| 1. |
- Zou, Linqing, et al.
(author)
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Assessing the upper elevational limits of vegetation growth in global high-mountains
- 2023
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In: Remote Sensing of Environment. - : Elsevier BV. - 0034-4257. ; 286
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Journal article (peer-reviewed)abstract
- The upper elevational limits of vegetation growth in global high-mountains have been the focus for monitoring and assessment of climate change impacts on terrestrial ecosystems. However, existing studies have relied on field surveys that do not allow for large-scale analysis. Although remote sensing data have been used for local and regional monitoring of the vegetation upper boundaries, a global synthesis of the treeline and vegetation line (the upper altitudinal threshold for the existence of trees and the transition line from vegetation to bare land or permanent snow cover, respectively) in high-mountain ecosystems is still missing. To fill this gap, we developed two independent methods based on (1) the relationship between a Sentinel-2 vegetation index and elevation and (2) the European Space Agency's 10 m resolution land cover dataset (WorldCover), respectively, to automatically identify the upper elevational limits of treeline and vegetation line for each one-quarter degree grid across the global high-mountain areas. We obtained highly consistent results from the two methods, both of which are spatially consistent with ground surveyed treeline elevations. Our results are in line with the current understanding of the global distributions of tree and vegetation lines, which are observed at the highest elevations in the Tibetan plateau and decreasing for increasing latitudes. We find that the tree and vegetation lines are aspect-dependent, reaching higher elevations on the equatorial-facing slopes than on the polar-facing slopes in high latitudes, and the opposite in the middle latitudes. Our analysis shows that mountain height is the dominant factor in determining the upper elevational limits of tree and vegetation lines across the globe, while climatic conditions and soil properties also play important roles at regional scales. Our study provides a framework for monitoring the tree and vegetation lines in global high-mountains and provides an important benchmark for further examining their long-term changes in response to climate change.
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| 2. |
- Yao, Tandong, et al.
(author)
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Recent Third Pole’s rapid warming accompanies cryospheric melt and water cycle intensification and interactions between monsoon and environment: multi-disciplinary approach with observation, modeling and analysis
- 2019
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In: Bulletin of The American Meteorological Society. - 0003-0007 .- 1520-0477. ; :March, s. 423-444
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Journal article (peer-reviewed)abstract
- The Third Pole (TP) is experiencing rapid warming and is currently in its warmest period in the past 2,000 years. This paper reviews the latest development in multidisciplinary TP research associated with this warming. The rapid warming facilitates intense and broad glacier melt over most of the TP, although some glaciers in the northwest are advancing. By heating the atmosphere and reducing snow/ice albedo, aerosols also contribute to the glaciers melting. Glacier melt is accompanied by lake expansion and intensification of the water cycle over the TP. Precipitation has increased over the eastern and northwestern TP. Meanwhile, the TP is greening and most regions are experiencing advancing phenological trends, although over the southwest there is a spring phenological delay mainly in response to the recent decline in spring precipitation. Atmospheric and terrestrial thermal and dynamical processes over the TP affect the Asian monsoon at different scales. Recent evidence indicates substantial roles that mesoscale convective systems play in the TP’s precipitation as well as an association between soil moisture anomalies in the TP and the Indian monsoon. Moreover, an increase in geohazard events has been associated with recent environmental changes, some of which have had catastrophic consequences caused by glacial lake outbursts and landslides. Active debris flows are growing in both frequency of occurrences and spatial scale. Meanwhile, new types of disasters, such as the twin ice avalanches in Ali in 2016, are now appearing in the region. Adaptation and mitigation measures should be taken to help societies’ preparation for future environmental challenges. Some key issues for future TP studies are also discussed.
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| 3. |
- Yao, Yunjun, et al.
