Glaciers are formed from snow that has compacted to form ice, which then deforms and flows downhill. Glaciers comprise two zones: an accumulation zone where accumulation of mass from snowfall exceeds ice loss, and an ablation zone where ice loss (ablation) from melting and other mechanisms exceeds accumulation. Where glaciers end in a lake or the ocean, ice loss can occur through melting where the ice meets the water, and via calving when chunks of the glacier break off.

Glacier mass balance – the amount of mass gained or lost by the glacier – is commonly expressed as the annual thickness change averaged over the glacier area, expressed in metres water equivalent. One metre water equivalent is approximately the same as one tonne per square metre. Ice loss from glaciers contributed around 21% of the total sea-level rise over the period 1993–2018, around half the contribution from expansion due to ocean warming (42%) but larger than contributions from melting of the ice sheets in Greenland (15%) and Antarctica (8%).

The mass balances of individual glaciers are affected by changes in temperature, precipitation, humidity and cloudiness. Variability in these can lead to positive mass balance in some years even where the mass balance is negative in the long run. The IPCC Sixth Assessment Report (AR6) concluded that human influence is very likely the main driver of the global retreat of glaciers since the 1990s, stating that “[t]he global nature of glacier retreat since the 1950s, with almost all of the world’s glaciers retreating synchronously, is unprecedented in at least the last 2000 years (medium confidence)”.

Melt rates are also strongly affected by the glacier albedo, the fraction of sunlight that is reflected by the glacier surface. Exposed glacier ice is darker and therefore has a lower albedo than the seasonal snowpack; it is also sensitive to darkening from mineral dust, black carbon, algal activity and fallout from forest fires. Reduced snow cover, long melt seasons and wildfire activity all concentrate darker material on the glacier surface, decreasing its albedo and thereby increasing the rate of melting.

INDICATOR CONSTELLATIONS