Ice sheet flow is the movement of ice across ice sheets and glaciers. Understanding the movement of ice to the oceans is fundamental for understanding sea-level rise.

One way to understand why ice moves is to look at features of the ice sheet. Ice sheets are dome-like: the centre of the ice sheet has the highest elevation and the thickest ice. The ice thins and gets lower towards the edge of the ice sheet. The Greenland and East Antarctic ice sheets are between 3000 - 4000m high and the West Antarctic and the Antarctic Peninsula Ice Sheets reach 2,500 meters high according to the National Snow and Ice Data Centre. Naturally, ice sheets flow downhill aided by gravity.

Other processes like snowfall, ablation, ocean bedrock topography and interactions with ocean water determine ice flow. Some areas of the ice sheet travel faster than others. For example, ice streams are corridors of fast flowing ice within an ice sheet, moving around 800 metres per year.

Understanding various features of ice sheets helps understand their behaviours and their stability. This gives scientists an overall picture of ice sheet dynamics.

Ice velocity increases where ice is thinning due to ocean warming.

It is important to measure ice velocity because an increase in ice velocity signals an area where ice is thinning due to ocean warming. Measuring ice velocity helps us to predict the onset and magnitude of ice sheet contribution to future sea level rise.

Ice sheet flows and velocity of the flow can be monitored using feature tracking. This is the principle of identifying a feature and comparing its position across two satellite images. This can be done manually, or algorithms can be used to do it automatically. Stationary objects such as bedrock outcrops are used to cross-correlate between images. Features that remain coherent in space and time, such as crevasses, are used as markers which persist in time and move with the ice. This method can be used with a variety of satellite data including optical images and SAR amplitude images, or a combination of the two.

Ice velocity maps are created to show ice speed by tracking moving features with SAR data, which can be obtained from the Sentinel-1 satellite.

One of the fastest flowing glaciers in the world is Jakobshavn Isbrae on the West Greenland Ice Sheet. This colossal ice stream flows at up to 10km per year during the winter. Its speed nearly doubles in the summer, when warmer air temperatures increase melt and lubricate the ice.

Velocity maps also show long term changes. For example, the Pine Island Glacier on the West Antarctic Ice Sheet has increased in velocity by over 42% over the last 25 years.

NASA and UC Irvine have recently produced a map of ice velocity in Antarctica which is 10 times more accurate than any previous map. It shows flows from over 80% of the continent, whereas older maps covered about 20% of the continent. They used data from six different satellite missions, from between 1994 to the present day, to create the map.

Dr Anna Hogg explains how SAR is used to monitor ice sheet velocity and the startling dynamics that have ben revealed through satellite data in Ice Sheet Velocity and Seasonal Variation. This video is available as part of ESA's Frozen Frontier: Monitoring The Greenland Ice Sheet from Space, another freely available MOOC produced by Imperative Space.

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