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Vegetation phenology interacts strongly with climate through the exchange of carbon, water, momentum, and energy between terrestrial ecosystem and atmosphere. These vegetation dynamics in Northern Hemisphere permafrost regions have substantial uncertainties in previous studies, partly due to differences in datasets. Thus, reliable land surface phenology (LSP) retrievals are crucial for understanding the effects of climate change on ecosystems and biosphere‐atmosphere‐hydrosphere interactions. We assessed various LSP datasets at different spatial resolutions for 2001–2014, generated by different methods based on different satellite observations. We also assessed the accuracy of LSP by comparing with CO2flux phenology. For start of growing season (SOS), the comprehensive evaluation indicated MODIS phenology showed better consistency and higher accuracy with flux‐derived phenology observations (R= 0.54, RMSE = 36.2 days, bias<5 days). For end of growing season (EOS), we cannot conclusively determine which LSP performs best. In the Northern Hemisphere permafrost regions, SOS occurred at 100–150 days (April–May), and EOS occurred at 260–320 days (September–November). During 2001–2014, SOS occurred earlier by 0.33 ± 0.30 days/yr. Significant trends were observed for 6.4%–27.6% of pixels, averaging −1.30 ± 1.16 days/yr. EOS occurred earlier by 0.25 ± 0.43 days/yr, with significant trends averaging −0.52 ± 0.94 days/yr. Among LSPs, variability in EOS trends was significant, with even the direction of trends differing. This study may provide insights into LSP data selection and further understanding of vegetation dynamics and its mechanisms in permafrost regions. To better understand permafrost ecosystem dynamics associated with global change, the changes in vegetation and their spatial patterns in the Northern Hemisphere permafrost regions must be carefully evaluated. Thus, reliable land surface phenology (LSP) retrievals are very important. We therefore collected a total of 13 LSPs to understand how these products perform relative to each other and to ground observations and to quantify their uncertainties. MODIS and different GIMMS products are slightly more consistent overall. Considering different metrics, regions, and parameters, the temporal trends and spatial patterns of phenology indicate an earlier start to the growing season, but the end of the growing season is inconsistent. Bigger vegetation changes are found in the continuous permafrost region. These findings allow us to clarify the applicability of the LSP products, to provide a reference for data selection, and insights into the understanding of vegetation dynamics and their mechanisms in permafrost regions. The average start and end of the growing season occurs on 131 ± 18 days and 291 ± 4 days in Northern Hemisphere permafrost regionsComparison of 13 land surface phenology products to CO2flux phenology indicates that the RMSEs ranges from 27.25ays to 77.84 daysBoth the start and end of the growing season are earlier by 1.30 ± 1.16 and 0.52 ± 0.94 days/yr in Northern Hemisphere permafrost regions The average start and end of the growing season occurs on 131 ± 18 days and 291 ± 4 days in Northern Hemisphere permafrost regions Comparison of 13 land surface phenology products to CO2flux phenology indicates that the RMSEs ranges from 27.25ays to 77.84 days Both the start and end of the growing season are earlier by 1.30 ± 1.16 and 0.52 ± 0.94 days/yr in Northern Hemisphere permafrost regions

The maximum annual freeze depth (MFD) is a primary indicator of the thermal state of frozen ground, affecting ecosystems, hydrological processes, vegetation growth, infrastructure, and human activities in cold regions. It is thus important to quantify the past, present, and future spatial and temporal variability of MFD at the hemispheric scale. We develop a data‐driven MFD simulation method within a machine learning framework, integrating MFD observations from meteorological stations and several environmental predictors, to analyze past and future scenarios in the Northern Hemisphere (NH). Based on ERA5 reanalysis estimates and historical to future CMIP6 scenarios, the NH MFD averaged 133 cm (ERA5) and 131 cm (CMIP6) during 1981–2010, and will vary 81–112 cm during 2015–2100 depending on the emission scenario. During 1950–2013, MFD decreased by 0.37 cm/a (ERA5) versus 0.22 cm/a (CMIP6), and is projected to decrease 0.16–0.69 cm/a by 2100. During 1981–2010, MFD decreased by an average of 19.1% (ERA5) and 13.9% (CMIP6), with a net change of −17 cm (ERA5) and −13 cm (CMIP6). Depending on the emission scenario, MFD will decrease 11% (−12 cm) to 42% (−19 cm) between 2015 and 2099 relative to the 1981–2010. Warming, increased moisture, warmer cold seasons, warmer warm seasons, shallower snow depths, and increased vegetation cover all lead to a reduction in MFD. The results from this novel machine learning approach provide useful insights regarding the fate of future frozen ground changes. Seasonally frozen ground covers approximately half of the exposed ground surface in the Northern Hemisphere and is found in areas of intense human activity. There, the moisture retention and occurrence of freezing and thawing significantly impact agricultural production and infrastructure. Maximum freeze depth is a key indicator of the status of seasonally frozen ground. We simulate and predict the spatial distribution of maximum freeze depth at the hemispheric scale and quantify the variability of maximum freeze depth over past and future periods. Depending on the choice of future emission scenario, average maximum freeze depth in the Northern Hemisphere will decrease by 11% (−12 cm) to 42% (−19 cm) between 2015 and 2099, relative to the base period (1981–2010). A novel machine learning approach is used to calculate 250 years of maximum freeze depth for Northern Hemisphere seasonally frozen groundUnder the SSP5‐8.5 scenario, the maximum freeze depth in the Northern Hemisphere will decrease by 42% by 2099The maximum freeze depth decreases the most in the snow climate zone A novel machine learning approach is used to calculate 250 years of maximum freeze depth for Northern Hemisphere seasonally frozen ground Under the SSP5‐8.5 scenario, the maximum freeze depth in the Northern Hemisphere will decrease by 42% by 2099 The maximum freeze depth decreases the most in the snow climate zone

