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.

Permafrost is one of the essential carbon pools in the world. Due to limited studies on historical soil moisture changes and the coupling relationship between soil moisture and temperature in permafrost regions, significant uncertainty exists in the carbon loss in permafrost predicted by different models under global warming scenarios. Based on the tree-ring width chronology of Pinus sylvestris var. mongholica Litv. growing in the southern edge of the Eurasian continuous permafrost zone, we reconstructed the summer (June-September) 0-1 m soil moisture variations from 1705 to 2009, which could explain 45.6% of the variance in the observed soil moisture. Overall, local precipitation and temperature exhibited statistically significant positive feedback (p < 0.001) to soil moisture before the 1950s, indicating that the warm/humid climate pattern was conducive to soil moisture conservation before the Anthropocene Epoch. However, the effect of temperature on soil moisture has shifted suddenly to negative since the 1950s, implying that the positive soil moisture-temperature relationship during the past three centuries has been disrupted by the unprecedented warming in the Modern Warm Period. Furthermore, we found that the temporal relationship of the soil moisture-temperature (15-year sliding correlation) in the study area is negatively regulated by the global mean temperature variations (p < 0.01). The regime shift between soil moisture and temperature might be attributed to the superimposed influence of natural and anthropogenic factors since the 1950s. Although the warming leads to the melting of the permafrost layer, and thus the increase in soil moisture content, the enhanced evapotranspiration caused by warming up results in more water loss and drier soil. This study provides historical evidence of shifted soil moisture-temperature coupling in the permafrost zone, warning that soil moisture in the permafrost region may further decline under global warming scenarios, thereby affecting vegetation growth and forest carbon sequestration potential.

Although climate change has convincingly been linked to the evolution of human civilization on different temporal scales, its role in influencing the spatial patterns of ancient civilizations has rarely been investigated. The northward shift of the ancient Silk Road (SR) route from the Tarim Basin (TB) to the Junggar Basin during -420-850 CE provides the opportunity to investigate the relationship between climate change and the spatial evolution of human societies. Here, we use a new high-resolution chironomidbased temperature reconstruction from arid China, combined with hydroclimatic and historical datasets, to assess the possible effects of climate fluctuations on the shift of the ancient SR route. We found that a cooling/drying climate in the TB triggered the SR route shift during -420-600 CE. However, a warming/ wetting climate during -600-850 CE did not inhibit this shift, but instead promoted it, because of the favorable climate-induced geopolitical conflicts between the Tubo Kingdom and the Tang Dynasty in the TB. Our findings reveal two distinct ways in which climate change drove the spatial evolution of human civilization, and they demonstrate the flexibility of societal responses to climate change. (c) 2024 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.

Tree-ring width chronologies are a critically important material to reconstruct past precipitation variability on the northeastern Tibetan Plateau (NTP). However, temperature signals are often encoded in these chronologies, which complicate the precipitation reconstructions and should be carefully assessed. Here, a dataset of 487 Qilian juniper (Juniperus przewalskii Kom.) tree-ring width series from 16 sites on the NTP were collected to investigate the influence of different temperature signals on the precipitation reconstructions. Correlation analysis showed that all tree-ring series recorded similar precipitation information, but had positive (p 0.05, Group1), weak (p 0.05, Group2), and negative (p < 0.05, Group3) correlations with temperature, respectively. In view of this, all tree-ring series were divided into three groups to develop chronologies to reconstruct local precipitation. During the calibration period of 1957?2011 CE, the Group1 reconstruction had the fastest uptrend, which almost overlapped the observed precipitation; the Group2 reconstruction showed a slower uptrend, whereas the Group3 reconstruction lacked an uptrend. As a result, we get different results when the reconstructions were used to assess the current precipitation status over the past millennium. The Group1 (Group2) reconstructions showed that the recent 20 (10) years were the highest precipitation period over the past millennium, whereas the Group3 reconstruction did not capture this phenomenon. Therefore, we caution that the temperature effects should be evaluated carefully before tree-ring width chronologies being employed to study past precipitation variability.

In mountainous regions, global warming will likely affect the frequency and magnitude of geomorphic processes. This is also the case for rockfall, one of the most common mass movements on steep slopes. Rainfall, snowmelt, or freeze-thaw cycles are the main drivers of rockfall activity, rockfall hazards are thus generally thought to become more relevant in a context of climate change. At high elevations, unequivocal relationships have been found between increased rockfall activity, permafrost thawing and global warming. By contrast, below the permafrost limit, studies are scarcer. They mostly rely on short or incomplete rockfall records, and have so far failed to identify climatically induced trends in rockfall records. Here, using a dendrogeomorphic approach, we develop two continuous 60-year long chronologies of rockfall activity in the Vercors and Diois massifs (French Alps); both sites are located clearly below the permafrost limit. Uncertainties related to the decreasing number of trees available back in time were quantified based on a detailed mapping of trees covering the slope across time. Significant multiple regression models with reconstructed rockfalls as predictors and local changes in climatic conditions since 1959 extracted from the SAFRAN reanalysis dataset as predictants were fitted to investigate the potential impacts of global warming on rockfall activity at both sites. In the Vercors massif, the strong increase in reconstructed rockfall can be ascribed to the recolonization of the forest stand and the over-representation of young trees; changes that are observed should not therefore be ascribed to climatic fluctuations. In the Diois massif, we identify annual precipitation totals and mean temperatures as statistically significant drivers of rockfall activity but no significant increasing trend was identified in the reconstruction. All in all, despite the stringency of our approach, we cannot therefore confirm that rockfall hazard will increase as a result of global warming at our sites.

