The shape of it
Snow falls in the interior and does not melt. It compacts to firn, then ice, and the weight pushes it outward and downward. The ice sheet is a very slow river system: high in the middle, flowing to the edges, funnelled through gaps in the coastal mountains as outlet glaciers. Some end on land. The important ones end in the sea.
The centre is high, cold and dry. The margins are lower, warmer, and this is where the action is. A rim of bare rock and tundra along the coast, wider in the south and west, narrow in the north-east where the ice reaches almost to the shore.
Two ways to lose ice
Surface. Summer sun melts snow and ice at the margins. Water runs across the surface, pools in lakes, plunges down moulins to the bed and out to the sea. Some refreezes in the firn; much of it leaves. In a warm summer the melt zone climbs far up the slope and, in rare events, reaches the summit.
Discharge. Ice flowing into the ocean at marine-terminating glaciers. Icebergs calve from the front. Warm water melts the submerged face. If the glacier speeds up, thins, or retreats off a bedrock rise, discharge climbs.
The balance sheet: snowfall in, surface runoff and discharge out. Both losses have grown. Snowfall has not kept pace. The sheet is shrinking.
| Process | Where | Driver | Timescale |
|---|---|---|---|
| Surface melt and runoff | Low margins, especially west and south | Warm summers, low albedo, clear skies | Seasonal, immediate |
| Calving and submarine melt | Marine-terminating outlets | Warm fjord water, loss of ice shelves and sea-ice buttressing | Years to decades |
| Ice flow response | Whole drainage basins | Thinning and retreat at the front pulling ice from inland | Decades to centuries |
The surface
Clean snow reflects almost everything. Bare ice reflects less. Wet ice, less again. Along the western margin a broad band darkens each summer: old dust, soot, and above all algae that bloom on the melting surface and pigment themselves dark against the light. This dark zone soaks up heat and melts faster, which feeds the algae, which darkens it further.
Melt lakes form in the same hollows year after year. When one drains through a crack, it can empty in hours, driving water to the bed and briefly lifting and speeding the ice above. Firn, the porous old snow, acts as a sponge for meltwater until it saturates; where refrozen ice layers have sealed it, water runs off instead of soaking in.
At Summit Station, on the highest point of the ice sheet, conditions are usually far below freezing all year. Melt there is exceptional and noteworthy when it happens.
The outlets
Sermeq Kujalleq, known on older charts as Jakobshavn Isbræ, drains into the fjord at Ilulissat on the west coast. Among the fastest-flowing glaciers anywhere, and the source of the icebergs that fill Disko Bay. The fjord is a UNESCO World Heritage Site. The glacier has retreated far up the fjord over the past century and its front now sits in deep water.
In the north-east, Zachariae Isstrøm and Nioghalvfjerdsfjorden Glacier drain a vast inland basin. Zachariae lost its floating ice shelf and accelerated. In the north-west, Petermann Glacier has lost large pieces of its floating tongue in single calving events. In the south-east, Helheim and Kangerlussuaq glaciers respond quickly to warm water in their fjords, thinning and retreating, then partially recovering when the water cools.
The common thread: warm, salty Atlantic-origin water at depth in the fjords. It reaches the glacier face, melts it from beneath, undercuts it, and triggers calving. Glaciers grounded in deep water are the most exposed. Where a glacier front retreats onto shallower bedrock it can stabilise. Where the bed deepens inland, retreat can run and run.
Sea level
All of it melted would raise global sea level by several metres. Nobody expects that this century. The concern is the pace of contribution: Greenland is among the largest single sources of present sea level rise, alongside thermal expansion of the oceans and the retreat of mountain glaciers.
The rise is not even. An ice sheet's gravity pulls the sea towards it; as the sheet shrinks, water near Greenland falls while water far away, in the tropics and the southern hemisphere, rises by more than the global average. The land under the ice also rebounds. Coasts a hemisphere away feel Greenland more than Greenland's own coast does.
Against Antarctica
Greenland is smaller and warmer. Much of its loss is surface melt, which responds to summer temperature within the season. Antarctica is colder, loses ice almost entirely through the ocean, and holds far more of it. Greenland is the faster story; Antarctica the larger one. Both matter for the same reason.
Watching
Satellite gravimetry weighs the whole sheet. Altimetry measures its height. Weather stations on the ice track melt. Fjord moorings measure the water that reaches the glaciers.
Weather stations and mass balance: Programme for Monitoring of the Greenland Ice Sheet. Daily surface conditions: Polar Portal.