World Day for Glaciers

Glaciers begin where more snow falls in winter than melts in summer, year after year.

Over time, layers of snow compact into dense ice. Under their own weight, these thick ice masses deform and slowly flow downhill under gravity, which is why glaciologists describe them as “rivers of ice.”

Their movement can be just a few centimeters to several meters per day, depending on slope, ice thickness, and temperature.  

Mountain glaciers act as natural water towers. They store water as ice in cold seasons and release meltwater during warm and dry periods, which helps stabilize river flows used for drinking water, irrigation, and hydropower.

UNESCO and the UN note that roughly half of humanity depends in some way on water coming from mountain regions, where glaciers and snowpacks buffer droughts and seasonal shortages downstream.  

In the short term, accelerated melting can temporarily increase river flows. Over time, however, as glaciers lose mass, their ability to supply meltwater in dry seasons declines.

This “peak water” effect means communities may first see higher flows and flood risk, followed by reduced summer water supplies that threaten drinking water, agriculture, ecosystems, and hydropower reliability.  

Both mountain glaciers and the Greenland and Antarctic ice sheets are adding water to the oceans as they lose ice.

While ice sheets contain far more total ice, small glaciers scattered around the world currently contribute a substantial share of observed sea level rise because they are losing mass very quickly.

UN and scientific assessments highlight glacier retreat, alongside ice sheet loss and ocean warming, as a key driver of rising seas that threaten coastal communities worldwide.  

As glaciers retreat, they often leave or dam lakes with ice or loose rock.

If one of these natural dams fails due to heavy rain, landslides, ice collapse, or seismic activity, large volumes of water can rush downstream with little warning.

These glacial lake outburst floods can destroy homes, roads, and farmland and pose serious risks to mountain communities, which is why the UN and glacier monitoring services emphasize early warning systems and hazard mapping.  

Researchers use a combination of ground measurements, aerial surveys, and satellite observations.

On selected glaciers, they measure ice thickness, stake snow and ice melt, and map positions of the glacier front. Satellite data then help extrapolate trends to larger regions by tracking changes in glacier area, surface elevation, and mass over time.

International networks coordinated by groups such as the World Glacier Monitoring Service provide standardized, long‑term records that feed into UN climate and water assessments.  

No. Almost all monitored glaciers are losing ice, but the speed of retreat varies widely by region, elevation, climate, and local conditions such as debris cover. Some low‑latitude and low‑altitude glaciers are disappearing especially quickly, while a few high, cold glaciers are thinning more slowly.

UN and scientific reports stress that even in regions where change seems gradual, long‑term trends point toward continued loss unless global greenhouse gas emissions are sharply reduced.