(author)
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Assessment and simulation of global terrestrial latent heat flux by synthesis of CMIP5 climate models and surface eddy covariance observations
- 2016
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In: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 223, s. 151-167
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Journal article (peer-reviewed)abstract
- The latent heat flux (LE) between the terrestrial biosphere and atmosphere is a major driver of the global hydrological cycle. In this study, we evaluated LE simulations by 45 general circulation models (GCMs) in the Coupled Model Intercomparison Project Phase 5 (CMIP5) by a comparison with eddy covariance (EC) observations from 240 globally distributed sites from 2000 to 2009. In addition, we improved global terrestrial LE estimates for different land cover types by synthesis of seven best CMIP5 models and EC observations based on a Bayesian model averaging (BMA) method. The comparison results showed substantial differences in monthly LE among all GCMs. The model CESM1-CAM5 has the best performance with the highest predictive skill and a Taylor skill score (S) from 0.51-0.75 for different land cover types. The cross-validation results illustrate that the BMA method has improved the accuracy of the CMIP5 GCM's LE simulation with a decrease in the averaged root-mean-square error (RMSE) by more than 3 W/m2 when compared to the simple model averaging (SMA) method and individual GCMs. We found an increasing trend in the BMA-based global terrestrial LE (slope of 0.018 W/m2 yr-1, p <0.05) during the period 1970-2005. This variation may be attributed directly to the inter-annual variations in air temperature (Ta), surface incident solar radiation (Rs) and precipitation (P). However, our study highlights a large difference from previous studies in a continuous increasing trend after 1998, which may be caused by the combined effects of the variations of Rs, Ta, and P on LE for different models on these time scales. This study provides corrected-modeling evidence for an accelerated global water cycle with climate change.
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| 4. |
- Yao, Yunjun, et al.
(author)
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Estimation of high-resolution terrestrial evapotranspiration from Landsat data using a simple Taylor skill fusion method
- 2017
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In: Journal of Hydrology. - : Elsevier BV. - 0022-1694. ; 553, s. 508-526
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Journal article (peer-reviewed)abstract
- Estimation of high-resolution terrestrial evapotranspiration (ET) from Landsat data is important in many climatic, hydrologic, and agricultural applications, as it can help bridging the gap between existing coarse-resolution ET products and point-based field measurements. However, there is large uncertainty among existing ET products from Landsat that limit their application. This study presents a simple Taylor skill fusion (STS) method that merges five Landsat-based ET products and directly measured ET from eddy covariance (EC) to improve the global estimation of terrestrial ET. The STS method uses a weighted average of the individual ET products and weights are determined by their Taylor skill scores (S). The validation with site-scale measurements at 206 EC flux towers showed large differences and uncertainties among the five ET products. The merged ET product exhibited the best performance with a decrease in the averaged root-mean-square error (RMSE) by 2–5 W/m2 when compared to the individual products. To evaluate the reliability of the STS method at the regional scale, the weights of the STS method for these five ET products were determined using EC ground-measurements. An example of regional ET mapping demonstrates that the STS-merged ET can effectively integrate the individual Landsat ET products. Our proposed method provides an improved high-resolution ET product for identifying agricultural crop water consumption and providing a diagnostic assessment for global land surface models.
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| 5. |
- Yuan, Wenping, et al.
(author)
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Differentiating moss from higher plants is critical in studying the carbon cycle of the boreal biome.
- 2014
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 5
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Journal article (peer-reviewed)abstract
- The satellite-derived normalized difference vegetation index (NDVI), which is used for estimating gross primary production (GPP), often includes contributions from both mosses and vascular plants in boreal ecosystems. For the same NDVI, moss can generate only about one-third of the GPP that vascular plants can because of its much lower photosynthetic capacity. Here, based on eddy covariance measurements, we show that the difference in photosynthetic capacity between these two plant functional types has never been explicitly included when estimating regional GPP in the boreal region, resulting in a substantial overestimation. The magnitude of this overestimation could have important implications regarding a change from a current carbon sink to a carbon source in the boreal region. Moss abundance, associated with ecosystem disturbances, needs to be mapped and incorporated into GPP estimates in order to adequately assess the role of the boreal region in the global carbon cycle.
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