Glacial Retreat

The retreat of glaciers observed since 1850 is a well documented consequence of climate change and one of the most sensitive indicators of a changing climate, visible in mountain glaciers around the world. It follows from a glacier's mass balance, the relationship between the snow it accumulates and the ice it loses to melting; when melting outpaces the accumulation of new snow, a glacier's mass balance turns negative and it retreats, a condition now true of nearly every glacier on Earth. Human-driven warming has accelerated the process well beyond the pace of natural deglaciation: over the twenty-six years from 1993 to 2018 alone, the world's glaciers lost an estimated 5,500 gigatons of ice, about 210 gigatons a year, with the steepest proportional losses in mid-latitude ranges such as the Himalayas, the Rockies, the Alps, and the Andes. The consequences documented alongside this retreat include rising sea levels from meltwater runoff, threats to the water supply of the billions of people who depend on glacial flows, particularly in the Himalayas and Andes, disruption to cold-water ecosystems and fisheries, and a growing risk of glacial lake outburst floods as unstable moraines give way.