Permafrost is an important indicator of climate change that affects, e.g., ecosystems, hydrology, and infrastructure in cold regions. Interactions between the surface and atmosphere in permafrost regions are complex and vary seasonally due to the contrasting energy and moisture conditions of frozen versus unfrozen ground. Differences in surface‐atmosphere connections have been documented between permafrost types, with significantly increased moisture feedbacks in areas of continuous Eurasian permafrost, but no such relationship in other regions. This study elucidates the processes responsible for these observed land‐atmosphere feedbacks in Eurasian permafrost regions. The Weather Research and Forecasting model is used to determine the varying effects of discontinuous versus continuous permafrost on different weather scenarios. For two 72‐hour periods, surface fluxes, boundary layer characteristics, and corresponding meso‐to‐synoptic scale meteorological features varied when comparing the two experiments: one with a saturated active layer representing continuous permafrost, and another witht1 a drier surface layer representing discontinuous permafrost. Depending on the weather regime, an increase in boundary layer moisture in response to increased latent heat fluxes over continuous permafrost increases precipitation and low‐level cloudiness. The effects of changing soil conditions are evident as far aloft as 500 hPa, as a mid‐level low over continuous permafrost is stronger and moves southward. Incipient moisture associated with continuous permafrost allows for different land‐atmosphere interactions than over discontinuous permafrost. Continued permafrost degradation as a result of climate change may lead to increased dryness in the region, altering the surface energy balance and associated weather and climate feedbacks. Changes in soil characteristics consistent with different permafrost (PF) types lead to modifications of lower troposphere moisture profileSimulations over discontinuous PF show decreased low‐level cloudiness and decreased precipitation relative to continuous PFGeophysical impacts of degrading PF, including atmospheric feedbacks, should be a devoted focus of research in the future

In cold regions characterized by perennially frozen soil, climate warming has caused permafrost degradation, which is manifested by surface deformation and deepening of the active layer. Interferometric synthetic aperture radar (InSAR) is a common method to obtain large‐scale surface deformation, especially in mountainous terrain where it is difficult to install a large number of monitoring sites. Here, we used Sentinel‐1A SAR data acquired from March 2017 to March 2021 to monitor surface deformation and estimate active layer thickness (ALT) using two methods—temperature‐based and soil moisture‐based—in the permafrost regions of the Heihe River Basin on the northeast Qinghai‐Tibetan Plateau. We find that the seasonal deformation amplitude ranges 10–60 mm, and the annual mean vertical deformation trend ranges from −40 to 30 mm/a. We verify the ALT estimates with observations and find that the method based on soil moisture has higher accuracy and is therefore more applicable in this particular study area. The ALT gradually decreases from the southeast to the northwest, ranging from 0.3 to 6.5 m, with an average value of 2.47 m. These results contribute to better quantifying the response of permafrost change and to understanding the spatial distribution of ALT on a large scale, serving as guidance for engineering and other applications. The effects of global warming have led to the degradation of permafrost. To quantify changes in permafrost, we use InSAR (Interferometric Synthetic Aperture Radar) technology to monitor the surface deformation in permafrost area. The main manifestation of permafrost degradation is the deepening of the active layer (AL). Therefore, we investigate the feasibility of obtaining AL thickness in the permafrost zone based on InSAR. Mean AL thickness in the Heihe River Basin is 2.47 m and ranges 0.3–6.5 m. Compared with temperature‐based inversion models, the inversion model for ALT based on the soil moisture content has a higher accuracy. Surface deformation based on interferometric synthetic aperture radar can be used to quantify active‐layer thickness (ALT) with high resolutionMean active layer thickness in the Heihe River Basin is 2.47 m and ranges from 0.3 to 6.5 mThe inversion model for ALT based on the soil moisture content has a higher accuracy when combined with the temperature‐based inversion model Surface deformation based on interferometric synthetic aperture radar can be used to quantify active‐layer thickness (ALT) with high resolution Mean active layer thickness in the Heihe River Basin is 2.47 m and ranges from 0.3 to 6.5 m The inversion model for ALT based on the soil moisture content has a higher accuracy when combined with the temperature‐based inversion model

