In this video, Louise Sandberg Sørensen, Senior Researcher at DTU, explains how to measure mass balance. She explains how scientists have measured mass changes on the Greenland Ice Sheet using GRACE theory.

The mass balance of an ice sheet is the difference between snow input and loss of mass, due to factors such as melting, ablation and calving.

Ice sheets contain vast amounts of water, so it is important to monitor mass balance because significant changes could have severe consequences for sea water levels.

There are different methods of monitoring mass balance of ice sheets:

• Altimetry mass balance: observing changes in glacier elevation
• IOM mass balance: comparing outflow and melt to snowfall accumulation
• GRACE theory mass balance: detecting changes in the Earth’s gravitational field over ice sheets

GRACE theory mass uses data from the GRACE satellites, which measure anomalies in the Earth’s gravity. As ice sheets melt, they lose mass - which alters the Earth’s gravity.

The twin GRACE satellites circle the globe 15 times a day. They sense minute variations in Earth’s gravitational pull. When the first satellite passes over a region of slightly stronger gravity, it is pulled slightly ahead of the trailing satellite - causing the distance between the two satellites to increase. By measuring the changing distance between the two satellites and combining that data with precise positioning from GPS instruments, scientists can construct a detailed map of Earth’s gravity anomalies.

Using measurements from GRACE, scientists measures ice loss in all of Earth’s land ice between 2003 and 2010, with particular emphasis on glaciers and ice caps outside of Greenland and Antarctica. The total ice mass lost over the period studied was about 4.3 trillion tonnes, which added about 12 millimetres to global sea level.

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Featured Educator:

• Dr Louise Sandberg Sørensen

(This video was filmed remotely during COVID-19 lockdown conditions)

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