What are Glaciers/Ice Sheets?

Glaciers are made up of fallen snow that, over many years, compresses into large, thickened ice masses. Glaciers form when snow remains in one location long enough to transform into ice. What makes glaciers unique is their ability to flow. Due to sheer mass, glaciers flow like very slow rivers. Some glaciers are as small as football fields, while others grow to be dozens or even hundreds of kilometers long.

Presently, glaciers occupy about 10 percent of the world's total land area, with most located in polar regions like Antarctica, Greenland, and the Canadian Arctic. Glaciers can be thought of as remnants from the last Ice Age, when ice covered nearly 32 percent of the land, and 30 percent of the oceans. Most glaciers lie within mountain ranges that show evidence of a much greater extent during the ice ages of the past two million years, and more recent indications of retreat in the past few centuries.

An ice cap is a dome-shaped glacier mass flowing in all directions, such as the ice cap on Ellesmere Island in the Canadian Arctic. An ice sheet is a dome-shaped glacier mass exceeding 50,000 square kilometers. The world's ice sheets are confined to Greenland and Antarctica.

How are Glaciers/Ice Sheets Measured?

Since 2002, the twin satellites of GRACE have made a complete map of Earth’s gravitational field every 30 days. Gravity is determined by mass. While most of the planet’s mass — its land and core — doesn’t move much in 30 days, its water and ice do, causing Earth’s gravity to shift. By tracking these changes, GRACE and GRACE-FO can identify how much ice sheets and glaciers are shrinking.

GRACE data are used extensively to determine mass changes of the world’s land ice (ice sheets, icefields, ice caps and mountain glaciers). Land ice continually adds mass through precipitation and loses mass via meltwater runoff and calving of solid ice into the ocean. If losses are greater than gains, land ice loses mass, causing sea level to rise. Over the last decade or so, losses from land ice have been implicated in causing two-thirds of the observed rise in sea level.