Climate warming causes permafrost degradation that not only leads to the release of greenhouse gases to the atmosphere, but also to soil moisture increases due to ground ice melt. These processes are particularly prevalent in peat‐rich and ice‐rich permafrost regions. Peat is important because of its high organic matter content and soil moisture. Although previous work has focused on the importance of two factors, their precise quantitative impact on permafrost is still not clear. Here we apply the Geophysical Institute Permafrost Laboratory model and sensitivity experiments to quantify the role of organic matter and soil moisture on permafrost, with a case study focused on the northeastern Tibetan Plateau. We verify that organic matter and soil moisture has a cooling effect in the warm season and an insulating effect in the cold season. The average thawing onset was delayed 10 days at 0.05–1.4 m depths, when organic matter content increases from 0% to 90%. Freezing onset occurs slightly earlier. Furthermore, active layer thickness (ALT) decreased by 0.40 m. Soil moisture has similar effect on permafrost as organic matter, but ALT changes have a higher magnitude, decreasing by 0.46 m. The results show that both organic matter and soil moisture have an insulating effect on permafrost. Further, the magnitude of impact is larger as organic matter or soil moisture increase. These results can be helpful in assessing permafrost carbon mitigation with climate change. Permafrost can contain high amounts of organic matter and ground ice, and peat plays an important role in permafrost degradation. Permafrost degrades due to climate warming, which can lead to the release of carbon in the form of greenhouse gases, exacerbating the degradation of permafrost. Additionally, soil moisture also significantly impacts permafrost degradation. However, few studies have estimated the effects of organic matter and soil moisture on permafrost degradation. This paper therefore explores the quantitative effect of organic matter and soil moisture on permafrost through a series of model sensitivity experiments. We find that organic matter and soil moisture both insulate permafrost, cooling it in summer and warming it in winter. These results are important for mitigating the effects of climate change on permafrost and carbon feedbacks. The insulating effect of organic matter and soil moisture on permafrost is quantified using model sensitivity experimentsIncreasing organic matter and soil moisture both cool permafrost in summer and warm it in winterThese organic matter and soil moisture feedbacks refine our understanding of permafrost dynamics under climate change The insulating effect of organic matter and soil moisture on permafrost is quantified using model sensitivity experiments Increasing organic matter and soil moisture both cool permafrost in summer and warm it in winter These organic matter and soil moisture feedbacks refine our understanding of permafrost dynamics under climate change

Variations in seasonal soil freeze/thaw state are important indicators of climate change and influence ground temperature, hydrological processes, surface energy, and the moisture balance. Previous studies mainly focused on the active layer and permafrost, while seasonally frozen ground research in nonpermafrost regions has received less attention. In this study, we investigate the response of changes in seasonal soil freeze/thaw state to changes in air temperatures by combining observations from more than 800 stations with gridded mean monthly air temperature data across China. The results show that mean annual air temperature (MAAT) increased statistically significantly by 0.29 ± 0.03°C/decade from 1967 to 2013, with greater warming on the Qinghai‐Tibetan Plateau. There is a statistically significant decrease in the freeze/thaw cycle (FTC) at 0.39 ± 0.05 cycles/decade. In addition, there are strong negative correlations between FTC and MAAT. Estimating the soil freeze/thaw state classification based on the number of days in the month, we find that changes of mean annual area extent of seasonal soil freeze/thaw state decreased significantly for completely frozen (CF) ground, while the area extent of partially frozen (PF) and unfrozen (UF) ground both increased. Changes in mean monthly area extent of seasonal soil freeze/thaw state indicate that the extent of CF and UF area was decreasing and increasing, respectively. But for the extent of PF areas, both increasing and decreasing trends were observed. Quantifying the spatial pattern of the seasonal soil freeze/thaw, we find that CF and PF areas are located in northern China and the Tibetan Plateau from December to March, and UF areas are located in southern China. The variations of mean annual area extent departure of soil freeze/thaw states are consistent with MAAT changes in different land cover types across China. Seasonal soil freeze/thaw state is sensitive to climate changeThe area extent of soil completely frozen state is in decreasing with warming climateSoil freeze/thaw cycle is highly correlated with air temperature

