Glaciers outside of the ice sheets account for only a small fraction of the total volume of ice trapped on land, about 1%, but account for a disproportionate amount of sea-level rise. Over the past century nearly half of all sea-level rise can be attributed to the loss of mountain glacier ice. If melted completely, these glaciers would provide another 40 centimeters to sea level.
Glaciers exist at temperatures near the melting point of ice. This makes them especially vulnerable to changes in temperature, especially during the warm summer months when they melt and shed huge volumes of meltwater into rivers and oceans. This runoff provides a key source of water for consumption and irrigation to downstream communities in arid regions. When the amount of runoff plus the amount of calving (solid ice that is discharged into the ocean) exceeds the annual snowfall, there is a net transfer of mass from glaciers to oceans. This causes sea levels to rise. As temperatures have risen over the past century, the amount of water lost from glaciers into the ocean has increased.
About 216,000 thousand glaciers exist on seven continents, covering a total area of 706,000 square kilometers. The vast majority of ice volume is locked up on Arctic and Antarctic Islands, the North American West Coast, southern South America and South Asia with smaller glacier regions in Europe, New Zealand and a few glaciers on the highest mountains in the tropics.
There are several ways to determine a glacier’s mass balance, which is defined as the net change in mass over a given year (accumulation of snowfall minus the runoff and the calving). Historically, glacier mass balance was measured by installing a series of poles on the glacier surface, then measuring changes in the depth of the snow at each pole each year. Another approach, referred to as the geodetic method, measures the change in the volume of the glacier using repeat measurements of the glacier surface height from ground or airborne surveys. Local mass balance measurements can help provide estimates of global glacier contributions to sea-level rise.
In 2002 and 2003, NASA launched the GRACE and ICESat missions that revolutionized the ability to measure glacier mass balance on global scales. The GRACE mission provided precise measurements of gravity change due to the loss or gain of mass at glaciers. ICESat used lasers in space to provide detailed measurements of changes in glacier elevation that can be converted to changes in mass. ICESat ended in 2009, GRACE in 2017. In 2018, follow-on missions to both GRACE and ICESat (GRACE-FO and ICESat-2) were launched and are now providing even more detail than before.
In addition to measuring net mass change, researchers are working to advance understanding of key physical processes that is needed to improve modeling used for projecting glacier change. Such processes include energy and mass exchange with the atmosphere, glacier flow and glacier calving. Understanding of such processes is being advanced with NASA satellite observations of surface reflectance (MODIS), snow fall (CloudSat) and surface motion (Landsat and NISAR).
There is broad scientific consensus that glaciers will continue to contribute large volumes of water to the world’s ocean over the coming century and beyond. But large uncertainties remain in how much and how fast. Further progress in constraining future rates of glacier loss will come from improved historical and present-day estimates of global glacier mass balance, which will provide the context for recent changes as well as improved data for calibration of glacier mass-balance models. Likely the largest source of data to reconstruct glacier changes over the past 50 to 60 years is airborne and declassified satellite stereo imagery.
The highly variable terrain of alpine regions creates complex patterns in meteorological conditions that pose significant challenges to accurately quantifying precipitation, temperature, and other variables, so these forces can be accurately represented in glacier evolution models. At present, 20 to 50% of the uncertainty in projections of global glacier changes between now and 2100 is due to the degree of difference among existing global climate models, or GCMs. Significant improvement to model quality can be achieved by combining glacier models with GCM climate projections that have been downscaled using regional climate models (RCMs) at the highest possible spatial resolution.