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 degrees 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 degrees C in cold conditions (i.e., mean annual ground temperature below -5 degrees C) and as much as -2.6 degrees 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.

We use a spatially distributed and physically based energy and mass balance model to derive the ostrem curve, which expresses the supraglacial debris-related relative melt alteration versus the debris thickness, for the Djankuat Glacier, Caucasus, Russian Federation. The model is driven by meteorological data from two on-glacier weather stations and ERA-5 Land reanalysis data. A direct pixel-by-pixel comparison of the melt rates obtained from both a clean ice and debris-covered ice mass balance model results in the quantification of debris-related relative melt-modification ratios, capturing the degree of melt enhancement or suppression as a function of the debris thickness. The main results show that the distinct surface features and different surface temperature/moisture and near-surface wind regimes that persist over debris-covered ice significantly alter the pattern of the energy and mass fluxes when compared to clean ice. Consequently, a maximum relative melt enhancement of 1.36 is modeled on the glacier for thin/patchy debris with a thickness of 0.03 m. However, insulating effects suppress sub-debris melt under debris layers thicker than a critical debris thickness of 0.09 m. Sensitivity experiments show that especially within-debris properties, such as the thermal conductivity and the vertical debris porosity gradient, highly impact the magnitude of the sub-debris melt rates. Our results also highlight the scale-dependency as well as the dynamic nature of the debris thickness-melt relationship for changing climatic conditions, which may have significant implications for the climate change response of debris-covered glaciers. The presence of rocks, boulders and sediments on top of glacier ice can highly modify the behavior of mountain glaciers. As such, compared to a clean ice surface, a debris-covered ice surface is subject to a modified melting regime. In our study, we quantify this melt-modification effect for the Djankuat Glacier, a well-studied glacier situated in the Caucasus. The results are expressed by a so-called ostrem curve that quantifies the debris-related melt-modification effect and compares it to the corresponding debris thickness. Here, we present the first attempt to construct such a glacier-specific ostrem curve through sophisticated 2D glacier-wide energy and mass balance modeling. Our results show that the energy and mass balance at the glacier surface are greatly modified due to the debris, resulting in different melting regimes over both surface types. Hence, ice melt is enhanced for thin and patchy debris layers, whereas melt is increasingly suppressed for thick and continuous debris layers due to an insulating effect. The degree of melt modification and the shape of the ostrem curve are found to depend on the debris properties, the spatio-temporal distribution of the debris, and the local climatic conditions. Quantifying such melt-modification effects is important to more accurately understand and assess the behavior of (partly) debris-covered glaciers under a future warming climate. We use a spatially distributed and physically based energy and mass balance model to derive the ostrem curve for the Djankuat Glacier The sub-debris melt rates are especially sensitive to within-debris properties, such as the thermal conductivity, the debris porosity and its gradient The relative melt suppression of the debris cover is modeled to increase in a warming climate, regardless of the changes in debris thickness

Spin-up is essential to provide initial conditions for land surface models (LSM) when they cannot be given reliably as in the application to regional permafrost change studies. In this study, the impacts of spin-up strategy including total spin-up length and cycling scheme on modeling of permafrost dynamics on the Qinghai-Tibet Plateau (QTP) were evaluated through two groups of experiments using a modified Noah LSM. The first group aims to test different total spin-up lengths and the second group for different cycling schemes. The results show that the presence of permafrost prolongs the convergence of the model. Vertically, the slowest convergence is observed at the permafrost table. The insufficiency of total spin-up length is prone to underestimate permafrost area and overestimate the degradation rate. Different cycling schemes considerably affect the resulting initial thermal fields and result in degradation rates with a difference of 3.37 x 10(3) km(2)/a on the QTP, which exceeds the difference (2.92 x 10(3) km(2)/a) in the degradation rates reported in existing studies. The multi-year cycling scheme is generally preferred, but overlong cycle length should be avoided to prevent the introduction of climate change trends in the spin-up period. We recommend a spin-up strategy of a 500-year cycling with the first 5- to 10-year of forcing for modeling permafrost on the QTP with the Noah LSM. Our findings highlight the importance of the spin-up strategy, which is usually neglected in present LSM-based permafrost modeling studies.

