Hydrological Response of Alpine Wetlands to ClimateWarming in the Eastern Tibetan Plateau

Alpine wetlands in the Tibetan Plateau (TP) play a crucial role in the regional hydrological cycle due to their strong influence on surface ecohydrological processes; therefore, understanding how TP wetlands respond to climate change is essential for projecting their future condition and potential vulnerability. We investigated the hydrological responses of a large TP wetland complex to recent climate change, by combining multiple satellite observations and in-situ hydro-meteorological records. We found different responses of runoff production to regional warming trends among three basins with similar climate, topography and vegetation cover but different wetland proportions. The basin with larger wetland proportion (40.1%) had a lower mean runoff coefficient (0.173 ± 0.006), and also showed increasingly lower runoff level (−3.9% year−1, p = 0.002) than the two adjacent basins. The satellite-based observations showed an increasing trend of annual non-frozen period, especially in the wetland-dominated region (2.64 day·year−1, p \u3c 0.10), and a strong extension of vegetation growing-season (0.26–0.41 day·year−1, p \u3c 0.10). Relatively strong increasing trends in evapotranspiration (ET) (~1.00 mm·year−1, p \u3c 0.01) and the vertical temperature gradient above ground surface (0.043 °C·year−1, p \u3c 0.05) in wetland-dominant areas were documented from satellite-based ET observations and weather station records. These results indicate recent surface drying and runoff reduction of alpine wetlands, and their potential vulnerability to degradation with continued climate warming

Characteristics of water and heat change during the freezing-thawing process at an alpine steppe in seasonally frozen ground of the Northern Tibetan plateau

Introduction: Permafrost and seasonally frozen soil are widely distributed on the Qinghai–Tibetan Plateau, and the freezing–thawing cycle can lead to frequent phase changes in soil water, which can have important impacts on ecosystems.Methods: To understand the process of soil freezing-thawing and to lay the foundation for grassland ecosystems to cope with complex climate change, this study analyzed and investigated the hydrothermal data of Xainza Station on the Northern Tibet from November 2019 to October 2021.Results and Discussion: The results showed that the fluctuation of soil temperature showed a cyclical variation similar to a sine (cosine) curve; the deep soil temperature change was not as drastic as that of the shallow soil, and the shallow soil had the largest monthly mean temperature in September and the smallest monthly mean temperature in January. The soil water content curve was U-shaped; with increased soil depth, the maximum and minimum values of soil water content had a certain lag compared to that of the shallow soil. The daily freezing-thawing of the soil lasted 179 and 198 days and the freezing-thawing process can be roughly divided into the initial freezing period (November), the stable freezing period (December–early February), the early ablation period (mid-February to March), and the later ablation period (March–end of April), except for the latter period when the average temperature of the soil increased with the increase in depth. The trend of water content change with depth at all stages of freezing-thawing was consistent, and negative soil temperature was one of the key factors affecting soil moisture. This study is important for further understanding of hydrothermal coupling and the mechanism of the soil freezing-thawing process

Policy implications of warming permafrost

Permafrost is perennially frozen ground occurring in about 24% of the exposed land surface in the Northern Hemisphere. The distribution of permafrost is controlled by air temperature and, to a lesser extent, by snow depth, vegetation, orientation to the sun and soil properties. Any location with annual average air temperatures below freezing can potentially form permafrost. Snow is an effective insulator and modulates the effect of air temperature, resulting in permafrost temperatures up to 6°C higher than the local mean annual air temperature. Most of the current permafrost formed during or since the last ice age and can extend down to depths of more than 700 meters in parts of northern Siberia and Canada. Permafrost includes the contents of the ground before it was frozen, such as bedrock, gravel, silt and organic material. Permafrost often contains large lenses, layers and wedges of pure ice that grow over many years as a result of annual freezing and thawing of the surface soil laye

Surface energy fluxes in complex terrain

The emphasis of the 1985 NASA project activity was on field measurements of wind data and heat balance data. Initiatives included a 19 station mountaintop monitoring program, testing and refining the surface flux monitoring systems and packing and shipping equipment to the People's Republic of China in preparation for the 1986 Tibet Experiment. Other work included more extensive analyses of the 1984 Gobi Desert and Rocky Mountain observations plus some preliminary analyses of the 1985 mountaintop network data. Details of our field efforts are summarized and results of our data analyses are presented

