A8 Changes in the Hydrological Cycle

Topic

Glaciers and mountain snowpacks act as freshwater reservoirs: they accumulate solid precipitation during winter and spring and release that water as meltwater during the summer, thereby regulating the flow of rivers originating in arid and semi-arid regions that lack other equivalent water sources. Climate change profoundly alters the hydrological cycle sustained by these glaciers and snowpacks, modifying the timing of water delivery throughout the year, the total volume of water delivered, and the predictability of glacial water flow from one season to the next.

The central concept of this glacial hydrological cycle is "peak water": under progressive warming, the total meltwater discharge from a glacier initially increases—because more ice melts than the glacier accumulates during winter—until that discharge reaches a temporary maximum, known as peak water. Once peak water is passed, meltwater discharge begins to decline because the glacier's volume has shrunk to the point where insufficient ice remains to sustain previous melt rates; this drop evolves into a permanent decline in summer streamflow, affecting the water supply for millions of people in regions such as the Andes, the Hindu Kush-Himalayas, and the Alps.

The physics underlying peak water involves three processes: the glacier's mass balance, which compares winter snow accumulation with summer ice melt; the path meltwater takes through the subglacial channel system before entering the river; and the hydrology of the entire river basin. Within this mass balance, the temperature response to altitude determines the fraction of precipitation falling as snow versus rain: as warming progresses, the snowline rises, and this higher snowline reduces the proportion of precipitation falling as solid snow—thereby diminishing the ice accumulation available to sustain meltwater flow in subsequent summers.

Current hydrological projections anticipate that peak water will occur before 2050 in most of the world's mountain regions; consequently, the very glaciers and snowpacks that currently regulate river flow in the aforementioned arid and semi-arid regions will, in the coming decades, shift from a transient peak-water phase to a subsequent permanent decline, reducing the freshwater supply available to millions of people.

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