Abstract
In recent decades, significant changes have occurred in high-latitude areas, particularly to the cryosphere. Sea ice extent and thickness have declined. In land areas, glaciers and ice ...sheets are experiencing negative mass balance changes, and there is substantial regional snow cover variability. Subsurface changes are also occurring in northern soils. This study focuses on these changes in the soil thermal regime, specifically the seasonally frozen ground region of Eurasia. We use a database of soil temperatures at 423 stations and estimate the maximum annual soil freezing depth at the 387 sites located on seasonally frozen ground. Evaluating seasonal freeze depth at these sites for 1930–2000 reveals a statistically significant trend of −4.5 cm/decade and a net change of −31.9 cm. Interdecadal variability is also evident such that there was no trend until the late 1960s, after which seasonal freeze depths decreased significantly until the early 1990s. From that point forward, likely through at least 2008, no change is evident. These changes in the soil thermal regime are most closely linked with the freezing index, but also mean annual air temperatures and snow depth. Antecedent conditions from the previous warm season do not appear to play a large role in affecting the subsequent cold season’s seasonal freeze depths. The strong decrease in seasonal freeze depths during the 1970s to 1990s was likely the result of strong atmospheric forcing from the North Atlantic Oscillation during that time period.
Hydrologic cycle intensification is an expected manifestation of a warming climate. Although positive trends in several global average quantities have been reported, no previous studies have ...documented broad intensification across elements of the Arctic freshwater cycle (FWC). In this study, the authors examine the character and quantitative significance of changes in annual precipitation, evapotranspiration, and river discharge across the terrestrial pan-Arctic over the past several decades from observations and a suite of coupled general circulation models (GCMs). Trends in freshwater flux and storage derived from observations across the Arctic Ocean and surrounding seas are also described.
With few exceptions, precipitation, evapotranspiration, and river discharge fluxes from observations and the GCMs exhibit positive trends. Significant positive trends above the 90% confidence level, however, are not present for all of the observations. Greater confidence in the GCM trends arises through lower interannual variability relative to trend magnitude. Put another way, intrinsic variability in the observations tends to limit confidence in trend robustness. Ocean fluxes are less certain, primarily because of the lack of long-term observations. Where available, salinity and volume flux data suggest some decrease in saltwater inflow to the Barents Sea (i.e., a decrease in freshwater outflow) in recent decades. A decline in freshwater storage across the central Arctic Ocean and suggestions that large-scale circulation plays a dominant role in freshwater trends raise questions as to whether Arctic Ocean freshwater flows are intensifying. Although oceanic fluxes of freshwater are highly variable and consistent trends are difficult to verify, the other components of the Arctic FWC do show consistent positive trends over recent decades. The broad-scale increases provide evidence that the Arctic FWC is experiencing intensification. Efforts that aim to develop an adequate observation system are needed to reduce uncertainties and to detect and document ongoing changes in all system components for further evidence of Arctic FWC intensification.
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BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Active layer thickness (ALT) is a critical metric for monitoring permafrost. How soil moisture influences ALT depends on two competing hypotheses: (a) increased soil moisture increases the ...latent heat of fusion for thaw, resulting in shallower active layers, and (b) increased soil moisture increases soil thermal conductivity, resulting in deeper active layers. To investigate their relative influence on thaw depth, we analyzed the Field Measurements of Soil Moisture and Active Layer Thickness (SMALT) in Alaska and Canada dataset, consisting of thousands of measurements of thaw depth and soil moisture collected at dozens of sites across Alaska and Canada as part of NASA’s Arctic Boreal Vulnerability Experiment (ABoVE). As bulk volumetric water content (VWC) integrated over the entire active layer increases, ALT decreases, supporting the latent heat hypothesis. However, as VWC in the top 12 cm of soil increases, ALT increases, supporting the thermal conductivity hypothesis. Regional temperature variations determine the baseline thaw depth while precipitation may influence the sensitivity of ALT to changes in VWC. Soil latent heat dominates over thermal conductivity in determining ALT, and the effect of bulk VWC on ALT appears consistent across sites.
In this study, we report on the spatial and temporal distribution of seasonal snow depth derived from passive microwave satellite remote-sensing data (e.g. SMMR from 1978 to 1987 and SMM/ I from 1987 ...to 2006) in China. We first modified the Chang algorithm and then validated it using meteorological observation data, considering the influences from vegetation, wet snow, precipitation, cold desert and frozen ground. Furthermore, the modified algorithm is dynamically adjusted based on the seasonal variation of grain size and snow density. Snow-depth distribution is indirectly validated by MODIS snow-cover products by comparing the snow-extent area from this work. The final snow-depth datasets from 1978 to 2006 show that the interannual snow-depth variation is very significant. The spatial and temporal distribution of snow depth is illustrated and discussed, including the steady snow-cover regions in China and snow-mass trend in these regions. Though the areal extent of seasonal snow cover in the Northern Hemisphere indicates a weak decrease over a long period, there is no clear trend in change of snow-cover area extent in China. However, snow mass over the Qinghai–Tibetan Plateau and northwestern China has increased, while it has weakly decreased in northeastern China. Overall, snow depth in China during the past three decades shows significant interannual variation, with a weak increasing trend.
