Confused about ice shelf decay and sea ice increase?
Explaining Antarctic ice types and their different responses to climate changes
Antarctica's Brunt Ice Shelf photographed in October 2011 from NASA’s DC-8 research aircraft during an Operation IceBridge flight. The ice shelf is the thick snow-covered ice in the background. This is made up of ice that has formed from snowfall on the Antarctic continent and been pushed out past the coast by gravity. The very thin ice next to the ice shelf is sea ice, formed by freezing of seawater. The tendrils of “greasy” slicks on the ocean show new sea ice being formed. Credit: Michael Studinger/NASA.
There is some confusion among the public about the differences between Antarctica’s ice shelves (the topic of a recent paper by us in Science), sea ice, and the ice sheet. Some commenters are concerned about apparent contradictions between our new study of volume loss from ice shelves and reported increases in Antarctic sea ice extent. This comment explains the different ice types, and explains why the observed different behavior of each type is not contradicting the others.
You can learn more about different types of ice at NSIDC including how they form, how we measure them, and why they are important.
Sea ice forms as ocean water freezes, mainly during winter. Sea ice around Antarctica is typically about 1 meter thick and is often covered with snow. Most sea ice forms close to the Antarctic coast in winter, then is carried north by winds and ocean currents into the warmer Southern Ocean where it ultimately melts in summer. The area covered by sea ice around Antarctica changes from about 19 million square km in winter to about 3 million square km in summer. Because sea ice is made up of ocean water and is floating (ice is about 10% less dense than water), changing the amount of sea ice does not affect sea level (Archimedes principle).
Antarctic land ice is ice which has accumulated over thousands of years on the continent through snowfall to form the massive Antarctic Ice Sheet. The weight of new snow compresses the older snow below it to form solid ice. The ice sheet is around 2 km thick on average, in some places up to 4 km thick. The ice sheet spreads under its own weight to form glaciers that flow slowly downhill towards the ocean. When they reach the ocean, they lift off the bedrock and begin to float, forming ice shelves. The total area of the Antarctic ice sheet is about 15.5 million square km, of which about 10%, or 1.5 million square km, is floating ice shelves.
Our Science paper (Paolo et al., 2015) is about the ice shelves. Ice shelves are typically about one kilometer thick at the point where they first start to float (the “grounding line”) and a few hundred meters thick at the front edge. Ice shelves are attached to the main ice sheet but move slowly forward (about one km per year) pushed by the ice flowing into them from the continent.
Schematic diagram of an Antarctic ice shelf showing the processes causing the volume changes measured by satellites. Ice is added to the ice shelf by glaciers flowing off the continent and by snowfall that compresses to form ice. Ice is lost when icebergs break off the ice front, and by melting in some regions as warm water flows into the ocean cavity under the ice shelf. Under some ice shelves, cold and fresh meltwater rises to a point where it refreezes onto the ice shelf. Credit: Helen Amanda Fricker, Professor, Scripps Institution of Oceanography, UC San Diego
Because an ice shelf is floating, like sea ice, changing its volume does not directly affect sea level (Archimedes principle again). As our article describes, however, reducing the ice shelves can lead to faster flow of grounded ice into the ocean which does raise sea level, like adding extra ice cubes to a glass of water.
The three types of ice – sea ice, floating ice shelves and the grounded ice sheet – respond to climate changes in different ways. Antarctic sea-ice extent is increasing, contrary to the trends of ice loss in all other major ice components on the planet; Arctic sea ice, the Greenland and Antarctic ice sheets including their ice shelves, mountain glaciers and ice caps, permafrost and snow cover.
The most likely cause of faster melting of Antarctica’s ice shelves is the observed increase in “westerly” winds around the continent. Because of Earth’s rotation, these winds drive surface water away from Antarctica. Denser and warmer water that is normally quite a long way below the surface responds by rising (a process called “upwelling”), then flowing towards the coast and under the ice shelves where the warmer water increases melting.
The strengthening westerly winds also force new sea ice away from the continent, creating regions of exposed water called “polynyas”. These are regions of very rapid sea-ice growth in winter because the cold atmosphere can rapidly suck heat out of the ocean to cool the ocean enough to freeze it. Therefore, stronger winds lead to greater sea ice formation in winter. An increase in the amount of snowfall makes the sea ice thicker, so that it can survive longer and spread out further before it melts in summer. Other processes affecting Antarctic sea ice are explained in an article in The Conversation and another at Skeptical Science.
We conclude that the loss of ice from ice shelves that our article describes and the increased extent of Antarctic sea ice are both consistent with observed climate changes around Antarctica.
Earth & Space Research (www.esr.org)
Fernando Paolo and Helen Fricker
Scripps Institution of Oceanography / University of California, San Diego