Warming, increase in precipitation, and irrigation enhance greening in High Mountain Asia

Abstract

High-Mountain Asia exhibits one of the highest increases in vegetation greenness on Earth, subsequently influencing the exchange of water and energy between the land surface and the atmosphere. Given the strong interactions between the hydrosphere, the biosphere, and the cryosphere, understanding the drivers of greening in this highly complex region with significant land cover heterogeneity is essential to assess the changes in the regional water budget. Here, we perform a holistic multivariate remote sensing analysis to simultaneously examine the primary components of the terrestrial water cycle from 2003 to 2020 and decipher the principal drivers of greening in High-Mountain Asia. We identified three drivers of greening: (1) precipitation drives greening in mid and low elevation areas covered by evergreen and mixed forests (e.g., Irrawaddy basin), (2) decreases in snow enhance greening in most of the hydrologic basins, and (3) irrigation induces greening in irrigated lands (Ganges–Brahmaputra and Indus).

Irrigation and warming drive the decreases in surface albedo over High Mountain Asia

“…Though HMA experiences greening at high rates 27 , LAI only controls surface albedo changes in forests of Ganges–Brahmaputra, Yangtze, Irrawaddy, Song Hong, and Si (Fig. 3 a).…”

“…Land structure heterogeneity over HMA is tremendous, with elevation ranging from the sea level to the world’s highest point, different climatic conditions (westerlies and monsoons), footprints of human activities, and large variations in land cover types. HMA experiences strong changes in land surface characteristics caused by greening 25 , 26 , decreases in snow 21 , and irrigation 27 . Because greening changes the optical and structural properties of the vegetation canopy and increases the Leaf Area Index (LAI), it affects surface albedos 28 – 30 .…”

Surface albedo decreases from anthropogenic impacts over High Mountain Asia with implications of positive radiative forcing feedbacks

“…Hence the VIS starts to decrease. With the increases in LAI, more incoming solar energy is reflected and scattered by the vegetation canopy and only a small proportion of the incoming radiation reaches the ground 73 . The VIS remains at its lowest value (~0.05) for two consecutive months August and September when the average LAI and soil moisture have their highest values 2.5 and 0.29 respectively.…”

“…Though HMA experiences greening at high rates 73 , LAI only controls surface albedo changes in forests of Ganges-Brahmaputra, Yangtze, Irrawaddy, Song Hong, and Si. Because of its dense canopy and high precipitation (970 to 1200 mm/year) leading to high soil moisture, annual averages of NIR and VIS surface albedos in the Yangzte are low equal to 0.275 and 0.187 respectively (Figure 4b).…”

“…Land structure heterogeneity over HMA is tremendous, with elevation ranging from the sea level to the world's highest point, different climatic conditions (westerlies and monsoons), footprints of human activities, and large variations in land cover types. HMA experiences strong changes of land surface characteristics caused by greening 34,35 , decreases in snow 30 , and irrigation 73 . Because greening changes the optical and structural properties of the vegetation canopy and increases the Leaf Area Index (LAI), it affects the surface albedos 25,36,37 .…”