Global climate models (GCM) are frequently used to understand and predict future climate change, but most GCMs do not attempt to represent permafrost dynamics and its potentially critical feedbacks ...on climate. In this paper, we evaluate the Community Land Model (CLM3), which is a land‐surface scheme, against observations and identify potential modifications to this model that improve fidelity of permafrost and soil temperature simulations. These modifications include increasing the total soil depth by adding new layers, incorporating a surface organic layer, and modifying the numerical scheme to include unfrozen water dynamics and more realistic phase change representation.
A simplified, data-constrained approach to estimate the permafrost carbon-climate feedback Koven, C. D.; Schuur, E. A. G.; Schädel, C. ...
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
11/2015, Letnik:
373, Številka:
2054
Journal Article
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We present an approach to estimate the feedback from large-scale thawing of permafrost soils using a simplified, data-constrained model that combines three elements: soil carbon (C) maps and profiles ...to identify the distribution and type of C in permafrost soils; incubation experiments to quantify the rates of C lost after thaw; and models of soil thermal dynamics in response to climate warming. We call the approach the Permafrost Carbon Network Incubation-Panarctic Thermal scaling approach (PInc-PanTher). The approach assumes that C stocks do not decompose at all when frozen, but once thawed follow set decomposition trajectories as a function of soil temperature. The trajectories are determined according to a three-pool decomposition model fitted to incubation data using parameters specific to soil horizon types. We calculate litterfall C inputs required to maintain steady-state C balance for the current climate, and hold those inputs constant. Soil temperatures are taken from the soil thermal modules of ecosystem model simulations forced by a common set of future climate change anomalies under two warming scenarios over the period 2010 to 2100. Under a medium warming scenario (RCP4.5), the approach projects permafrost soil C losses of 12.2-33.4 Pg C; under a high warming scenario (RCP8.5), the approach projects C losses of 27.9-112.6 Pg C. Projected C losses are roughly linearly proportional to global temperature changes across the two scenarios. These results indicate a global sensitivity of frozen soil C to climate change (γ sensitivity) of −14 to −19 Pg C °C−1 on a 100 year time scale. For CH4 emissions, our approach assumes a fixed saturated area and that increases in CH4 emissions are related to increased heterotrophic respiration in anoxic soil, yielding CH4 emission increases of 7% and 35% for the RCP4.5 and RCP8.5 scenarios, respectively, which add an additional greenhouse gas forcing of approximately 10-18%. The simplified approach presented here neglects many important processes that may amplify or mitigate C release from permafrost soils, but serves as a data-constrained estimate on the forced, large-scale permafrost C response to warming.
Models of sub‐sea permafrost evolution vary significantly in employed physical assumptions regarding the paleo‐geographic scenario, geological structure, thermal properties, initial temperature ...distribution, and geothermal heat flux. This work aims to review the underlying assumptions of these models as well as to incorporate recent findings, and hence develop an up‐to‐date model of the sub‐sea permafrost dynamics at the Laptev Sea shelf. In particular, the sub‐sea permafrost model developed here incorporates thermokarst and land‐ocean interaction theory, and shows that the sediment salinity and a temperature‐based parametrization of the unfrozen water content are critical factors influencing sub‐sea permafrost dynamics. From the numerical calculations, we suggest development of open taliks may occur beneath submerged thaw lakes within a large area of the shelf.
Key Points
Review of the underlying assumptions of previous models
Development of an up‐to‐date model of sub‐sea permafrost in Laptev Sea
Development of open taliks underneath submerged thaw lakes on the shelf
A thin layer of soil used in many coupled global climate models does not resolve the heat reservoir represented by underlying ground material. Under representation of this feature leads to ...unrealistic simulation of temperature dynamics in the active layer and permafrost. Using the Community Land Model (CLM3) and its modifications we estimate a required thickness of soil layers to calculate temperature dynamics within certain errors. Our results show that to compute the annual cycle of temperature dynamics for cold permafrost, the soil thickness should be at least 30 meters. Decadal‐to‐century time scales require significantly deeper soil layers, e.g. hundreds of meters. We also tested a new geometrical configuration of the soil layer geometry which is called slab permafrost. This configuration is represented by a thick soil layer underneath the traditional resolved soil layer. The model configuration with 30 m deep resolved soil layer and a 30 to 100 m thick slab shows results that favorably compare with our benchmark model which has a fully resolved 300 m‐deep soil layer.
A variational data assimilation algorithm is developed to reconstruct thermal properties, porosity, and parametrization of the unfrozen water content for fully saturated soils. The algorithm is ...tested with simulated synthetic temperatures. The simulations are performed to determine the robustness and sensitivity of algorithm to estimate soil properties from in-situ high resolution-in-time temperature records in the active layer, and once-a-year measurements in a relatively deep borehole. The algorithm is applied to estimate soil properties at several sites along the Dalton Highway. The presented approach is quite general and can be applied to many problems requiring finding an optimal set of soil properties, and uncertainties in found values.
By assuming the flow is uniform along the narrow long bays, the 2-D nonlinear shallow-water equations are reduced to a linear semi-axis variable-coefficient 1-D wave equation via the generalized ...Carrier–Greenspan transformation. The run-up of long waves in constantly sloping U-shaped and V-shaped bays is studied both analytically and numerically within the framework of the 1-D nonlinear shallow-water theory. An analytic solution, in the form of a double integral, to the resulting linear wave equation is obtained by utilizing the Hankel transform, and consequently the solution to the tsunami run-up problem is developed by applying the inverse generalized Carrier–Greenspan transform. The presented solution is a generalization of the solutions found by Carrier et al. (2003) and Didenkulova and Pelinovsky (2011) for the case of a plane beach and a parabolic bay, respectively. The shoreline dynamics in U-shaped and V-shaped bays are computed via a double integral through standard integration techniques.
