The collapse of a giant ice shelf in Antarctica in 2002 was the result of warmer air temperatures new research concludes, allowing scientists to identify two clear ways in which ice shelves become unstable.

Single summer

At the beginning of February 2002, the Larsen-B ice shelf stretched across 3,250 square kilometers, an area larger than Luxemburg. By the end of the following month it was gone.

In the space of a single Antarctic summer the entire 220-metre thick ice shelf disintegrated into the sea.

Since the dramatic collapse, scientists have been trying to understand how the shelf could disappear so quickly.

Collapse of the Larsen-B ice shelf in photos (from top left to bottom right). NASA

Surface warming

An  ice shelf can form when a glacier on land reaches the coast and flows into the ocean. If the ocean is cold enough the ice doesn’t melt. Instead, it forms a permanent floating sheet of ice, as the glacier continues to ‘feed’ into the ice shelf.

Scientists had previously thought that Larsen-B collapsed so quickly because warming oceans caused the ice sheet to become thin and unstable.

When an ice shelf is melted from below by warmer sea temperatures, the thinning ice loses support from the sea bed as it retreats. This can make it unstable and liable to collapse (as shown in figure 3  below).

However, the new paper in Science says this isn’t what happened to Larsen-B.

Glacier-ice shelf interactions. NSIDC

The collapse of the ice shelf allowed scientists to map and take samples of the sea floor underneath. They found that the ‘grounding line’, the area where the ice sheet touched the sea floor, hadn’t moved for 12,000 years. That’s the sign of a stable ice sheet. (See figure 1 in the diagram above.)

So the collapse probably wasn’t caused by warming oceans stressing the ice sheet. Instead, the researchers say, it’s likely to be the result of the surface of the ice sheet melting.

The process by which surface melt can break up ice shelves is reasonably well understood. As temperatures rise, warmer air melts ice into pools on the surface of the shelf. The water in these pools seeps into cracks in the ice and gradually erodes and widens them, creating huge crevasses, a process known as ‘hydrofracturing’. (See figure 2 in the diagram above.)

Once large enough, crevasses can cause parts of the ice shelf to split and break off. This is how icebergs are formed, in a process called ‘calving’. (See figure 4.)

The research suggests Larsen-B collapsed because of rapid and widespread calving, due to the amount of meltwater that had formed on its surface.

Sea-ice in the former Larsen-B area in April 2006. Michele Rebesco.

Antarctic Peninsula

Ice shelves can last for thousands of years, being fed by the continual flow from the glacier, with chunks of ice occasionally calving into the sea.

The Larsen ice shelf is one of many that surround 75 per cent of the Antarctic continent, sitting in the Weddell sea off the Antarctic Peninsula.

The Larsen shelf used to have four parts.

But Larsen-A collapsed in 1995 and Larsen-B in 2002. Larsen-C, the largest part, and Larsen-D are all that remain.

Both are thought to be stable, although Larsen-C has been thinning.

Acceleration

As ice shelves break up and melt, they add fresh water to the surrounding Antarctic seas. The loss of ice through thinning of ice shelves in Antarctica has been estimated at 280 billion tonnes per year between 2003 and 2008.

Larsen-B ice shelf. Michele Rebesco.

Because ice shelves float, their collapse does not directly raise sea levels, although it can have an effect on local sea levels.

But there are important implications. Once an ice shelf is gone, the glaciers that back it up are free to flow more quickly into the ocean, increasing the rate of sea level rise (see figure 4 in earlier diagram).

Research suggests that glaciers behind ice shelves may accelerate as much as five times following a rapid ice shelf collapse.

Ice shelves therefore play an important role in maintaining the stability of ice sheets on land. This research has helped scientists assess the risks such collapses pose to land-based ice sheets. Professor Eugene Domack, co-author of the paper, tells us:

“It allows the science community to make clear two distinct mechanisms for ice shelf and ice sheet instability: undermelt and grounding line instability, and surface warming and hydrofracturing.”

Domack also highlights that satellite and aircraft measurements alone may not reveal what is happening to an ice shelf:

“It points to the importance of direct sampling of grounding lines beneath these glacier systems in order to understand the true dynamic that may be at play.”