Observational data collection on permafrost and active layer freeze–thaw cycle is extremely limited in the upper reaches of the Heihe River (URHHR) in the Qilian Mountains of the north-eastern ...Qinghai-Tibet Plateau. It acts as a bottleneck, restricting the hydrological effects of the changes in the permafrost and active layer in the Heihe River Basin. Using soil temperature, moisture and air temperature data collected from the four active layer observation sites (AL1, AL3, AL4 and AL7) established in the alpine permafrost regions in the URHHR, from 2013 to 2014, the region's active layer freeze–thaw cycle and the soil hydrothermal dynamics were comparatively analysed. As the elevation increased from 3700 m a.s.l. to 4132 m a.s.l., the mean annual ground temperatures (MAGTs) of the active layer and the active layer thicknesses (ALTs) decreased, the onset date of soil freeze of the active layer occurred earlier and the soil freeze rate increased. However, the onset date of soil thaw and the thaw rate did not exhibit significant trends. Compared to the thaw process, the duration of the active layer freeze process was significantly shortened and its rate was significantly higher. The soil freeze from bottom to top did not occur earlier than that from top to bottom. Furthermore, as elevation increased, the proportion of the bottom-up freeze layer thickness increased. The soil moisture in the thaw layer continuously moved to the freeze front during the active layer's two-way freeze process, causing the thaw layer to be dewatered. The seasonal thaw process resulted in significant reduction of the soil water content in the thaw layer, accounting for the high ice content in the vicinity of the permafrost table. Controlled by elevation, the active layer's seasonal freeze–thaw cycle was also affected by local factors, such as vegetation, slope, water (marsh water and super-permafrost water), lithology and water (ice) content. This study provides quantitative data that identify, simulate and predict the hydrological effects of the changes in the permafrost and active layer of the Heihe River Basin.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Despite the response of ground temperatures to increases in air temperature, discontinuous and isolated bodies of permafrost are strongly affected by lateral heat fluxes from adjacent unfrozen zones. ...However, many current simulations consider heat flow in only one-dimension (1D) and thus cannot represent two or three-dimensional effects. To address this issue, we use fine-scale in situ measurements and two-dimension (2D) heat conduction simulations and compare the simulated ground thermal evolution and degradation of island permafrost to a 1D simulation. The 2D simulation reasonably represented seasonal and interannual features of the ground thermal regime, such as temperature profile, thawing/frozen depth, and permafrost body width. Lateral thawing of the island permafrost was dominant, and was about one order of magnitude greater than the modelled vertical thaw. In the vertical direction, downward permafrost thaw caused by increase in air temperature rise was restricted by phase change of ground ice near the permafrost table during the simulation period, contributing little to the overall permafrost degradation. Rates of permafrost degradation in the 1D simulation were 1.6–1.8 times lower than the simulation considering lateral heat fluxes. The field monitoring and numerical simulation highlight the importance of coupling lateral heat transfer in permafrost models.
•The lateral thaw is one order of magnitude greater than the vertical thaw.•Permafrost degradation near the lower limits is underestimated by 1D simulations.•Near the limits of permafrost, lateral heat flow should be considered in simulations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Within the gas hydrate drilling project in the Qilian Mountain permafrost region, a temperature-depth profile measured from borehole DK-12 in Juhugeng of Muri Coalfield, Tianjun County, Qinghai ...Province, China, was analyzed to infer recent climate changes. The long-term surface temperature and thermal gradient were retrieved from borehole temperature measurements. The ground surface temperature (GST) changes were reconstructed by inversion of transient temperature perturbations through solving an inverse heat conduction problem using the Tikhonov method. Based on the instability of this kind of inverse problem and the nature of method-dependent features of borehole paleothermometry, we initially applied the Tikhonov regularization technique to obtain a stable past GST variation pattern with relatively low resolution. The inversion results showed that this region experienced temperature fluctuation with a total warming of 3°C (±1.6°C) from 1400 to the 2010s and a more exacerbated warming starting from the 1960s. The GST trend fit the surface air temperature observation trend from the nearest Yeniugou meteorological station. This work fills the gap created by limited meteorological records in the Muli area and extends knowledge of ground surface temperature trends going back more than ten centuries.
