Watch out for icebergs!

Sailing around Cape Horn, Statsraad Lehmkuhl navigates the Drake Strait, the 800-kilometer stretch of sea between South America and Antarctica. Most of Antarctica is covered by ice, and it is not uncommon to see ice drifting in the waters here.

So, the lookout at Statsraad Lehmkuhl looks not only for whales now, but also for icebergs.

Antarctica

The southernmost continent of the globe is huge, 14.4 million km², one third larger than Europe, 9.5 percent of the land area on earth. Most of Antarctica is covered by a huge glacier, 1.6 kilometers thick on average. As much as 70 percent of the globe's freshwater is found here, enough water for the sea to rise by 60 meters if all of it melted.

The glacier that covers Antarctica is in constant motion. The snow that falls inside the continent turns into ice, which slowly moves towards the coast. In some places the glacier extends far into the sea, like large shelves of ice.

Breakdown

From time to time, large pieces of the ice shelves break. Waves and wind split them into smaller pieces, ending up as icebergs floating in the ocean. Ice is lighter than water, so the icebergs do not sink, but float deep in the sea. Only ten percent of an iceberg is above water, and it is these white specks that the lookout on board the Statsraad Lehmkuhl tries to spot.

In mid-March, the Conger ice shelf collapsed completely. Satellite images taken on March 23 show that what was a 1,200 square kilometer large continuous ice surface just a few weeks ago, is now a soup of large and small icebergs.

Surprised

The Conger ice shelf is part of the larger Shackleton ice shelf, on the east coast of Antarctica. It has diminished somewhat in the last couple of years, but the collapse came as a surprise to the scientists.

- East Antarctica is the highest, coldest, driest place in the world and mostly thought of as stable, writes Catherine Walker, a glaciologist at the Woods Hole Oceanographic Institution, writing about the incident.

On March 18 the temperature did rise to minus 11.8 degrees in this area, 40 degrees above what is normal for the season. Tore Hattermann, an oceanographer at the Polar Institute, suspects that the sudden warming triggered the collapse.

- Higher temperature means more precipitation, and a lot of heavy snow may have been the drop that caused the cup to overflow, literally. But an event like this does not happen overnight, it is the temperature in the sea that is most important. If the sea temperature rises, the ice melts from below, and in recent years we have indications of this happening also in eastern Antarctica, he says.

The ice shelves are important, they regulate how much ice from the glaciers inland that ends up in the sea.

The sea level

Before the last ice age, the sea level was between six and nine meters higher than today. Climate scientists Rob DeConto at the University of Massachusetts and David Pollard at Pennsylvania State University struggled for a long time to figure out why. Was it so hot that large amounts of the ice in the Arctic and Antarctic melted away?

No, it was not that hot 120,000 years ago. The researchers knew that the ice that covers much of Greenland was only a few hundred meters thinner than today.

Could the reason behind the high sea level be found in Antarctica. The ice shelves were the answer.

The sea lifts and supports the ice shelves, so that they slow down the movement of the viscous ice masses behind. DeConto and Pollard took the braking effect of the ice shelves completely away from their computer model. Then everything fell into place, and they were able to model the sea level correctly.

A sea level six to nine meters higher did not require extremely high temperatures, warm enough for the ice shelves to disappear was all it took.

Taking the cap off

As the sea gets warmer, the shelves melt from the underside, and become thinner. If it gets hot enough, the ice shelves also melt from the top.

The meltwater flows into cracks in the ice, widens them, and large pieces of the increasingly thinner ice break off. If the shelves disappear completely, the result is like taking the cap off a lying bottle. The ice on land moves faster towards the coast, breaks off, and more ice flows into the sea.

Scary

DeConto and Pollard used the new model to estimate what would happen to the sea level if global warming continued. The result was frightening. The ice in Antarctica is more unstable than scientists previously thought.

The UN Climate Panel report from 2013 estimates that melting ice in Antarctica will cause the sea level to rise by only a few centimeters in the years to 2100.

The model developed by DeConto and Pollard shows that the Climate Panel's estimates may be too low. The sea level can be a whole meter higher in the year 2100, and 15 meters higher in 2500.

- It will literally change the look of our planet, said Rob DeConto to Nature.

The latest report from the Climate Panel, published in 2021, has adjusted the figures, and now takes into account more extreme scenarios.

Impossible to stop

Several of the ice shelves in Antarctica have collapsed in recent years. In 2002, 3,250 square kilometers of the Larsen ice shelf disappeared. In 2008, more than 400 square kilometers of the Wilkins ice sheet collapsed. And now 1,200 square kilometers of the Conger ice shelf is breaking up.

If a total breakdown first starts, it is impossible to stop.

- The ice shelves will not return until the sea gets colder, and it can take thousands of years, DeConto told Nature.

The good news is that the model of DeConto and Pollard shows that we can avoid a complete collapse in Antarctica. But it requires that the global temperature rise slows down, and do not rise above two degrees.

An underwater mountain plateau

In a few days Tore Hattermann leaves for South-America. But this time not to study the ice conditions.

On April 8, he and fellow scientists from the Norwegian Polar Institute will be on board when Statsraad Lehmkuhl departs from Ushuaía, with the course set for Puerto Montt in Chile.

- One of the important goals for our trip is to investigate the Sars Guyot, says Tore Hattermann.

The Sars Guyot is an underwater mountain plateau. The top is only a few hundred meters below sea level in the Drake Strait, midway between South America and Antarctica.

The wind system that blows in a circle around Antarctica sets up a strong ocean current, and Hattermann and his colleagues wants to investigate how the Sars Guyot affects this current.

- It pushes water from the depths up to the surface. The deep water is rich in nutrients, and we will try to find out how it influence the organisms that live here, Hattermann says.

The plan is to stop Statsraad Lehmkuhl completely, and take water samples at various depths down to the top of the plateau.