Glaciers and water nexus, glaciers contribution to global freshwater.

 Glacial and snowmelt provide essential water supplies to Rivers and basins at seasonal and annual timescales, both within and downstream from mountains (Adler et al., 2022; Hock et al., 2019). Changes in water flows pose a growing risk to the water, food and livelihood security of billions of people (UNESCO, 2025). River basins with a large glacier and cryosphere (snow) component that are at high risk include the Amu Darya, Columbia, Fraser, Ganges-Brahmaputra, Indus, Po, Rhine, Rio Negro, Syr, Darya and Tarim. The impacts will be particularly felt in regions that strongly depend on glacier and snowmelt with pronounced dry seasons, including Central Asia, South Asia, tropical and subtropical western South America and southwestern North America (Hock et al., 2019; Immerzeel et al., 2020).

 Key rivers that are heavily influenced by water sources from mountains (90 percent of the mean annual flow) include the Amu Darya, Colorado, Nile, Orange and Rio Negro. Rivers that depend on mountain waters for more than 70 percent of their flow include the Euphrates, Indus, São Francisco, Senegal and Tigris. Major cities that are critically dependent on mountain waters include Addis Ababa, Barcelona, Bogotá, Jakarta, Kathmandu, La Paz, Lima, Los Angeles, Melbourne, Mexico City, New Delhi, New York, Quito, Rio de Janeiro and Tokyo (Kohler et al., 2015; UNESCO, 2025). Hotspot regions with large lowland populations highly dependent upon mountain waters include the Amu and Syr Darya, Brahmaputra, Colorado, Euphrates-Tigris, Ganges, Indus, Mekong, Niger, Nile, Pearl, Rhine, Yangtze and Yellow river basins (Immerzeel et al., 2020).

Glacial melt provides an important source of water for seasonal bufferingwithin mountains and downstream. During the dry season, meltwater from glaciers plays a vital role in supplying irrigated agriculture and drinking water. Glacial melt can act as a drought buffer, enhancing mountain and downstream resilience during periods of water scarcity (Adler et al., 2022).Snowmelt accounts for most cryospheric contributions to streamflow in most river basins with a cryosphere component and is often substantially greater than glacier melt. The contributions of glacier melt to available water supply vary in importance. Snow cover has decreased in nearly all mountain regions, especially in spring and summer, with an expected further decrease in the coming decades. The magnitude and timing of snowmelt have already changed considerably, with trends in snow water equivalent being predominately negative around the world in the past few decades (Adler et al., 2022; UNESCO, 2025)