Seasonally frozen ground (SFG) in the Northern Hemisphere (NH) plays a significant role in the earth system via changes in the freeze‐thaw cycle. Previous studies primarily focus on permafrost; however, the SFG response to climate change on a hemispheric scale is uncertain due to a lack of observations. We rectify this with a newly assembled comprehensive database of 1,220 stations with daily observations. To quantify the spatiotemporal characteristics of SFG in response to climate change, we calculate eight variables with these observations: the first date of soil freeze (FFD), freezing duration (FDR), maximum freeze depth (MFD), the date of maximum freeze depth (MFDD), the last date of soil thaw (TLD), thawing duration (TDR), freeze‐thaw duration (FTDR), and actual number of freezing days (AD). During the variables' common 1986–2005 period, MFD decreased 8.9 cm (9% change). FFD was later by 5.3 days (2% change), MFDD and TLD were earlier by 14.5 days (27% change) and 24.7 days (22% change), respectively, and FDR and TDR decreased by 9 days (11% change) and 4.6 days (10% change). FTDR and AD decreased 18.1 days (14% change) and 12.1 days (10% change), respectively. The spatial pattern of freeze‐thaw variables depends on latitude and elevation, and varies by climatic zone: FTDR increases, going from the warm temperate climate, to the arid climate, and the snow and polar climates. The variability in freeze‐thaw changes is mainly driven by air temperature and latitude, while precipitation, soil moisture, snow depth, and elevation are relatively insignificant at the hemispheric scale. Although seasonally frozen ground (SFG) covers 51% of the continental land areas in the Northern Hemisphere (NH), few studies have focused on this cryospheric parameter largely due to a prior lack of data. We remedy this by assembling a comprehensive database of over a thousand stations with daily soil temperature observations. We present a hemispheric‐scale assessment of SFG changes across the NH, to provide a better understanding of the spatial and temporal variability and its potential driving factors. We define eight variables to comprehensively describe all aspects of SFG variability during 1986–2005, and find earlier thawing, later freezing, shallower freeze depths, and an overall shorter freezing period and longer thawed period. These changes are mostly due to a warming climate, with spatial patterns that correspond to latitude and climate regions. These findings can be useful to inform future variability, as climate continues to change. A comprehensive station database with soil temperature observations for the entire Northern Hemisphere is assembledThe temporal and spatial characteristics of seasonally frozen ground in response to climate change are quantifiedSeasonal soil freeze/thaw status is sensitive to climate change and mainly driven by air temperature changes and latitude A comprehensive station database with soil temperature observations for the entire Northern Hemisphere is assembled The temporal and spatial characteristics of seasonally frozen ground in response to climate change are quantified Seasonal soil freeze/thaw status is sensitive to climate change and mainly driven by air temperature changes and latitude

Vegetation is closely tied to climate change, hydrological processes, the carbon cycle, and the energy balance. However, in cold regions, both climate and vegetation changes are also closely coupled to permafrost. The association between amplified warming and greening in Northern Hemisphere permafrost regions is not clearly understood. We therefore produce an extended 1982–2015 normalized difference vegetation index record based on concatenated Global Inventory Modeling and Mapping Studies and Moderate‐Resolution Imaging Spectroradiometer data to quantify the spatiotemporal patterns of vegetation variability. We also establish spatiotemporal permafrost warming patterns based on a thawing index, active layer thickness, soil temperature, the first thaw date, and thawing days. Next, we establish the association between vegetation and warming permafrost. We find that normalized difference vegetation index increases in approximately 70% of Northern Hemisphere permafrost regions during the growing season (April–October) and in 72% of the region during autumn. Warming permafrost shows a positive relationship with greening. A higher thawing index, greater active layer thickness, higher soil temperature, and also increased precipitation are linked with the observed greening. An earlier first thaw date and an increased number of thawing days also correlate with vegetation greening. These findings underscore the sensitivity of vegetation to warming permafrost. Continued permafrost warming will result in further greening of Northern Hemisphere cold regions (and vice versa), with important implications for the permafrost soil‐vegetation‐atmosphere carbon cycles, and the water, nutrient, and energy budgets. A greening trend is evident in Northern Hemisphere permafrost regionsPermafrost warming has a strong association with greening in cold regions