A glacier is a large, persistent body of dense ice that moves under its own weight across a land surface. Glaciers are freshwater; they represent the largest reservoir of readily available freshwater on the planet. This massive frozen storage is composed of water virtually free of salt and other minerals, a direct result of the natural process by which the ice forms and accumulates.
How Glacial Ice Forms
The freshwater composition of glaciers begins with the hydrological cycle through evaporation. Water evaporates from the ocean surface, leaving salt and dissolved minerals behind. This pure water vapor condenses to form clouds and precipitates as snow, and the accumulation of this fresh snow on land is the first step in glacial ice formation.
The transformation from light, fluffy snow to dense, crystalline glacier ice is a multi-stage process driven by pressure and time. As new layers of snow fall, the weight of the overlying snow compresses the layers beneath, squeezing out the air between the snowflakes. This pressure causes the snow crystals to compact into a granular material called firn, which is snow that has survived at least one melt season.
The firnification process continues as weight increases, causing the grains to lock together and density to rise. When the ice reaches a density where air pockets are sealed off into tiny bubbles, it officially becomes glacial ice. This compaction and recrystallization process takes decades to centuries, creating a massive body of pure water.
Glaciers as Global Water Reservoirs
Glaciers and ice sheets represent the largest store of freshwater on Earth, holding approximately 69% of the world’s total freshwater supply. Although only about 2.1% of all water on the planet is frozen in glaciers, this small fraction is a large component of the usable freshwater resource. The sheer volume of ice is immense, with the vast majority contained within the two great ice sheets covering Antarctica and Greenland.
The Antarctic and Greenland ice sheets contain more than 99% of the freshwater ice on the planet, covering millions of square kilometers and reaching several kilometers thick. Smaller mountain glaciers, often referred to as “water towers,” are important for local populations.
These smaller glaciers in mountainous regions, such as the Himalayas, Andes, and Rocky Mountains, serve as natural, reliable reservoirs. They store winter precipitation and release meltwater gradually during the warmer, drier summer months. This seasonal melt is a source of drinking water, irrigation for agriculture, and hydropower generation for millions of people living downstream.
Differentiating Glaciers, Sea Ice, and Icebergs
Different forms of frozen water exist in polar and oceanic environments, and their origins determine their composition. Glaciers are masses of land-based, freshwater ice that form from compacted snow and flow under their own weight. This land origin ensures their freshwater nature.
Sea ice, in contrast, forms when ocean water freezes directly on the surface. Although the freezing process, known as brine rejection, forces most of the salt out of the growing ice crystals, sea ice still retains small pockets of concentrated salt water called brine. Consequently, sea ice is considered saline or briny, and not freshwater.
Icebergs are distinct from both, as they are large chunks of ice that have broken, or “calved,” off the edge of a glacier or an ice shelf and float in the ocean. Because icebergs originate from land-based glaciers, they are composed of dense, non-saline freshwater.
Consequences of Melting Freshwater Ice
The melting of freshwater ice from glaciers and ice sheets is a significant contributor to global sea level rise. Since this ice is stored on land, its meltwater adds new volume to the oceans. Meltwater from glaciers, excluding the large ice sheets, is currently the second-largest contributor to sea level rise, following the thermal expansion of ocean water.
The average loss of ice from glaciers is estimated at hundreds of billions of tonnes per year, a rate that has accelerated over the last few decades. This influx of freshwater also has localized effects on ocean systems, as the reduced salinity of the surface water can impact ocean currents and marine ecosystems.
The loss of this frozen reservoir threatens the water security of communities that depend on seasonal glacial melt. While initial increased melting can lead to temporary flooding, the long-term trend of glacier shrinkage means that a reliable summer water source will eventually diminish. This decline poses a threat to agriculture, drinking water supplies, and hydroelectric power generation for millions of people in glacier-fed regions.