Sea Ice & Icebergs Explained
Ice sheets are continent-scale masses of land ice that cover vast areas of Greenland and Antarctica. They store most of Earth’s freshwater ice, flow slowly toward the ocean, feed glaciers and ice shelves, and strongly influence global sea level.
What Are Ice Sheets?
Ice sheets are enormous bodies of glacier ice that cover more than 50,000 square kilometers of land. Today, Earth has two major ice sheets: the Greenland Ice Sheet and the Antarctic Ice Sheet.
Unlike sea ice, which forms when ocean water freezes, ice sheets form from snowfall on land. Over thousands of years, snow compresses into dense glacier ice that slowly spreads outward under its own weight.
How Do Ice Sheets Form?
Ice sheets form where snowfall persists year after year and summer melting does not remove all accumulated snow. Layers of snow build up, compress into firn, then transform into glacier ice.
- Snow accumulation: snow survives from year to year in cold regions.
- Compaction: older snow is buried and squeezed into firn.
- Glacier ice formation: air is forced out and dense ice develops.
- Ice-sheet flow: gravity spreads the ice outward from high domes.
- Outlet glaciers: fast-moving ice streams carry ice toward the ocean.
Greenland Ice Sheet vs. Antarctic Ice Sheet
The Greenland Ice Sheet covers most of Greenland and is strongly affected by surface melting during summer. The Antarctic Ice Sheet is much larger and includes East Antarctica, West Antarctica and the Antarctic Peninsula.
| Ice Sheet | Main Features | Key Concerns |
|---|---|---|
| Greenland Ice Sheet | Smaller, warmer, strong summer surface melt | Meltwater runoff, outlet glacier retreat, iceberg discharge |
| West Antarctic Ice Sheet | Much ice rests below sea level | Ocean-driven melting, ice-shelf thinning, marine ice-sheet instability |
| East Antarctic Ice Sheet | Largest and coldest ice reservoir on Earth | Long-term stability, coastal glacier changes, ocean warming at margins |
How Ice Sheets Move
Ice sheets may look frozen and still, but they are always moving. Gravity pulls thick ice outward from high interior domes toward lower coastal margins.
Some zones move slowly, while outlet glaciers and ice streams can flow much faster. These fast-moving corridors feed
ice shelves,
calving fronts and
icebergs.
Why Ice Sheets Melt and Lose Ice
Ice sheets lose mass when melting, runoff, calving and ocean-driven ice loss exceed snowfall gains. This balance between accumulation and loss is called ice-sheet mass balance.
- Surface melting: warmer air melts snow and ice on the surface.
- Meltwater runoff: liquid water drains into rivers, lakes, crevasses and the ocean.
- Basal melting: geothermal heat or warm ocean water melts ice from below.
- Calving: glaciers and ice shelves break off icebergs.
- Ice-shelf loss: weakened shelves allow inland ice to flow faster seaward.
Surface Meltwater, Blue Lakes and Rivers on Ice
During warm periods, meltwater can form bright blue lakes, streams and rivers on ice-sheet surfaces. Some water drains through cracks called moulins, reaching the bed of the ice sheet and influencing glacier motion.
Surface meltwater is especially visible on Greenland, but meltwater events also occur on Antarctic ice shelves and margins during warm summers.
Basal Melting, Geothermal Heat and Warm Ocean Water
Ice can also melt from below. Beneath ice sheets, geothermal heat from Earth’s interior can warm the base. Along marine margins, relatively warm ocean water can enter cavities beneath floating ice shelves and melt them from underneath.
This is especially important for parts of West Antarctica, where ocean-driven melting can thin ice shelves and reduce their ability to hold back inland glaciers.
Ice Sheets and Sea-Level Rise
Ice sheets matter because they sit on land. When land ice melts or flows into the ocean, it adds water to the sea and contributes to global sea-level rise.
This differs from floating sea ice, which does not directly raise sea level when it melts. Ice sheets are therefore one of the most important long-term controls on future coastlines, storm-surge risk and coastal flooding.
Ice Sheets During the Last Glaciation
During the last glacial maximum, ice sheets covered much larger areas of North America, northern Europe and Asia. These ancient ice sheets shaped landscapes, carved valleys, depressed Earth’s crust and stored enough water to lower global sea level.
Comparing past ice sheets with modern skylines helps show how enormous continental ice masses can become during colder climate periods.
How Scientists Monitor Ice Sheets
Scientists monitor ice sheets using satellites, aircraft, radar, GPS, ice cores, field measurements and gravity missions. These tools reveal changes in ice thickness, surface height, flow speed, meltwater, snowfall and mass loss.
- Satellite altimetry: measures ice-surface height changes.
- Gravity measurements: detect changes in total ice mass.
- Radar: maps internal layers and bedrock beneath ice.
- Ice cores: preserve ancient snowfall, gases and climate records.
- GPS and imagery: track glacier speed and retreat.
Fossils and Ancient Landscapes Beneath Ice Sheets
Ice sheets can preserve ancient soils, sediments, plants and landscapes beneath kilometers of ice. Discoveries beneath Greenland and Antarctica reveal how polar regions changed through past warm and cold periods.
These hidden records help scientists understand ice-sheet sensitivity, long-term climate history and how polar ice responded during earlier warm intervals.
Ice Sheets vs. Ice Shelves vs. Sea Ice
| Feature | Where It Forms | Sea-Level Role |
|---|---|---|
| Ice sheet | On land from compacted snowfall | Can raise sea level when mass is lost to the ocean |
| Ice shelf | Floating extension of land ice | Little direct effect when melting, but can speed inland ice flow |
| Sea ice | Frozen seawater on the ocean surface | Little direct effect when melting because it already floats |
Related Polar Ice Topics
Ice sheets connect directly to
Sea Ice & Icebergs Explained,
Ice Shelves Explained,
Glacier Calving Explained,
Icebergs Explained,
Sea Ice Explained and
Ocean Temperature & Climate Oscillations Explained.
FAQ: Ice Sheets Explained
What is an ice sheet?
An ice sheet is a continent-scale mass of glacier ice covering more than 50,000 square kilometers of land.
Where are Earth’s ice sheets today?
Earth’s two major modern ice sheets are the Greenland Ice Sheet and the Antarctic Ice Sheet.
Are ice sheets the same as sea ice?
No. Ice sheets form from snowfall on land, while sea ice forms when ocean water freezes.
Do ice sheets raise sea level when they melt?
Yes. Because ice sheets sit on land, meltwater and ice loss from them add water to the ocean and raise sea level.
What is ice-sheet mass balance?
Ice-sheet mass balance is the difference between ice gained through snowfall and ice lost through melting, runoff, calving and glacier flow into the ocean.
Why is West Antarctica important?
Much of the West Antarctic Ice Sheet rests below sea level, making it vulnerable to ocean-driven melting and marine ice-sheet instability.
