Lesson 4 of 6 · glaciers and ice sheets
Measure ice that stores water and flows
10 minute lesson · 3 checks along the way
Your goal for this lessonExplain glacier motion and mass balance while distinguishing melting, discharge and changes in surface height.
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Solid ice can move
A is persistent land ice that moves under its own weight. Snow surviving repeated seasons becomes compacted and transformed into ice. Over large distances and long periods, that ice deforms, and some glaciers also slide over their beds. A solid material need not be rigid forever. Surface slope, thickness, temperature, water near the bed, and resistance from the surrounding terrain influence the flow.
cover much larger regions than individual valley glaciers, spreading outward from broad interiors through outlets and ice streams. Greenland and Antarctica contain Earth’s present ice sheets. Their boundaries include grounded ice resting on a bed and, in places, floating extensions called . The grounding line separates those conditions. It can move, so a map must state which boundary and date it represents.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Account for gains and losses separately
Snowfall adds mass. Runoff that leaves the ice system and ice discharged across its boundary remove mass. Melting at the surface is not automatically an equal loss: some water refreezes or remains stored inside. Likewise, ice flowing from an upstream region into a downstream region is internal if both lie within the chosen whole-system boundary. depends on the definition of the system, not just the sign of one visible change.
Floating shelves can provide resistance that limits the flow of grounded ice behind them. Thinning or loss of a resisting shelf can change that flow, depending on geometry and contact with surrounding features. An iceberg separating from a shelf is therefore different from snowfall failing to replace ice lost over a full year. A dramatic photograph documents an event; it does not by itself measure the annual balance of the entire ice sheet.
Operation IceBridge photographed the calving front of Greenland’s Sermeq Kujatdleq glacier from a NOAA P-3 aircraft; the image was published in May 2016. Aircraft structure crosses the top of this aerial photograph. The front marks an ice boundary, not an annual mass-loss measurement.
Look closely
Locate the front and the separated pieces beyond it. What would you need before converting this view into an annual ice-export estimate?
See what to notice
An export estimate needs ice thickness, density, the movement or volume crossing a defined boundary, and an interval. One photograph does not establish those rates or distinguish every contribution to the glacier’s total mass balance.
Work through a decision
Worked balance: a shrinking inventory can still receive snow
Consider an invented ice region starting with 1,000 gigatonnes of stored mass. One gigatonne is one billion metric tonnes. Over one year it gains 300 gigatonnes of snow, loses 200 as net meltwater runoff, and exports 150 as ice across its downstream boundary. Assume these are the complete external transfers, and that the runoff already excludes water that refroze within the region.
The change is 300−200−150 = −50 gigatonnes, leaving 950. Subtracting an additional gross surface-melt total would count some of the same loss twice. Snow can continue falling during a year with a negative mass balance; gaining material and gaining total mass are different claims. The invented annual balance also gives no exact terminus position without information about geometry and ice motion.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Let independent measurements constrain the result
ICESat-2 measures surface elevation along its observing tracks, while GRACE Follow-On detects changes in Earth’s gravity field related to redistributed mass. Neither simply photographs total ice mass. Elevation changes can include snow compaction and density changes; gravity interpretation requires separating ice signals from other mass changes and Earth’s response. Velocity measurements and snow observations add further checks on what is crossing the boundary.
To test a reported retreat, compare dates, seasonal conditions, elevation, motion, and mass estimates. A retreating front can coexist with ice flowing forward toward it, because removal at the front can outpace that motion. A growing surface area need not imply increasing volume. Describing each measured quantity precisely makes pictures, laser profiles, and gravity observations parts of one physical explanation rather than interchangeable evidence.
Check 3 of 3
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Put it together
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