonal snow cover, which can be flexibly combined with a range of classes representing subsurface materials, each with their own set of process representations (e.g., soil with and without water balance, glacier ice).
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