Currently, we know little about accumulation of soil carbon and nitrogen in permafrost‐affected wetlands on the Qinghai–Tibet Plateau (QTP). In this study, we analyze the vertical distribution of ...concentrations, stocks, and apparent accumulation rates of soil organic carbon (SOC) and total nitrogen (TN) in a wetland underlain by ice‐rich permafrost in the Headwater Area of the Yellow River (HAYR) on the northeastern QTP in the context of Holocene environmental change. SOC and TN stocks at depths of 0–216 cm were 80.0 kg C m−2 and 6.7 kg N m−2, respectively. During the past 7.3 kyr, the general regional climate trend in the HAYR was cooling and drying, as indicated by the decline in chemical weathering in the soil profile. Overall, SOC and TN concentrations increased during this period. Meanwhile, an intense period of SOC and TN accumulation occurred at 1,110–720 yr BP, in contrast to much lower apparent accumulation rates of SOC and TN for the other periods during the past 7.3 kyr. This suggests that the accumulation of SOC and TN in permafrost‐affected wetlands was also affected by local environmental factors, such as soil material deposition rate, in addition to climatic controls as exerted mainly by temperature and precipitation. This study may help integrate relevant studies on plateau wetlands into global models and estimates to better simulate and predict interactions between the carbon cycle and climate changes on a global scale.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Snow properties and their changes are crucial to better understanding of hydrological processes, soil thermal regimes, and surface energy balances. Reliable data and information on snow depth and ...snow water equivalent (SWE) are also crucial for water resource assessments and socio-economic development at local and regional scales. However, these data are extremely limited and unreliable in northern Xinjiang, China. This study thus aims to investigate spatial variations of snow depth, SWE, and snow density based on winter snowfield surveys during 2015 through 2017 in the Altai Mountains, northwestern China. The results indicated that snow depth (25–114 cm) and SWE (40–290 mm) were greater in the alpine Kanas-Hemu region, and shallow snow accumulated (9–42 cm for snow depth, 26–106 mm for SWE) on the piedmont sloping plain. While there was no remarkable regional difference in the distribution of snow density. Snow property distributions were strongly controlled by topography and vegetation. Elevation and latitude were the most important factors affecting snow depth and SWE, while snow density was strongly affected by longitude across the Altai Mountains in China. The influence of topography on snow property distributions was spatially heterogenous. Mean snow depth increased from 13.7 to 31.2 cm and SWE from 28.5 to 79.9 mm, respectively, with elevation increased from 400 to 1000 m a.s.l. on the piedmont sloping plain. Snow depth decreased to about 15.1 cm and SWE to about 28.5 mm from 1000 to 1800 m a.s.l., then again increased to about 98.1 cm and 271.7 mm on peaks (∼2000 m a.s.l.) in the alpine Kanas-Hemu. Leeward slopes were easier to accumulate snow cover, especially on north-, east-, and southeast-facing slopes. Canopy interception was also the cause of the difference in snow distribution. Snow depth, SWE, and snow density in forests were reduced by 8%–53%, 2%–67% and −4% to +48%, respectively, compared with surrounding open areas. Especially when snow depth was less than 40 cm, snow depth and SWE differences in forests were more exaggerated. This study provides a basic data set of spatial distributions and variations of snow depth, SWE and snow density in the Altai Mountains, which can be used as an input parameter in climate or hydrological models. These first-hand observations will help to better understand the relationship between snow, topography and climate in mountainous regions across northern China and other high-mountain Asian regions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
We assess the correspondence between precipitation products from atmospheric reanalyses (ERA‐40, NCEP‐1, and NCEP‐2), the Climate Prediction Center (CPC) Merged Analyses of Precipitation (CMAP‐1 and ...CMAP‐2), and the Global Precipitation Climatology Project Version 2 (GPCP‐2) with adjusted observational precipitation (AOP) from China for 1979–2001 and also for ERA‐40 and NCEP‐1 over 1958–1978. In general, we conclude that CMAP‐1 and GPCP‐2 agree more closely with AOP than the reanalysis products do, although ERA‐40 data agree more closely with AOP than NCEP data. The percentages of precipitation differences (PPDs) across China between annual ERA‐40, NCEP‐1, NCEP‐2, CMAP‐1, CMAP‐2, and GPCP‐2 data and AOP are −12, 22, 14, −8, −7, and −15%, respectively, for 1979–2001. Although relatively small biases are evident for China as a whole, maximum PPDs, usually occurring around the Qinghai‐Tibetan Plateau, can exceed 1000%, indicating a strong terrain dependence of gridded precipitation data. GPCP‐2, although characterized by greater underestimation for most of China compared with CMAP‐1, exhibits a smaller biases range and hence may be better than CMAP‐1. Compared with the NCEP‐1 system, NCEP‐2 represents an improvement as NCEP‐2 precipitation agrees more closely with AOP than NCEP‐1 data. However, the coherence of NCEP‐2 precipitation needs further improvement. In addition, we find worse consistency and accuracy and larger positive biases in some parts of China for CMAP‐2 versus CMAP‐1, illustrating an advantage of including reanalysis data in CMAP, as CMAP‐1 does. CMAP‐1 could be further improved if they used the more skillful ERA‐40 precipitation instead of the NCEP/NCAR data.
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