Are Warm Ocean Currents Melting the Ice in Antarctica?

We went to Antarctica on a research ship to set out instruments, which stayed in the water and took measurements for 2 years. While the instruments were out, we went to a research facility and had a swimming pool full of water turn around on a big merry-go-round for 2 months. We did all this to understand whether warm currents are melting ice in Antarctica [1]. What was the answer? Let us start from the beginning…

and grows thicker as snow falls on it. The parts of an ice sheet that float on the ocean but are still attached to the ice sheet are called ice shelves. They can extend for many kilometers away from the land.

ICE SHELVES
Large ice sheets that flow o land and float on the ocean, but are still connected to the ice that is still resting on land. They can be several hundreds of meters thick.
There, big ice bergs break o and leave a wall of ice that reaches m ( -, ft) down into the ocean.
It is important to distinguish ice shelves and ice bergs from sea ice, resting on land, but their o shore parts float on the ocean. Those parts act as a stopper that keeps the rest of the ice on land. If the floating ice melts, more ice will slide from land into the ocean and cause sea-level rise.
The worst-case estimates predict that the sea level could rise cm/year ( in/year). This is really fast! Imagine a spot near the ocean where you get slightly wet toes right now. Forty years from now, the water level could already be above your head! It is important to be able to predict how fast sea level will rise so that we can prepare for it. Therefore, we need to understand how much ice is really melting and what causes melting.

THE OCEAN IS MELTING ANTARCTICA'S ICE
Ice has been melting faster over the last decades because of climate change, due to warmer air temperatures and because more ice is all around Antarctica. In some places it brings relatively warm water ( -• C; -• F) toward the ice shelves [ ], warm enough to melt ice if water and ice contact each other. But do they?
Since the sea floor around Antarctica is full of narrow, deep valleys, those canyons could funnel the warm water toward the ice, similar to water slides that funnel you down into the swimming pool. If the currents manage to flow underneath the ice shelves, this could increase melting from below. The thinner the floating ice shelves become, the faster ice will slide o Antarctica.
But it is di cult for warm currents to get underneath the floating ice shelves. At their thinnest parts, they still reach -m deep into the water. Imagine the ocean as a room with the ocean surface as its ceiling: the ceiling would suddenly drop by -m where the ice shelves start. This change in height makes it more di cult for currents to flow underneath. In our imaginary room, people wanting to go underneath the dropped ceiling might have to duck or even crawl, which some might be better at than others. Similarly, there are di erent kinds of currents-those that stay close to the ground and dive below an obstacle, and those that cannot. But which sort of currents do we have around Antarctica, and does the warm water actually get close enough to the ice to melt it? There are several ways to find out.

FIGURING OUT HOW FAST THE ICE IS MELTING, AND WHY
It seems like we could easily take a research ship, sail to Antarctica, and observe the currents directly. But there are several reasons why this is not easy. The weather there is bad and the ocean is covered in sea ice during winter, threatening ships, and crews. Therefore, data taken from research ships only exists in selected locations for short periods of time, and only in summer.
An alternative are instruments that stay in the ocean for long periods of time (Figure ). Moorings are anchored to a fixed location on the MOORING Oceanographic instruments that are anchored to the sea floor and stay in the ocean for a certain time period to collect data. Moorings can measure ocean currents and the temperature and salinity of sea water. ocean floor, thus giving measurements in that location specifically. Floats are drifting with the currents and therefore provide data only where the currents take them. Instruments can also be mounted on seals, giving data wherever the seals choose to swim. Gliders are like small submarines and move slowly, remotely controlled through the water, but need a research ship nearby. And, even for instruments, it is dangerous to be too close to the ice edge-there is a lot of both skill and luck involved in deploying and recovering instruments! It stays exciting until the very end: will we find the instruments again, get them back on board, and will they actually have recorded for the full period of time they were in the ocean? The data can only be read from the instruments when they are safely back on board the ship.
A second approach to understanding the warm currents and ice shelves is to simulate the system by building it in miniature (imagine a model railway). Then, we can change the shapes of the ice shelves or the canyons in our model, for example, to understand the impact of each change on the current's behavior in the real world.

MEASURING DIRECTLY IN THE OCEAN
We set out moorings with instruments that can tell us about water temperature and the direction and strength of ocean currents at three sites over a period of years: one right at the front of the ice shelf and two along a canyon that funnels water towards the ice shelf. Data from two moorings showed water flowing toward the ice shelf. The third mooring, closest to the ice shelf, showed the current turning just before reaching the ice shelf. That means the current's warm water does not continue straight underneath the ice shelf. Instead, it turns and flows along the front of the ice shelf before flowing back into the kids.frontiersin.org October | Volume | Article | open ocean. Therefore, the ice is not melting as much as it would if the current went underneath the ice shelf.

RECREATING OCEAN CURRENTS IN A MINIATURE WORLD
In the lab in Grenoble, France, we found the explanation for why the current turns around (Figure ). We used a m diameter pool that rotates, simulating Earth's rotation. We built a plastic canyon to represent our area of interest in Antarctica. We then pumped water into the canyon to create a current. The end of the canyon was covered by a plastic "ice shelf" that we could rise, lower, and tilt to create di erent conditions. We made the currents visible by mixing little plastic particles into the water and lighting them with lasers. Following where particles moved between photographs of the laser-lit particles, we could reconstruct the currents.
For an ice shelf that starts with a steep step, the current nibbles at the ice edge, but it is forced to turn around without flowing underneath the ice. With only very little water movement underneath the ice, there is little melting there. However, if the shape of the ice sheet is changed so that it starts at the sea surface and then gradually reaches deeper into the water, it is easier for currents to move under the ice. An ice shelf of that shape will melt faster. Also, if the structure of the current changes such that only the lower part is moving, it might behave di erently, and more water might be able to get under the ice shelf.

PREDICTING THE FUTURE
Now that we know how the shape of the ice shelves as well as the type of currents approaching them influence how fast ice melts, we can use that to help predict future sea levels.

ANNA WÅHLIN
Professor Anna Wåhlin is a physical oceanographer. She is fascinated by water and ice, and how water and ice interact in the polar seas on Earth. To understand the ocean, she goes on research cruises to collect data, but she also works with laboratory experiments and computer models. She enjoys teaching students about the physics of the oceans at her university, which is situated in Gothenburg, a small city on the West coast of Sweden.