In the past several decades, the trend of rainfall have been significantly increasing in the Qinghai-Tibet Plateau, which inevitably leads to a change in the surface energy balance processes and thermal-moisture status of the permafrost active layers. However, the influence of mechanisms and associated effects of increasing rainfall on active layers are still poorly understood. Therefore, in this study, a validated coupled numerical water-vapor-heat model was applied for simulating the surface energy components, liquid and vapor water migration, and energy transfer within the permafrost active layer under the action of increasing rainfall in the case of an especially wet year. The obtained results demonstrate that the surface heat flux decreases with the increase in rainfall, and the dominant form of energy exchange between the ground and atmosphere becomes the latent heat flux, which is beneficial for the preservation of permafrost. The increasing rainfall will also cause the migration of liquid and vapor water, and the migration of liquid will be more significant. The liquid and vapor water migration caused by the increasing rainfall is also accompanied by energy transfer. With the increase in rainfall, the decrease in total soil heat flux directly leads to a cooling effect on the soil, and then the upper limit of the frozen soil rises, which alleviates the degradation of permafrost. These results provide further insights into engineering structures, regional ecological climate change, hydrology, and environmental issues in permafrost regions.

In the past three decades, an abrupt, pan-Arctic acceleration of ice wedge melting has transformed tundra landscapes, spurring the formation of hummock-like features known as high-centered polygons (HCPs). This rapid geomorphic transition profoundly alters regional hydrology and influences surface emissions of CO2 and CH4. In Arctic Alaska, most recent instances of ice wedge degradation have arrested within 15-20 years of inception, stabilizing HCP microtopography. However, feedbacks between ground surface deformation and permafrost stability are incompletely understood, limiting our capacity to predict trajectories of landscape evolution in a still warmer future. Here, we use field data from a site near Prudhoe Bay, Alaska, to develop a modeling-based framework for assessing the strength of positive (i.e., exacerbating) feedbacks on ice wedge degradation, focusing on the importance of heterogeneity in surface drainage and microtopographic conditions. Our simulations suggest that, when troughs are narrow, positive feedbacks on ice wedge melting (associated with thermokarst pool formation) are relatively weak. Positive feedbacks are markedly stronger beneath wide troughs, such as those that form above older, larger ice wedges. Seasonal thaw abruptly accelerates once a talik begins to form beneath wide and deep thermokarst pools. Once a talik initiates, winter severity and snowpack thickness increase in importance as predictors of thaw intensity in summer. Our results indicate that meter-scale heterogeneity in polygonal microtopography potentially exerts strong, nonlinear controls on thermokarst trajectories. These findings are useful for predicting future thermokarst dynamics and for interpreting the results from coarser-resolution land surface models operating at greater spatial and temporal scales.

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.

Multiple studies demonstrate Northwest Alaska and the Alaskan North Slope are warming. Melting permafrost causes surface destabilization and ecological changes. Here, we use thermistors permanently installed in 1996 in a borehole in northwestern Alaska to study past, present, and future ground and subsurface temperature change, and from this, forecast future permafrost degradation in the region. We measure and model Ground Surface Temperature (GST) warming trends for a 10 year period using equilibrium Temperature-Depth (TD) measurements from borehole T96-012, located near the Red Dog Mine in northwestern Alaska part of the Arctic ecosystem where a continuous permafrost layer exists. Temperature measurements from 1996 to 2006 indicate the subsurface has clearly warmed at depths shallower than 70 m. Seasonal climate effects are visible in the data to a depth of 30 m based on a visible sinusoidal pattern in the TD plots that correlate with season patterns. Using numerical models constrained by thermal conductivity and temperature measurements at the site, we show that steady warming at depths of similar to 30 to 70 m is most likely the direct result of longer term (decadal-scale) surface warming. The analysis indicates the GST in the region is warming at similar to 0.44 +/- 0.05 degrees C/decade, a value consistent with Surface Air Temperature (SAT) warming of similar to 1.0 +/- 0.8 degrees C/decade observed at Red Dog Mine, but with much lower uncertainty. The high annual variability in the SAT signal produces significant uncertainty in SAT trends. The high annual variability is filtered out of the GST signal by the low thermal diffusivity of the subsurface. Comparison of our results to recent permafrost monitoring studies suggests changes in latitude in the polar regions significantly impacts warming rates. North Slope average GST warming is similar to 0.9 +/- 0.5 degrees C/decade, double our observations at RDM, but within error. The RDM warming rate is within the warming variation observed in eastern Alaska, 0.36-0.71 degrees C/decade, which suggests changes in longitude produce a smaller impact but have warming variability likely related to ecosystem, elevation, microclimates, etc. changes. We also forward model future warming by assuming a 1D diffusive heat flow model and incorporating latent heat effects for permafrost melting. Our analysis indicates similar to 1 to 4 m of loss at the upper permafrost boundary, a similar to 145 +/- 100% increase in the active layer thickness by 2055. If warming continues at a constant rate of similar to 0.44 +/- 0.05 degrees C/decade, we estimate the 125 m thick zone of permafrost at this site will completely melt by similar to 2150. Permafrost is expected to melt by similar to 2200, similar to 2110, or similar to 2080, if the rate of warming is altered to 0.25, 0.90, or 2.0 degrees C/decade, respectively, as an array of different climate models suggest. Since our model assumes no advection of heat (a more efficient heat transport mechanism), and no accelerated warming, our current prediction of complete permafrost loss by 2150 may overestimate the residence time of permafrost in this region of Northwest Alaska. (C) 2016 The Authors. Published by Elsevier B.V.

