Donglin Guo

A projection of permafrost degradation on the Tibetan Plateau during the 21st century

[1]xa0The current distribution and future change of permafrost on the Tibetan Plateau were examined using the Community Land Model version 4 (CLM4) with explicit treatment of frozen soil processes. When forced off-line with archived high-resolution data from The Abdus Salam International Centre for Theoretical Physics Regional Climate Model version 3 nested within the Model for Interdisciplinary Research on Climate 3.2 HiRes, the CLM4 produced a near-surface permafrost area of 122.2 × 104 km2 for the Tibetan Plateau. This area compares reasonably with area estimates of 126.7 × 104 km2 for the Plateau frozen soil map. In response to the simulated strong Plateau warming (approximately 0.58°C per decade over the Tibetan Plateau for the period from 1980 to 2100 under the A1B greenhouse gas emissions scenario), the near-surface permafrost area is projected to decrease by approximately 39% by the mid-21st century and by approximately 81% by the end of the 21st century. The near-surface permafrost area exhibits a significant decreasing linear trend, with a rate of decrease of 9.9 × 104 km2 per decade. The simulated deep permafrost area remains longer than the near-surface permafrost for the same period. The active layer thickness of 0.5–1.5 m found in the present-day increases to approximately 1.5–2.0 m by the period of 2030–2050. This increase will continue and reach a level of 2.0–3.5 m by the period of 2080–2100. Surface runoff decreases but subsurface runoff increases, both relative to the difference between precipitation and evapotranspiration. This is related to the fact that the decrease in ground ice content, as caused by permafrost degradation, facilitates the percolation of more water to deeper soil layers, thus resulting in the reallocation of runoff. These results provide useful references for evaluating the level of permafrost degradation in response to climate warming on the Tibetan Plateau.

CMIP5 permafrost degradation projection:A comparison among different regions

The considerable impact of permafrost degradation on hydrology and water resources, ecosystems, human engineering facilities, and climate change requires us to carry out more in-depth studies, at finer spatial scales, to investigate the issue. In this study, regional differences of the future permafrost changes are explored with respect to the regions (high altitude and high latitude, and in four countries) based on the surface frost index (SFI) model and multimodel and multiscenario data from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Results show the following: (1) Compared with seven other sets of driving data, Climatic Research Unit air temperature combined with Climate Forecast System Reanalysis snow data (CRU_CFSR) yield a permafrost extent with the least absolute area bias and was thus used in the simulation. The SFI model, driven by CRU_CFSR data climatology plus multimodel mean anomalies, produces a present-day (1986–2005) permafrost area of 15.45u2009×u2009106u2009km2u2009decade−1, which compares reasonably with observations of 15.24u2009×u2009106u2009km2u2009decade−1. (2) The high-altitude (Tibetan Plateau) permafrost area shows a larger decreasing percentage trend than the high-latitude permafrost area. This indicates that, in terms of speed, high-altitude permafrost thaw is faster than high-latitude permafrost, mainly due to the larger percentage sensitivity to rising air temperature of the high-altitude permafrost compared to the high-latitude permafrost, which is likely related to their thermal conditions. (3) Permafrost in China shows the fastest thaw, which is reflected by the percentage trend in permafrost area, followed by the United States, Russia, and Canada. These discrepancies are mainly linked to different percentage sensitivities of permafrost areas in these four countries to air temperature change. (4) In terms of the ensemble mean, permafrost areas in all regions are projected to decrease by the period 2080–2099. Under representative concentration pathway (RCP)4.5, permafrost retreats toward the Arctic, and the thaw in every region mainly occurs at the southern edge of the permafrost area. Under RCP8.5, almost no permafrost is expected to remain in China, the United States, and the Tibetan Plateau. Permafrost in Russia will remain mainly in the western part of the east Siberian Mountains, and permafrost in Canada will retreat to the north of 65°N. Possible uncertainties in this study are primarily attributed to the climate models coarse horizontal resolution. The results of the present study will be useful for understanding future permafrost degradation from the regional perspective.

