Glaciers
Flow Velocities
Ice Thickness
Mass Change
Glaciers are masses of land ice that form where snow accumulates, compacts, and transforms into ice over time. They range in size from small cirque glaciers located in high mountain environments to large valley glaciers extending through entire mountain systems. Unlike ice sheets, glaciers are generally constrained by local topography and flow in specific directions, often descending along mountain slopes or through valleys like slow-moving rivers of ice. An ice sheet is the largest type of glacier, covering more than 50,000 km² and extending over continental-scale areas. Outlet glaciers originate within an ice sheet and flow outward through topographic depressions, transporting ice from the interior of the ice sheet toward its margins and the ocean.
Temporal changes of the volume and mass of glaciers are sensitive indicators of climate change, and strongly affect the global water cycle, surface energy budget, sea level change, ocean circulation patterns, and hence global climate. A precise monitoring of the behaviors of these ice bodies is essential for improved understanding of the Earth’s system, and interactions between its sub-components.
With observations from dedicated satellite gravity missions (CHAMP, GRACE, GRACE Follow-On, and GOCE), as well as remote sensing techniques from satellites and aircrafts, a precise monitoring of the mass and volume changes of the ice sheets and larger glaciers is now possible. The three following methods are currently applied to measure the mass balance and volume changes of these ice bodies:
- comparing net snow accumulation (assessed by atmospheric modeling) with ice discharge to the ocean by glacier flow, assessed through measurements of glacier flow velocities by interferometric SAR (InSAR);
- using altimetry over the ice surface to measure changes of the thickness of ice sheets and glaciers from radar (e.g., Cryosat) and laser (e.g., ICESat) altimeters (geometric method);
- measuring temporal gravity changes based on data from satellite gravity missions, directly providing estimates of mass change and mass transport (gravimetric method);
Both, the geometric and gravimetric methods require correction for changes in the elevation of rock beneath the ice. This is not necessary for the first method, which however relies on assessing the small difference between two large quantities, each with considerable uncertainty. Since the gravimetric method is limited by a spatial resolution of about 200 to 300 kilometers with current satellite missions, it does not allow for monitoring mass changes of smaller glaciers.
As an example, results of these three methods are shown for an outlet glacier system in the Amundsen Sea embayment in West Antarctica, comprising the Pine Island glacier and the Thwaites-Haynes-Pope-Smith-Kohler glacier complex. This outlet system is of great interest to scientists because it is changing rapidly; it is thinning, accelerating and receding, all of which contribute directly to sea level rise.
Data Sources
Data sets available at the European Space Agency (ESA)
Data sets available at the US National Aeronautics and Space Administration (NASA)
Further Information