The long-term and continuous permafrost temperature data is of great significance to the study of permafrost, climate, ecology, hydrology and engineering on the Qinghai-Tibet Plateau (QTP), but the available observed data sequence is no longer than 25 years. To address the gap, we first attempt to reconstruct the sequences of permafrost temperature at three monitoring sites including Xidatan, Wudaoliang and Tanggula along the QTP engineering corridor from 1920 to 2019, based on one of the most used permafrost models worldwide (i.e., Geophysical Institute Permafrost Lab version 2 (GIPL2)). The GIPL2 model and its parameterized schemes were first evaluated and calibrated using the ground temperature observations at the three sites. The monthly near surface ground temperature at the depth of 5 cm after calibration and correction based on monitoring data was used as forcing dataset to simulate the temperature change of the permafrost from 1900 to 2019. The temperature change sequences since 1920 were selected to discuss the changes of permafrost on the QTP, and its responses to climate change. Results showed that (i) the GIPL2 model can well simulate the thermal state of permafrost on the QTP with low simulation errors (below 0.1 degrees C) at the depth of zero annual amplitude; (ii) the annual average ground temperature at different depths for all three sites experienced warming trends from 1920 to 2019, in which the average warming rate was 0.07 degrees C/10 a (0.05-0.09) at the depth of zero annual amplitude (15 m). Besides, the site with the largest warming rate at the shallow layer (3 m) was found in Wudaoliang, while the deep layer (30 m) was in Xidatan; (iii) the permafrost temperature at the shallow layer increased rapidly since 1980. Nevertheless, the response times of the thermal conditions to climate change varied with soil layers, among which the deep layer lagged by about 20 years compared to the shallow layer; (iv) permafrost thicknesses for the Xidatan, Wudaoliang and Tanggula sites were decreased by 13.9 m, 4.6 m and 4.7 m respectively. The average deepening rate of the permafrost table and rising rates of permafrost base for the three sites were 0.6 cm/a and 10.27 cm/a, respectively. More specifically, the deepening rate of the permafrost table was 0.5 cm/a for Xidatan, 0.6 cm/a for Wudaoliang and 0.7 cm/a for Tanggula, and the rising rate of the permafrost base was 13.4 cm/a for Xidatan and 4.0 cm/a for both Wudaoliang and Tanggula. Compared with that in Wudaoliang and Tanggula, the permafrost in Xidatan was relatively unstable and its response to climate change was more sensitive. Although the simulations of the GIPL2 model could be impacted by the accuracy of the forcing data (e.g., 5 cm ground temperature), the reconstructed permafrost temperature changes from 1920 to 2019 were consistent with the observations over the past 40 years. Besides, our results also confirmed the continuous warming phenomenon of permafrost on the QTP since 1920. These findings can well fill the narrow gap relating to the short sequence and discontinuity of the permafrost temperature dataset on the QTP, and provide a baseline of permafrost changes to the scientific community for a better understanding of the changes in the cryosphere, ecosystem, water resources, and even climate. Nevertheless, some limitations in temperature reconstruction and model processing were noted. In the future, multiple aspects including accurate forcing data and complex factors (e.g., heat convection and lateral heat flow exchange) should be considered comprehensively in the model to reduce the uncertainties of ground temperature simulations.

Ground subsidence and uplift caused by the annual thawing and freezing of the active layer are important variables in permafrost studies. Global positioning system interferometric reflectometry (GPS-IR) has been successfully applied to retrieve the continuous ground surface movements in permafrost areas. However, only GPS signals were used in previous studies. In this study, using multiple global navigation satellite system (GNSS) signal-to-noise ratio (SNR) observations recorded by a GNSS station SG27 in Utqiagvik, Alaska during the period from 2018 to 2021, we applied multiple GNSS-IR (multi-GNSS-IR) technique to the SNR data and obtained the complete and continuous ground surface elevation changes over the permafrost area at a daily interval in snow-free seasons in 2018 and 2019. The GLONASS-IR and Galileo-IR measurements agreed with the GPS-IR measurements at L1 frequency, which are the most consistent measurements among all multi-GNSS measurements, in terms of the overall subsidence trend but clearly showed periodic noises. We proposed a method to reconstruct the GLONASS- and Galileo-IR elevation changes by specifically grouping and fitting them with a composite model. Compared with GPS L1 results, the unbiased root mean square error (RMSE) of the reconstructed Galileo measurements reduced by 50.0% and 42.2% in 2018 and 2019, respectively, while the unbiased RMSE of the reconstructed GLONASS measurements decreased by 41.8% and 25.8% in 2018 and 2019, respectively. Fitting the composite model to the combined multi-GNSS-IR, we obtained seasonal displacements of - 3.27 +/- 0.13 cm (R-2 = 0.763) and - 10.56 +/- 0.10 cm (R-2 = 0.912) in 2018 and 2019, respectively. Moreover, we found that the abnormal summer heave was strongly correlated with rain events, implying hydrological effects on the ground surface elevation changes. Our study shows the feasibility of multi-GNSS-IR in permafrost areas for the first time. Multi-GNSS-IR opens up a great opportunity for us to investigate ground surface movements over permafrost areas with multi-source observations, which are important for our robust analysis and quantitative understanding of frozen ground dynamics under climate change.