SNOW COVER AND DEPTH OF FREEZE-THAW ON THE TIBETAN PLATEAU: A CASE STUDY FROM 1997 TO 1998

An increase in snow cover on the Tibetan Plateau has been observed over the past 50 years. The frequency of snow disasters on the Plateau has also increased. Much of the Qinghai-Xizang (Tibetan) Plateau is underlain by permafrost. Some observation sites of the GEWEX Asian Monsoon Experiment (GAME)-Tibet are located in the Naqu-Anduo area, and have been operating since 1997. A data set obtained from the GAME-Tibet measurement network was used to examine variations in the active layer, with emphasis on the heavy snow cover in 1997/1998. Analysis in this study is focused on air and soil temperature, soil moisture, freeze-thaw depths, and snow depth in years of normal (1998/1999) and abnormally heavy (1997/1998) snow cover. Results indicate that anomalously deep snow cover influenced the pattern of soil freeze-thaw depth through its control over ground temperature. Although air temperature was lower than normal during this period, ground temperatures were appreciably warmer under the heavy snow cover, which retarded frost penetration at depth. Snow cover can also contribute to the cooling process through its high albedo. Because the anomalously heavy snow load lasted well beyond the normal date of meltout, soil thaw developed later than usual during the summer of 1998. Interannual variations in snow depth have significant implications for the maintenance of Tibetan permafrost.

Will the Tibetan Plateau warming depend on elevation in the future

Elevation-dependent warming, greater warming at higher elevations, tends to accelerate the ablation of solid water reserves on the Tibetan Plateau and is thus expected to affect the sustainable water supply of the plateau. In the context of a global climate that is predicted to continue to warm, whether elevation-dependent warming exists on the Tibetan Plateau in the future and, if so, what its characteristics and mechanisms are, are important issues that have not yet been fully assessed. Using six sets of high-resolution outputs from dynamical downscaling simulations based on regional climate models, we investigated the future situation regarding the elevation dependency of climate warming on the Tibetan Plateau. The simulated air temperature trends from the six simulations are validated using meteorological station observations. The trends from only two simulations are selected for analysis due to their statistically significant correlation with the observations. The warming rate first increases to a peak and then slightly declines along with elevation increasing from 2000m to 5600m. The peak of the warming rate is reached at variable elevations (4400–5200m), which depends on the intensity of the warming. The elevation at which this peak occurs increases when the warming intensifies. Such elevation-dependent warming is mostly caused by the decrease in upward short-wave radiation due to the depletion of snow based on surface energy budget analysis. These results provide some understanding of the future elevation-dependent warming on the Tibetan Plateau, which will be useful for evaluating the sustainability of water resources of the Tibetan Plateau water-affected area.

Comparison of a very-fine-resolution GCM with RCM dynamical downscaling in simulating climate in China

Regional climate simulation can generally be improved by using an RCM nested within a coarser-resolution GCM. However, whether or not it can also be improved by the direct use of a state-of-the-art GCM with very fine resolution, close to that of an RCM, and, if so, which is the better approach, are open questions. These questions are important for understanding and using these two kinds of simulation approaches, but have not yet been investigated. Accordingly, the present reported work compared simulation results over China from a very-fine-resolution GCM (VFRGCM) and from RCM dynamical downscaling. The results showed that: (1) The VFRGCM reproduces the climatologies and trends of both air temperature and precipitation, as well as inter-monthly variations of air temperature in terms of spatial pattern and amount, closer to observations than the coarse-resolution version of the GCM. This is not the case, however, for the inter-monthly variations of precipitation. (2) The VFRGCM captures the climatology, trend, and inter-monthly variation of air temperature, as well as the trend in precipitation, more reasonably than the RCM dynamical downscaling method. (3) The RCM dynamical downscaling method performs better than the VFRGCM in terms of the climatology and inter-monthly variation of precipitation. Overall, the results suggest that VFRGCMs possess great potential with regard to their application in climate simulation in the future, and the RCM dynamical downscaling method is still dominant in terms of regional precipitation simulation.