Deep learning estimation of northern hemisphere soil freeze-thaw dynamics using satellite multi-frequency microwave brightness temperature observations

Satellite microwave sensors are well suited for monitoring landscape freeze-thaw (FT) transitions owing to the strong brightness temperature (TB) or backscatter response to changes in liquid water abundance between predominantly frozen and thawed conditions. The FT retrieval is also a sensitive climate indicator with strong biophysical importance. However, retrieval algorithms can have difficulty distinguishing the FT status of soils from that of overlying features such as snow and vegetation, while variable land conditions can also degrade performance. Here, we applied a deep learning model using a multilayer convolutional neural network driven by AMSR2 and SMAP TB records, and trained on surface (~0–5 cm depth) soil temperature FT observations. Soil FT states were classified for the local morning (6 a.m.) and evening (6 p.m.) conditions corresponding to SMAP descending and ascending orbital overpasses, mapped to a 9 km polar grid spanning a five-year (2016–2020) record and Northern Hemisphere domain. Continuous variable estimates of the probability of frozen or thawed conditions were derived using a model cost function optimized against FT observational training data. Model results derived using combined multi-frequency (1.4, 18.7, 36.5 GHz) TBs produced the highest soil FT accuracy over other models derived using only single sensor or single frequency TB inputs. Moreover, SMAP L-band (1.4 GHz) TBs provided enhanced soil FT information and performance gain over model results derived using only AMSR2 TB inputs. The resulting soil FT classification showed favorable and consistent performance against soil FT observations from ERA5 reanalysis (mean percent accuracy, MPA: 92.7%) and in situ weather stations (MPA: 91.0%). The soil FT accuracy was generally consistent between morning and afternoon predictions and across different land covers and seasons. The model also showed better FT accuracy than ERA5 against regional weather station measurements (91.0% vs. 86.1% MPA). However, model confidence was lower in complex terrain where FT spatial heterogeneity was likely beneath the effective model grain size. Our results provide a high level of precision in mapping soil FT dynamics to improve understanding of complex seasonal transitions and their influence on ecological processes and climate feedbacks, with the potential to inform Earth system model predictions

Response of seasonal soil freeze depth to climate change across China

Abstract. The response of seasonal soil freeze depth to climate change has repercussions for the surface energy and water balance, ecosystems, the carbon cycle, and soil nutrient exchange. Despite its importance, the response of soil freeze depth to climate change is largely unknown. This study employs the Stefan solution and observations from 845 meteorological stations to investigate the response of variations in soil freeze depth to climate change across China. Observations include daily air temperatures, daily soil temperatures at various depths, mean monthly gridded air temperatures, and the normalized difference vegetation index. Results show that soil freeze depth decreased significantly at a rate of −0.18 ± 0.03 cm yr−1, resulting in a net decrease of 8.05 ± 1.5 cm over 1967–2012 across China. On the regional scale, soil freeze depth decreases varied between 0.0 and 0.4 cm yr−1 in most parts of China during 1950–2009. By investigating potential climatic and environmental driving factors of soil freeze depth variability, we find that mean annual air temperature and ground surface temperature, air thawing index, ground surface thawing index, and vegetation growth are all negatively associated with soil freeze depth. Changes in snow depth are not correlated with soil freeze depth. Air and ground surface freezing indices are positively correlated with soil freeze depth. Comparing these potential driving factors of soil freeze depth, we find that freezing index and vegetation growth are more strongly correlated with soil freeze depth, while snow depth is not significant. We conclude that air temperature increases are responsible for the decrease in seasonal freeze depth. These results are important for understanding the soil freeze–thaw dynamics and the impacts of soil freeze depth on ecosystem and hydrological process. </jats:p

