Lesson 5 of 6 · sea ice and sea level change
Separate floating ice from rising shorelines
10 minute lesson · 3 checks along the way
Your goal for this lessonDistinguish ice origin, ocean volume and land motion when explaining sea-level change.
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Where the ice formed matters
forms when ocean water freezes. An iceberg originates from land ice that breaks away, often after passing into a floating shelf. Both may float at sea, but their histories differ. Sea-ice area, concentration, thickness, and volume also describe different quantities. A broad region of thin broken ice is not equivalent to the same area covered by thick compact ice, and seasonal expansion is not a new .
Floating ice already displaces water. In a simple freshwater-container model, melting freely floating freshwater ice leaves the water level unchanged because the melt replaces its displaced volume. Actual sea ice interacts with salty water, so small density-related effects complicate an exact zero statement. Its direct melting contribution is nevertheless very different from adding water previously stored on land. Losing sea ice also changes reflectivity and exchanges of energy between ocean and air.
Arctic sea ice photographed during Operation IceBridge in March 2011. The aerial view shows the arrangement of floes and openings at the surface. It supplies neither a depth section nor a full Arctic inventory.
Look closely
Compare the bright ice-covered patches with the openings. Which quantities can a calibrated overhead survey help estimate, and which crucial dimension is missing?
See what to notice
A georeferenced survey can estimate surface coverage and concentration, with stated sampling and thresholds. Thickness is not supplied by the overhead pattern alone, so equal coverage can conceal different ice volumes.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Separate the pathways that change sea level
Land-ice loss can add water to the ocean, while warming can expand water already there. Changes in land-water storage contribute too. The effect is not geographically uniform: currents, winds, Earth’s gravity field, and the solid Earth’s response redistribute sea-level change. Floating ice-shelf loss can indirectly matter by allowing grounded ice to flow faster, even when the shelf’s own displacement was already present.
Coastal experience depends on the height of the sea relative to the land. Subsiding land increases relative sea-level rise, while uplift can reduce or reverse it locally. Tide gauges and satellite altimeters therefore do not always report the same quantity without additional reference information. Sentinel-6 Michael Freilich supports the satellite record; local land-motion measurements help explain why a particular shoreline differs from a global mean.
Work through a decision
Worked contributions: state the reference
For an invented global-average calculation, deliver 360 gigatonnes of additional liquid freshwater from land into an ocean with fixed area 3.6×10¹⁴ square meters. Assume water density 1,000 kilograms per cubic meter and ignore shoreline shifts, gravity redistribution, and expansion. The added mass is 3.6×10¹⁴ kilograms, so its volume is 3.6×10¹¹ cubic meters. Dividing by ocean area gives 0.001 meter, or one millimeter.
This is an added-water calculation, not a rule that every gigatonne of floating ice gives the same contribution. In a separate fictional coastal example, sea height rises 3 millimeters per year relative to a fixed reference while the land sinks 2 millimeters per year relative to that same reference. rises 5 millimeters per year. A stable land assumption would miss two millimeters of the local change.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Compare records that measure the same thing
A photograph of sea ice can reveal floes and open-water leads, but it does not measure the thickness hidden below the surface. Repeated satellite observations, field measurements, and models constrain different parts of the inventory. An extent map normally uses a specified concentration threshold. Comparing maps requires the same definition and seasonal timing; a winter maximum cannot be directly treated as a summer minimum.
For sea level, separate short-term tides and storm surges from longer-term change before interpreting a trend. Then check whether the record describes local relative height, satellite-referenced ocean height, or a global average. These distinctions do not make observations contradictory. They explain how floating ice, land ice, ocean warming, and moving coastlines contribute to different parts of a connected planetary measurement problem.
Sea level depends on the reference
Original reference-frame schematic for the lesson’s fictional local rates. A positive sign means upward relative to the same fixed vertical reference; relative sea-level change equals sea motion minus land motion. Distances are exaggerated, with a consistent illustrative scale within the figure.
Open full-size diagram in a new tab: Sea level depends on the referenceCheck 3 of 3
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Put it together
Your lesson progress
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