# Remote Sensing of Environmental Changes in Cold Regions: Methods, Achievements and Challenges
## 3 Changes and Trends
### Tibetan Plateau  
The Tibetan Plateau (TP), the most extensive highland in the world, has an area of approximately \(2.5\times 106\) km\({}^{2}\) and an average elevation of over 4000 m. The TP also has the Earth's largest storage of ice outside of the north and south polar regions. The climate of the TP is changing rapidly with temperatures warming at a rate of around 0.36 \({}^{\circ}\)C/decade [3], which is twice the mean global trend [4]. Consequently, the impacts of climate change on the TP environment are most pronounced, leading to earlier onset of seasonal thawing, accelerating glacier melting, permafrost degradation, and complex changes in snow, lakes, and vegetation. Glacier melting has been observed over the TP and larger High Mountain Asia region. [PERSON] et al. [257] used ICESat to analyze glacier mass change in the Hindu Kush-Karakoram-Himalaya region during 2003-2008 and found a mass loss rate of \(-12.8\pm 3.5\) G\(\,\)yr\({}^{-1}\) in this region, which is faster than the rate previously estimated using GRACE [258]. For the whole TP region, [PERSON] et al. [259] estimated an overall mass loss rate of \(-15.6\pm 10.1\) G\(\,\)yr\({}^{-1}\) using ICESat observations from 2003 to 2009. For the same period, [PERSON] et al. [260] estimated a total mass change of \(-29\pm 13\) G\(\,\)yr\({}^{-1}\) over High Mountain Asia by integrating GRACE and ICESat observations. Global and localized satellite snow products have been used for studying environmental changes in the TP, including snow impacts on the regional water cycle, ecosystems, and atmospheric circulation [261]. Based on the MODIS snow cover product (MOD10A2) and observations from 37 meteorological stations, a significant trend of earlier onset of snow ablation during the 2001-2015 period was detected over the TP [262]. By analyzing MODIS snow cover data from 2001 to 2011, other research [263] found about 34.14% (5.56%) of the TP area having a declining (significant declining) trend in snow duration, while 24.75% (3.9%) of the region showed increasing snow duration. To further enhance the accuracy of TP snow products, [PERSON] et al. [264] integrated snow cover data from multiple sources to generate a gap-filled daily 5-km Tibetan Plateau snow cover extent record (TPSCE) from 1981-2016. As revealed by the TPSCE, the snow cover fraction increased in the northern interior TP river basins and upper reaches of the Yangtze, Mekong, and Brahmaputra River basins (Figure 7). Figure 7: Climatology and changes of the TP snow cover fraction (%) calculated using MCD10A1-TP (**a**, **b**) and TPSCE (**c**, **d**) from 2001–2014. Black dots in (**b**) and (**d**) indicate the changes are statistically significant at the 95% level. The figure was reproduced with permission from [264]. Beside the global FT products available from satellite microwave observations [160], [PERSON] et al. [265] developed an enhanced resolution (0.05 degree) FT product over the TP by merging MODIS LST with AMSR-E Tb records. [PERSON] et al. [266] analyzed changes in the soil FT cycle over the TP using SSM/I data from 1988-2007. They identified a trend toward earlier onset of soil thaw by approximately 14 days/decade and later onset of soil freeze by approximately 10 days/decade. The observed changes in FT patterns over the TP are also closely related to regional climate warming [265]. There are more than 1000 lakes with an area greater than 1 km\({}^{2}\) on the TP. Generally, the total TP lake area is expanding, from 41,800 km\({}^{2}\) in 2005-2006 [267] to 46,600 km\({}^{2}\) in 2015 [268]. The lake expansion can be directly measured through optical satellite remote sensing. [PERSON] et al. [269] analyzed the trend of lake area changes during 1960-2000 by integrating aerial photos, satellite images (Landsat and CBERS-1 (China-Brazil Earth Resources Satellite)), and topography information. They found that most lakes in the central TP were expanding, while the lakes in the source regions of the Yellow River were shrinking. For lakes in the central TP, [PERSON] et al.