Snow, glaciers and ice

Roughly 69% of all fresh water on Earth is frozen. Almost all of it is in the Antarctic and Greenland ice sheets, which are effectively inert on human timescales. The mountain snow and ice that is a small fraction of that total is what actually supplies people.

Snow water equivalent is the number that matters

The natural reservoir

Snowpack stores winter precipitation and releases it in spring and early summer. Glaciers do the same over years to centuries. In both cases the water arrives in the dry season — which is exactly when rainfall is lowest and demand highest.

That timing is the whole value. California’s Sierra Nevada snowpack, the Andes above Lima, the Himalaya above the Indus and Ganges plains, and the Alps above the Rhine and Po all function as free seasonal reservoirs. Replacing that storage with built infrastructure would be enormously expensive, and in most of these basins it has not been done.

Peak water

As a glacier retreats, meltwater contribution first increases — more ice is exposed and melting faster — and then declines permanently once the ice volume is depleted. The turning point is called peak water.

Many glacier-fed basins have already passed it. The temporary abundance beforehand is easy to mistake for security, and communities have built dependence on flows that are not sustainable. Once past peak water, the decline is permanent on any human timescale.

What warming changes

• More winter precipitation falls as rain rather than snow, so less is stored at all
• Snowpack melts earlier, shifting flow from summer into spring, when it is least useful and most likely to flood
• The snowline rises, reducing the area that accumulates snow in the first place
• Glaciers lose mass, passing peak water and then declining permanently
• The net effect in many basins is more water in winter, less in summer, and greater year-to-year variability — a worse distribution from the same or greater total