Voice of the Sea Ice: Antarctica’s heartbeat

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It's November 2024. Small red flags flap lazily in the breeze on the sea ice in front of Scott Base.

They mark a safe route through the pressure ridges - where sea ice squashes up against the land and is forced upwards into beautiful towering shapes.

These are slow-moving pillars and waves that, as the days and weeks go on, will continue to grow and bend and crack apart. Weddell seal mums and pups hang out near the gaps in the ice that provide the sea access they need.

Come February 2025 this solid ice is gone. It's a different scene - open ocean, with a pod of orca cruising by, on the hunt for the seals.

A person wearing black and orange cold-weather gear with the hood up stands on a flat expanse of ice criss-crossed with weaving patterns. In the background, jumbles of ice pile up. The buildings of Scott Base and black hills are visible in the background on the right, beneath a cloud-streaked blue sky.

Pressure ridges. Photo: Andrea Foley

The sea ice cycle

This is the annual sea ice cycle in Antarctica, a vital regulator of the planet's climate.

In winter, Antarctic sea ice covers a huge area, about 18 million square kilometres. Its white surface means it reflects sunlight back into space, preventing heat from getting into the Southern Ocean.

Plus, as the ice crystallises, salt is pushed out, forming super salty water. This dense, cold, briny water sinks to the bottom of the sea, flowing off the edge of the Antarctic continent - a yearly replenishment of the deep areas of the world's oceans, which drives global currents.

A wide shot of four figures either standing or kneeling on a white flat expanse of ice with a helicopter nearby. The sun is creating a lens flare in the blue sky, with a smattering clouds on the horizon, above a snow-covered hill.

The sea ice physics team at work. Photo: Inga Smith

But there are signs this cycle is changing.

Sea ice extent (the millions of square kilometres the sea ice covers throughout its cycle) hit a historic low in the 47-year satellite record in 2023, and both 2022 and 2024 weren't that much better.

With a minimum extent close to that historic low minimum of 2023, 2025 looks on course to be yet another poor year for Antarctic sea ice. And in February 2025, global sea ice (both the Arctic and Antarctic combined), hit a record low since satellite monitoring began.

A person in black and orange cold-weather gear, gloves and a hat kneels on a flat expanse of ice next to four black flags planted around a white buoy. The person is holding up something in their left hand towards the sky. The sky is overcast and foggy, obscuring the landscape on the horizon.

Antonia Radlwimmer prepares a sea ice tracking buoy. Photo: Inga Smith

Sea ice includes both land-fast ice and pack ice. Pack ice is the name for floating rafts of ice out in the sea that move with waves and tides. Land-fast ice is locked to the land - that's the kind of sea ice that occurs in McMurdo Sound, the patch of ocean between Ross Island (where Scott Base is located) and the mainland Antarctic coast. It forms a solid white expanse that researchers can camp and live and work on.

Because different things can impact sea ice freezing and melting - the frequency of storms in the winter that flush new sea ice out, a big iceberg hanging out in a sound that means more ice sticks around - researchers have to collect data across a long period to figure out how things are changing, and to use the trends to model what might happen in the future.

Four people in orange cold-weather gear engaged in various activities on a flat expanse of ice in front of a green-and-range Hagglund vehicle. Two people are wielding are long metal drill/pole, and the other two are crouched behind heaped up snow next to an array of flags. The sky is blue with a few bands of cloud.

Sea ice monitoring and coring. Photo: Inga Smith

Monitoring and measuring

New Zealand scientists have been monitoring the sea ice in McMurdo Sound for more than three decades.

The University of Otago Sea Ice Monitoring Station is placed in the ice as it starts to freeze. This long temperature sensor enables researchers to keep track of when and how the sea ice is forming, and how thick it is at any time. Sea ice cores are also collected and transported back to the lab in Dunedin to investigate this further.

A person wearing orange and black cold-weather gear with the hood up crouches on a flat expanse of ice with a conical snow-covered mountain in the background. The person is touching a core of ice on a green mat.

Sea ice core. Photo: Inga Smith

Sea ice trackers are placed on the frozen ice, ready to report back their location when the ice breaks apart and washes out of the sound. Using GPS and satellites, they give updates in real time.

All this information helps researchers put together a picture about the physics of the land-fast sea ice.

Two people kneeling on a flat white expanse of nows underneath a blue sky. They are both wearing goggles, beanies and orange-and-black warm clothing, and both are holding chunks of ice.

Antonia Radlwimmer (left) and Chris Pooley preparing a sea ice core for transport to the University of Otago Physics Antarctic Ice Lab. Photo: Inga Smith

Recent analysis indicates that land-fast ice thickness in McMurdo Sound has not yet felt any strong effects of climate change. Instead, its variability is impacted by more short-term factors like storm events, air temperature and wind speed.

However, there have been some anomalies in the past few years. McMurdo sea ice has formed much later than normal, which is making scientists increasingly concerned that this is the beginning of a very negative trend.

This series was made with travel support from the Antarctica New Zealand Community Engagement Programme.

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