Polar amplification has been a research focus in climate research in recent decades. However, little attention has been paid to Antarctic amplification (AnA). We have examined the variations in annual and seasonal temperature over the Antarctic Ice Sheet and its amplification based on reconstruction covering the period 2002-2018. The results show the occurrence of annual and seasonal AnA, with an AnA index greater than 1.39 with seasonal differences, and that AnA is strong in the austral winter and spring. Moreover, AnA displays regional differences, with the greatest amplification occurring in East Antarctica, with an AnA index greater than 1.51, followed by West Antarctica. AnA is always absent in the Antarctic Peninsula. In addition, amplification in East Antarctica is most conspicuous in spring, which corresponds to the obvious warming in this season; and the spring amplification signal is weakest for West Antarctica. When considering the influence of the ocean, the AnA becomes obvious, compared to when only the land is considered. Southern Annular Mode (SAM), surface pressure and westerlies work together to affect the temperature change over Antarctica and AnA; and SAM and surface pressure are highly correlated with the temperature change over East Antarctica. The picture reflects the accelerated changes in Antarctic temperature.

The annual mean temperature on the Tibetan Plateau (TP) has strongly increased over the past few decades, with larger warming in winter than in summer. Whether this different amplitude of change between seasons has persisted over longer time-scales in the past remains poorly understood, limiting our understanding of the mechanisms responsible. Here, we apply multivariate regression analysis and ensemble empirical mode decomposition (EEMD) to decompose winter (T-DJF) and summer (T-JJA) temperature reconstructions over the 1718-2005 CE period for the southeastern TP to investigate similarities and differences between winter and summer temperature changes, over multiple time-scales, as well as the driving factors behind the seasonal differences. The results reveal that the T-DJF and T-JJA reconstructions were significantly correlated throughout the study period, with the magnitude of the T-DJF variations approximately six times greater than the T-JJA variations. When the two reconstructions were decomposed over multiple time-scales, it was found that the consistency between winter and summer temperature reconstructions only existed at inter-annual scale. Assessing the driving factors, the main contributions to the T-JJA and T-DJF changes at the inter-annual and inter-decadal scales appear to be mainly the El Nino-Southern Oscillation (ENSO). The Pacific Decadal Oscillation (PDO) contribution was important to T-JJA and T-DJF changes at multi-decadal scales. Furthermore, we found that orbital parameters and the Atlantic Multidecadal Oscillation (AMO) was a major contributor to the changes in T-JJA and T-DJF at centennial scales, respectively. Both the T-JJA and T-DJF have a significant long-term increasing trend since c. 1850, mainly attributed to anthropogenic forcing. The detected similarities and differences between T-DJF and T-JJA at multiple time-scales provide new perspectives on the understanding the mechanisms behind climate change on the Tibetan Plateau and even entire East Asia.

Based on glacio-meteorological records, 7 years of in-situ mass balance data, and a temperature-index model, the long-term annual and seasonal mass balances of Shiyi Glacier in the northeast Tibetan Plateau (TP) were reconstructed from 1963/64 to 2016/17. Variations were then linked to local climatic and macroscale circulation changes. The model was calibrated based on in-situ mass balance data and was driven by daily air temperature and precipitation data recorded at nearby alpine meteorological stations. The results show that the reconstructed annual mass balance experienced an overall downward trend over the past 54 years, with a remarkably high mass loss rate during 1990/91-2016/17. Analysis of mass balance sensitivity and local climatic changes shows that the pronounced mass loss since the 1990s can be mainly attributed to cumulative positive temperature increases caused by air temperature increases and prolongation of the ablation season. From the perspective of macroscale circulation, the reconstructed annual mass balance values correlate well with zonal wind speeds (June to September) in the glacierized region. For the positive/negative phase of the annual mass balance, an inverse spatial pattern in relation to geopotential height change (low/high-pressure centres) and corresponding conversion of cyclonic/anti-cyclonic circulation were present in northern hemisphere mid-latitudes. Comparative analysis of existing long-term mass balance series over the TP indicates that asynchronous climatic changes in the different glacierized regions led to inconsistent interannual fluctuations in glacier mass balance.