Collapsing Ice Sheets

The Earth's polar ice sheets contain an incredible 33 million cubic kilometres of fresh water, more than 300 times the volume of all lakes combined. Although it was historically understood that these ice sheets were stable over long time periods, as scientists learn more about them it is becoming clear that ice sheets - like many other components of the Earth system - are capable of rapid change.

Two types of ice sheets

The three major ice sheets are the East Antarctic, West Antarctic and Greenland. The East Antarctic Ice Sheet is extremely thick, and frozen solidly to a dry base. As such, it is very stable and will almost certainly resist collapse. The other two ice sheets are, however, fundamentally different.

As shown in these cross sections, much of the West Antarctic Ice Sheet lies below sea level - in some places by more than 2km. Only its great thickness prevents it from floating. Because of this feature it, like the Greenland Ice Sheet, is called marine-based. To understand the importance of this in terms of sea level rise, it is necessary to understand how glaciers move.

The marine-based West Antarctic Ice Sheet is much less stable than its Eastern cousin.

Ice in motion

Despite the seemingly solid appearance of glaciers, they are constantly flowing under the force of gravity. Most glacier movement is on the order of metres per year, but Antarctic 'ice streams' and 'outlet glaciers' - which drain ice from the ice sheet interiors to the ocean - can move hundreds of times faster. Only recently have scientists begun to observe how quickly these outlet glaciers can react to climate.

As outlet glaciers deliver glacial ice to the sea, that ice melts and breaks off as large icebergs. This loss has been balanced over the millennia by the steady accumulation of snow over the ice sheet interior. New evidence from satellites shows that the delivery of ice to the sea has accelerated since the 1970s, and may be outstripping accumulation.

Icebergs up to 800m tall calve from the Jakobshavn outlet glacier in western Greenland, which flows into the ocean at up to 30m per day.

As ocean temperatures increase due to global warming, melting rates of marine glaciers will also increase, speeding the flow of outlet glaciers.

 

Sensitive sheets

Marine-based ice sheets are particularly sensitive to increased outlet flow. Floating ice shelves act as plugs, holding back the grounded ice behind. If those ice shelves collapse due to warming of the ocean, the glaciers feeding the ice shelf surge forward to fill the void. Faster flowing glaciers tend to become thinner, and if they thin beyond a certain threshold the base will begin to float.

Increasing the amount of floating ice contributes to sea level rise, just like adding ice cubes to a glass will increase the level of liquid within it. And as the area of the exposed base increases, the rate of melting increases in a self-accelerating cycle.

Change is underway
Ice shelves of the Antarctic Peninsula have been collapsing in recent years. Most recently, the Larsen B Ice Shelf suddenly disintegrated in early 2002. Meanwhile, the Pine Island Glacier - one of the main outlet glaciers for West Antarctica - has been thinning and sliding faster into the sea over the past decade. The grounding line of the Pine Island Glacier migrated inland by more than 1 km per year between 1992 and 1996, a period in which the Southern Ocean warmed by about 0.03ºC, testifying to the sensitivity of the ice sheet.

If marine ice sheets collapse, sea level could rise as much as 10 metres over the next few centuries. The resulting loss of coastal land could inundate cities, productive farmland, and even entire countries such as Bangladesh.

Sea level rise could be very rapid
Climate change is almost certain to bring a gradual rise in sea level as alpine glaciers melt, and as oceans warm and expand. However, beyond these relatively predictable processes lies the potential for rapid sea level rise: the spectre of polar ice sheet collapse.

At the end of the last ice age, sea level rose by up to 30 cm per decade, as the Laurentide ice sheet over Canada - also marine-based - collapsed. The possibility of a collapsing Greenland or the West Antarctic Ice Sheet producing sea level rise at a similar rate cannot be ruled out.

Massive sea level rise could prove one of the most devastating long-term effects of climate change. The only way to prevent leaving future generations a legacy of coastal destruction is to adopt climate change solutions today.

Melting Mountains
Back to Global Meltdown