# Remote Sensing of Environmental Changes in Cold Regions: Methods, Achievements and Challenges
## 3 Changes and Trends
### Tibetan Plateau
[270] analyzed Landsat TM/ETM+ and CBERS images, and found that lakes southeast of the Qiangtang area were expanding from 1975 to 2005. Another study [271] analyzed Landsat images from 1970 to 2010 and found a shrinking lake trend in the southwest TP contrasting with rapid lake expansion in the northeast TP. [PERSON] et al. [272] investigated lake extent fluctuations in the Hindu Kush-Himalaya-Tibetan (HKHT) regions over the past 40 years using Landsat images obtained from the 1970s to 2014. They showed that the TP lake trends are distinct from region to region, with the most intensive lake shrinking observed in northeastern HKHT (HKHT Interior, Tarim, Yellow, Yangtze), while the most extensive expansion was observed in the western and southwestern HKHT (Amu Darya, Ganges Indus, and Brahmaputra), largely caused by the proliferation of small lakes in high-altitude regions from the 1970s to 1995. Lake water levels measured from LIDAR/Radar altimetry can also be used to quantify lake changes. Based on 10-year TOPEX/Poseidon altimetry data, the water level of La'nga lake in the western TP was found to decrease steadily from 1993 to 2001, while that of Ngangzi lake in the eastern TP decreased from 1993 to 1997 and then increased monotonically afterwards [273]. Another study [274] used ICESat altimetry data to analyze water level changes from 2003-2009 for 74 TP lakes and identified an increasing lake level trend (\(\sim\)0.23 m yr\({}^{-1}\)) over 84% of the lakes represented. Consistent with the findings of [275], an average water level increase of 0.20 m yr\({}^{-1}\) was detected from GLAS data over 154 TP lakes for the same period [276]. By integrating multi- altimeter data from Envisat/RA-2, Cryosat-2/Siral, Jason-1/Poseidon-2, and Jason-2/Poseidon-3, [PERSON] et al. [277] found that water levels increased by about 0.275 m yr\({}^{-1}\) for over 82% of 51 TP lakes sampled, while major lake expansion and shrinking were identified over the northern and southern TP, respectively. A general greening trend from the 1980s to 2010s was detected from satellite remote sensing over the TP [278]. Seasonal analysis based on GIMMS (Global Inventory Monitoring and Modeling System) NDVI data further revealed that the largest NDVI increase occurred in autumn over 61% of the TP, while the smallest increase occurred in spring over 41% of the region [279]. Vegetation changes in the northeast, southwest, mid-eastern, and southern TP regions were driven by three different factors, including changes in surface air temperature, water availability, and solar radiation, respectively. Anthropogenic disturbances may offset climate-driven vegetation greening and exacerbate vegetation browning, while ecosystem conservation efforts contributed to vegetation recovery in the TP Three-River Headwaters Region [280]. Based on the NDVI data from 1982 to 2014, an advancing start of growing season, delayed end of season and increasing length of growing season were identified for meadow areas in the eastern TP; while the opposite changes were found for the steppe and sparse herbaceous or sparse shrub areas in the northwest and western edges of the TP. The satellite-observed phenology changes were driven by a number of environmental factors including temperature [281; 282; 283], precipitation [284], sunshine duration [285], and snow cover [286]; and may be partially attributed to aerosol contamination in the satellite observations [287].