Date Published: September 8, 2017
Publisher: Public Library of Science
Author(s): Li Tian, Jiquan Chen, Yangjian Zhang, Ben Bond-Lamberty.
The Tibetan Plateau has experienced higher-than-global-average climate warming in recent decades, resulting in many significant changes in ecosystem structure and function. Among them is albedo, which bridges the causes and consequences of land surface processes and climate. The plateau is covered by snow/ice and vegetation in the non-growing season (nGS) and growing season (GS), respectively. Based on the MODIS products, we investigated snow/ice cover and vegetation greenness in relation to the spatiotemporal changes of albedo on the Tibetan Plateau from 2000 through 2013. A synchronous relationship was found between the change in GSNDVI and GSalbedo over time and across the Tibetan landscapes. We found that the annual average albedo had a decreasing trend, but that the albedo had slightly increased during the nGS and decreased during the GS. Across the landscapes, the nGSalbedo fluctuated in a synchronous pattern with snow/ice cover. Temporally, monthly snow/ice coverage also had a high correspondence with albedo, except in April and October. We detected clear dependencies of albedo on elevation. With the rise in altitude, the nGSalbedo decreased below 4000 m, but increased for elevations of 4500–5500 m. Above 5500 m, the nGSalbedo decreased, which was in accordance with the decreased amount of snow/ice coverage and the increased soil moisture on the plateau. More importantly, the decreasing albedo in the most recent decade appeared to be caused primarily by lowered growing season albedo.
The magnitude of global warming in recent decades on the Tibetan Plateau—the largest plateau in the world—has surpassed the average of the northern hemisphere. The warming is causing a series of changes in terrestrial surface properties, which in turn feed back to regional and global climate . Among the various land surface properties, albedo is one of the most critical variables because it bridges land surface processes (e.g., land use, vegetation dynamics) and climate [2–4]. Within the climate system, albedo determines surface radiation balance and affects surface temperature. For the ecological systems, albedo affects energy balance (including evapotranspiration) through regulating the microclimatic conditions of plant canopies and their absorption of solar radiation. Consequently, albedo has been substantially studied for its role in global change science (e.g., [5–7]), including its role in the development of climate change scenarios. The magnitude and change in albedo are tightly related to land cover type and other surface properties such as snow/ice cover , vegetation [4, 9–10], landform , and soil moisture . Globally, albedo is reportedly decreasing in the Arctic Circle due to shrunken snow/ice cover , in boreal biomes , and in other regions. At high altitudes in mountainous areas, dense vegetation appears to reduce radiative forcing [4,8,15–16].
Surface albedo is a critical variable in understanding the changes and regulations of energy balance in local and regional ecosystems, which alter climate at corresponding scales [3, 19, 46]. The magnitude of solar irradiance varies greatly between GS and nGS, and with elevation [2, 39, 40]. With an average regional base elevation that is greater than 4000 m and rapidly rising temperatures on the Tibetan Plateau, the lessons learned from this study will have profound implications in assessing and modeling the changes in ecosystem structure and function as a consequence of global climate change and local land use.
Annual land surface albedo on the Tibetan Plateau has been decreasing since 2000. This decrease appeared to be caused mainly by lowered growing season albedo. A synchronous relationship was found for the change in the GSNDVI and GSalbedo over time, as well as across the Tibetan landscape gradient. In the nGS, minimum albedo occurred in December—the coldest month of the year—which has the lowest average snow/ice cover. At high elevations (≥6000 m), WnGSalbedo, non-GSSC, and soil moisture decreased during the study period. This study highlights not only the importance of monitoring ongoing NDVI and the effect of snow/ice on albedo closely, but more importantly, the change caused by vegetation on the Tibetan Plateau as well.