Permafrost Thaw and Associated Settlement Hazard Onset Timing over the Qinghai-Tibet Engineering Corridor

In permafrost areas, the timing of thermal surface settlement hazard onset is of great importance for the construction and maintenance of engineering facilities. Future permafrost thaw and the associated thermal settlement hazard onset timing in the Qinghai-Tibet engineering corridor (QTEC) were analyzed using high-resolution soil temperature data from the Community Land Model version 4 in combination with multiple model and scenario soil temperature data from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Compared to the standard frozen ground map for the Tibetan Plateau and ERA-Interim data, a multimodel ensemble reproduces the extent of permafrost and soil temperature change in the QTEC at a 1xa0m depth from 1986–2005. Soil temperature and active layer thickness increase markedly during 2006–2099 using CMIP5 scenarios. By 2099, the ensemble mean soil temperature at 15xa0m depth will increase between 1.0 and 3.6xa0°C in the QTEC. Using crushed-rock revetments can delay the onset of thermal settlement hazard for colder permafrost areas by approximately 17xa0years in the worst case scenario of RCP8.5. Nearly one-third of the area of the QTEC exhibits settlement hazard as early as 2050, and half of this one-third of the area is traversed by the Qinghai-Tibet highway/railway, a situation that requires more planning and remedial attention. Simulated onsets of thermal settlement hazard correspond well to the observed soil temperature at 15xa0m depth for seven grid areas in the QETC, which to some extent indicates that these timing estimates are reasonable. This study suggests that climate model-based timing estimation of thermal settlement hazard onset is a valuable method, and that the results are worthy of consideration in engineering design and evaluation.

Permafrost degradation and associated ground settlement estimation under 2 °C global warming

Global warming of 2u2009°C above preindustrial levels has been considered to be the threshold that should not be exceeded by the global mean temperature to avoid dangerous interference with the climate system. However, this global mean target has different implications for different regions owing to the globally nonuniform climate change characteristics. Permafrost is sensitive to climate change; moreover, it is widely distributed in high-latitude and high-altitude regions where the greatest warming is predicted. Permafrost is expected to be severely affected by even the 2u2009°C global warming, which, in turn, affects other systems such as water resources, ecosystems, and infrastructures. Using air and soil temperature data from ten coupled model intercomparison project phase five models combined with observations of frozen ground, we investigated the permafrost thaw and associated ground settlement under 2u2009°C global warming. Results show that the climate models produced an ensemble mean permafrost area of 14.01u2009×u2009106xa0km2, which compares reasonably with the area of 13.89u2009×u2009106xa0km2 (north of 45°N) in the observations. The models predict that the soil temperature at 6xa0m depth will increase by 2.34–2.67u2009°C on area average relative to 1990–2000, and the increase intensifies with increasing latitude. The active layer thickness will also increase by 0.42–0.45xa0m, but dissimilar to soil temperature, the increase weakens with increasing latitude due to the distinctly cooler permafrost at higher latitudes. The permafrost extent will obviously retreat north and decrease by 24–26% and the ground settlement owing to permafrost thaw is estimated at 3.8–15xa0cm on area average. Possible uncertainties in this study may be mostly attributed to the less accurate ground ice content data and coarse horizontal resolution of the models.

Estimates of Global Surface Hydrology and Heat Fluxes from the Community Land Model (CLM4.5) with Four Atmospheric Forcing Datasets

AbstractGlobal land surface hydrology and heat fluxes can be estimated by running a land surface model (LSM) driven by the atmospheric forcing dataset. Previous multimodel studies focused on the impact of different LSMs on model results. Here the sensitivity of the Community Land Model, version 4.5 (CLM4.5), results to the atmospheric forcing dataset is documented. Together with the model default global forcing dataset (CRU–NCEP, hereafter CRUNCEP), three newly developed, reanalysis-based, near-surface meteorological datasets (i.e., MERRA, CFSR, and ERA-Interim) with the precipitation adjusted by the Global Precipitation Climatology Project monthly product were used to drive CLM4.5. All four simulations were run at 0.5° × 0.5° grids from 1979 to 2009 with the identical initialization. The simulated monthly surface hydrology variables, fluxes, and the forcing datasets were then evaluated against various observation-based datasets (soil moisture, runoff, snow depth and water equivalent, and flux tower measu...