Escalating Carbon Export from High-Elevation Rivers in a Warming Climate

High-elevation mountains have experienced disproportionately rapid warming, yet the effect of warming on the lateral export of terrestrial carbon to rivers remains poorly explored and understood in these regions. Here, we present a long-term data set of dissolved inorganic carbon (DIC) and a more detailed, short-term data set of DIC, delta C-13(DIC), and organic carbon from two major rivers of the Qinghai-Tibetan Plateau, the Jinsha River (JSR) and the Yalong River (YLR). In the higher-elevation JSR with similar to 51% continuous permafrost coverage, warming (>3 degrees C) and increasing precipitation coincided with substantially increased DIC concentrations by 35% and fluxes by 110%. In the lower-elevation YLR with similar to 14% continuous permafrost, such increases did not occur despite a comparable extent of warming. Riverine concentrations of dissolved and particulate organic carbon increased with discharge (mobilization) in both rivers. In the JSR, DIC concentrations transitioned from dilution (decreasing concentration with discharge) in earlier, colder years to chemostasis (relatively constant concentration) in later, warmer years. This changing pattern, together with lighter delta C-13(DIC) under high discharge, suggests that permafrost thawing boosts DIC production and export via enhancing soil respiration and weathering. These findings reveal the predominant role of warming in altering carbon lateral export by escalating concentrations and fluxes and modifying export patterns

Interdecadal Changes in the Freeze Depth and Period of Frozen Soil on the Three Rivers Source Region in China from 1960 to 2014

On the basis of observed soil freeze depth data from 14 meteorological stations on the Three Rivers Source Region (TRSR) in China during 1960 to 2014, trends in the freeze depth, first date, last date, and duration of frozen soil were analyzed, together with other meteorological variables, such as air temperature, snow depth, and precipitation, observed at the same locations. The results showed the following. (1) A continuous, accelerated decreasing trend in freeze depth appeared in the TRSR during the 1985–2014 and 2000–2014 periods, compared with that during the 1960–2014 period. (2) The freeze first date had been delayed and the freeze last date had been advanced significantly. The advanced trends in freeze last date were more significant than the delayed trends in freeze first date. The freeze duration also experienced an accelerated decrease. (3) The freeze depth and period were strongly affected by air temperature, thawing index, and soil moisture (precipitation), but not by snow. The freeze depth, freeze first date, freeze last date, and duration also influenced each other. (4) These decreasing trends in freeze depth and duration are expected to continue given the increasing trends in air temperature and precipitation in this region

Sensitivity of soil organic carbon to the change in climate on the Tibetan Plateau

Soil organic carbon, as the main terrestrial component in the Earth’s carbon cycle, has a profound effect on the accumulation of CO2 in atmosphere and consequently on global warming. In the alpine grasslands of the Tibetan Plateau, the decompo- sition rate of soil organic carbon is controlled by several biotic and abiotic factors, which mostly change simultaneously and often leads to freezing and thawing cycles. However, it is highly uncertain whether the temperature sensitivity of decomposition around the freezing point of water is similar as in higher temperature ranges. The main objective of this dissertation is to evaluate the effects of simultaneous changes in three climate factors on soil organic carbon decomposition rates using an incubation experiment and a biogeochemical model. Due to the large divergence between empirical and model-based approaches in predicting the effects of abiotic factors on soil carbon dynamics, this dissertation first provides an approach to un- cover some sources of uncertainty in estimated SOC processes in alpine grasslands. In this study, I evaluated the complexity of the model required to represent the dynamics of carbon observed in incubation studies. Information theory metrics including AIC and BIC, as well as carbon particle mean transit time, were used as criteria to select models that better predict the data without making additional assumptions about model structure. These analyses showed that during the limited course of an incubation experiment, the amount of transfer between the different SOC pools is negligible and adding these parameters to the model could lead to over-parameterization. These findings suggest that carbon models with less parameterized structures, such as the one-pool model or the two-pool model with parallel structure, that does not account for transfers between pools, indeed have better predictive power in describing the decomposition of carbon fractions while following the principle of parsimony. The aforementioned information was later used to evaluate the sensitivity of SOC degradation rates to changes in soil temperature, soil moisture, and oxygen availabil- ity, especially at low temperatures. Functions from the Dual Arrhenius-Michaelis- Menten model (DAMM) were implemented in a one-pool model of SOC represented as first-order differential equation with time-dependent coefficients. A manipulative freeze-thaw cycle was imposed on a soil from Tibetan grasslands, in addition to soil moisture treatments that ranged from extremely dry to fully saturated, under both oxic and anoxic conditions. The intrinsic sensitivities indicated that temperature (energy) is the main factor limiting decomposition in cold environments, provided moisture and oxygen are sufficiently available. However, the intrinsic sensitivities related to soil moisture and oxygen concentration are only relevant in very narrow ranges when soils are nearly dry or partially anoxic, and small changes within these narrow ranges can result in large changes in decomposition rates