Thawing and freezing of Arctic soils is affected by many factors, with air temperature, vegetation, snow accumulation, and soil physical properties and soil moisture among the most important. We ...enhance the Geophysical Institute Permafrost Laboratory model and develop several high spatial resolution scenarios of changes in permafrost characteristics in the Alaskan Arctic in response to observed and projected climate change. The ground thermal properties of surface vegetation and soil column are upscaled using the Ecosystems of Northern Alaska map and temperature data assimilation from the shallow boreholes across the Alaska North Slope. Soil temperature dynamics are simulated by solving the 1‐D nonlinear heat equation with phase change, while the snow temperature and thickness are simulated by considering the snow accumulation, compaction, and melting processes. The model is verified by comparing with available active layer thickness at the Circumpolar Active Layer Monitoring sites, permafrost temperature, and snow depth records from existing permafrost observatories in the North Slope region.
Key Points
We develop scenarios of changes in permafrost characteristics in the Alaskan Arctic in response to projected climate change
The ground thermal properties are upscaled using an ecosystems map and data assimilation from the shallow boreholes
The model is tested against measured active layer thickness, permafrost temperature, and snow depth records
Modeling Long‐Term Permafrost Degradation Nicolsky, D. J.; Romanovsky, V. E.
Journal of geophysical research. Earth surface,
August 2018, 2018-08-00, 20180801, Letnik:
123, Številka:
8
Journal Article
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Permafrost, as an important part of the Cryosphere, has been strongly affected by climate warming, and a wide spread of permafrost responses to the warming is currently observed. In particular, at ...some locations rather slow rates of permafrost degradations are noticed. We related this behavior to the presence of unfrozen water in frozen fine‐grained earth material. In this paper, we examine not‐very‐commonly‐discussed heat flux from the ground surface into the permafrost and consequently discuss implications of the presence of unfrozen liquid water on long‐term thawing of permafrost. We conducted a series of numerical experiments and demonstrated that the presence of fine‐grained material with substantial unfrozen liquid water content at below 0°C temperature can significantly slow down the thawing rate and hence can increase resilience of permafrost to the warming events. This effect is highly nonlinear, and a difference between the rates of thawing in fine‐ and coarse‐grained materials is more drastic for lower values of heat flux incoming into permafrost. For high heat flux, the difference between these rates almost disappears. As near‐surface permafrost temperature increases towards 0°C and the changes in the ground temperature become less evident, the future observation networks should try to incorporate measurements of unfrozen liquid water content in the near‐surface permafrost and heat flux into permafrost in addition to the existing temperature observations.
Key Points
We examine and discuss implications of the unfrozen liquid water content on the long‐term thawing of permafrost
We demonstrate that the presence of material with substantial unfrozen liquid water content at below 0°C temperature can slow down thawing
Future observation networks should try to incorporate measurements of the unfrozen liquid water content in the near‐surface permafrost
Tsunamigenic earthquakes along the Alaska–Aleutian megathrust pose a major threat to many rural coastal communities in Alaska. To mitigate the impact of tsunamis and minimize loss of life, maximum ...potential runup and tsunami hazard zones need to be estimated despite a lack of the high-resolution digital elevation models. In this manuscript we present a methodology to approximate tsunami hazard zones in areas with poor topographic and bathymetric coverage. To manage a trade-off between the computational complexity and lack of quality data, we develop three scenario earthquakes with the intention that one of them characterizes impacts of a credible worst-case event. We then apply a safety factor to account for the limited number of considered scenarios and a coarse resolution elevation model used to simulate water dynamics near the community. The developed methodology is illustrated using a case study focused on the community of Adak, in the Andreanof Islands, Alaska. To validate the proposed method, we explore sensitivity of tsunami height with respect to the coseismic slip distribution and develop a comprehensive set of credible worst-case scenarios for the community. The tsunami hazard zone, developed with the use of the safety factor, is validated against high-resolution modeling of potential inundation according to the set of worst-case scenarios in a number of other coastal communities.
Permafrost, a key component of Arctic ecosystems, is currently affected by climate warming and anticipated to undergo further significant changes in this century. The most pronounced changes are ...expected to occur in the transition zone between the discontinuous and continuous types of permafrost. We apply a transient temperature dynamic model to investigate the spatiotemporal evolution of permafrost conditions on the Seward Peninsula, Alaska—a region currently characterized by continuous permafrost in its northern part and discontinuous permafrost in the south. We calibrate model parameters using a variational data assimilation technique exploiting historical ground temperature measurements collected across the study area. The model is then evaluated with a separate control set of the ground temperature data. Calibrated model parameters are distributed across the domain according to ecosystem types. The forcing applied to our model consists of historic monthly temperature and precipitation data and climate projections based on the Representative Concentration Pathway (RCP) 4.5 and 8.5 scenarios. Simulated near‐surface permafrost extent for the 2000–2010 decade agrees well with existing permafrost maps and previous Alaska‐wide modeling studies. Future projections suggest a significant increase (3.0°C under RCP 4.5 and 4.4°C under RCP 8.5 at the 2 m depth) in mean decadal ground temperature on average for the peninsula for the 2090–2100 decade when compared to the period of 2000–2010. Widespread degradation of the near‐surface permafrost is projected to reduce its extent at the end of the 21st century to only 43% of the peninsula's area under RCP 4.5 and 8% under RCP 8.5.
Key Points
The ecosystem‐type approach is applied to simulate permafrost dynamics on the Seward Peninsula
Improved regional scale model is used to simulate snow interception and scouring
Enhanced model calibration approach is implemented to allow for better use of historical borehole data
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