Simulated Historical (1901–2010) Changes in the Permafrost Extent and Active Layer Thickness in the Northern Hemisphere

A growing body of simulation research has considered the dynamics of permafrost, which has an important role in the climate system of a warming world. Previous studies have concentrated on the future degradation of permafrost based on global climate models (GCMs) or data from GCMs. An accurate estimation of historical changes in permafrost is required to understand the relations between changes in permafrost and the Earths climate and to validate the results from GCMs. Using the Community Land Model 4.5 driven by the CRUNCEP atmospheric dataset and observations of changes in soil temperature and active layer thickness and present day areal extent of permafrost, this study investigated the changes in permafrost in the Northern Hemisphere from 1901 to 2010. The results showed that the model can reproduce the inter-annual variations in the observed soil temperature and active layer thickness. The simulated area of present day permafrost fits well with observations, with a bias of 2.02 × 106 km2. The area of permafrost decreased by 0.06 (0.62) × 106 km2 decade−1 from 1901 to 2009 (1979 to 2009). A clear decrease in the area of permafrost was found in response to increases in air temperatures during the period from about the 1930s to the 1940s, indicating that permafrost is sensitive to even a temporary increase in temperature. From a regional perspective, high-elevation permafrost decreases at a faster rate than high-latitude permafrost; permafrost in China shows the fastest rate of decrease, followed by Alaska, Russia, and Canada. Discrepancies in the rate of decrease in the extent of permafrost among different regions were linked to the sensitivity of permafrost in the regions to increases in air temperatures rather than to the amplitude of the increase in air temperatures. An increase in the active layer thickness of 0.009 (0.071) m decade−1 was shown during the period 1901–2009 (1979–2009). These results are useful in understanding the response of permafrost to a historical warming climate and for validating the results from GCMs.

Evaluation of CORDEX regional climate models in simulating temperature and precipitation over the Tibetan Plateau

Abstract Using a regional climate model (RCM) is generally regarded as a promising approach in researching the climate of the Tibetan Plateau, due to the advantages provided by the high resolutions of these models. Whilst previous studies have focused mostly on individual RCM simulations, here, multiple RCMs from the Coordinated Regional Climate Downscaling Experiment are evaluated in simulating surface air temperature and precipitation changes over the Tibetan Plateau using station and gridded observations. The results show the following: (1) All RCMs consistently show similar spatial patterns, but a mean cold (wet) bias in the temperature (precipitation) climatology compared to station observations. The RCMs fail to reproduce the observed spatial patterns of temperature and precipitation trends, and on average produce greater trends in temperature and smaller trends in precipitation than observed results. The multi-model ensemble overall produces superior trends in both simulated temperature and precipitation relative to individual models. Meanwhile, RegCM4 presents the most reasonable simulated trends among the five RCMs. (2) Considerable dissimilarities are shown in the simulated quantitative results from the different RCMs, which indicates a large model dependency in the simulation of climate over the Tibetan Plateau. This implies that caution may be needed when an individual RCM is used to estimate the amplitude of climate change over the Tibetan Plateau. (3) The temperature (precipitation) in 2016–35, relative to 1986–2005, is projected by the multi-model ensemble to increase by 1.38 ± 0.09 °C (0.8% ± 4.0%) and 1.77 ± 0.28 °C (7.3% ± 2.5%) under the RCP4.5 and RCP8.5 scenario, respectively. The results of this study advance our understanding of the applicability of RCMs in studies of climate change over the Tibetan Plateau from a multiple-RCM perspective.