Climate change is expected to increase regional and global air temperatures and significantly alter precipitation regimes. These projected changes in meteorological conditions will likely influence subsurface thermal regimes. Increases in groundwater and soil temperatures could impact groundwater quality, harm groundwater-sourced ecosystems, and contribute to the geotechnical failure of critical infrastructure. Furthermore, permafrost thaw induced by rising subsurface temperatures will likely alter surface and subsurface hydrology in high altitude and/or latitude regions and exacerbate the rate of anthropogenic climate change by releasing stored carbon into the atmosphere. This contribution discusses the theory and development of subsurface heat transport equations for cold and temperate regions. Analytical solutions to transient forms of the conduction equation and the conduction-advection equation with and without freezing are detailed. In addition, recently developed groundwater flow and heat transport models that can accommodate freezing and thawing processes are briefly summarized. These models can be applied to simulate climate change-induced permafrost degradation and dormant aquifer activation in cold regions. Several previous reviews have focused on the impact of climate change on subsurface hydraulic regimes and groundwater resources, but this is the first synthesis of studies considering the influence of future climate change on subsurface thermal regimes in cold and temperate regions. The current gaps in this body of knowledge are highlighted, and recommendations are made for improving future studies by linking atmospheric global climate models to subsurface heat transport models that consider heat advection via groundwater flow. (C) 2014 Elsevier B.V. All rights reserved.

Air and ground temperatures measured in Eastern Siberia has been compiled and analyzed. The analysis of mean annual air temperatures measured at 52 meteorological stations within and near the East-Siberian transect during the period from 1956 through 1990 demonstrates a significant and statistically significant (at 0.05 level) positive trend ranging from 0.065 to 0.59 degrees C/ 10 yr. A statistically significant (at 0.05 level) positive trend was also observed in mean annual ground temperatures for the same period. The permafrost temperature reflects changes in air temperature on a decadal time scale much better than on an interannual time scale. Generally, positive trends in mean annual ground temperatures are slightly smaller in comparison with trends in mean annual air temperatures, except for several sites where the discordance between the air and ground temperatures can be explained by the winter snow dynamics. The average trend for the entire region was 0.26 degrees C/10 yr for ground temperatures at 1.6 in depth and 0.29 degrees C/10 yr for the air temperatures. The most significant trends in mean annual air and ground temperatures were in the southern part of the transect, between 55 degrees and 65 degrees N. Numerical modeling of ground temperatures has been performed for Yakutsk and Tiksi for the last 70 yr. Comparing the results of these calculations with a similar time series obtained for Fairbanks and Barrow in Alaska shows that similar variations of ground temperatures took place at the same time periods in Yakutsk and Fairbanks, and in Tiksi and Barrow. The decadal and longer time scale fluctuations in permafrost temperatures were pronounced in both regions. The magnitudes of these fluctuations were on the order of a few degrees centigrade. The fluctuations of mean annual ground temperatures were coordinated in Fairbanks and Yakutsk, and in Barrow and Tiksi. However, the magnitude and timing of these fluctuations were slightly different for each of the sites. (C) 2006 Elsevier B.V. All rights reserved.