The heartbeat of Antarctica
Step out on the sea ice just outside New Zealand’s Scott Base with researchers studying the physics of its annual cycle. Each year, a massive patch of ocean around Antarctica freezes and then melts again come summer – Antarctica’s heartbeat. In winter, the ice effectively more than doubles the size of this already massive continent, and it plays a huge role in controlling our planet’s climate.
Introducing Antarctica’s heartbeat
RNZ science communicator Dr Claire Concannon explores the sea ice cycle – Antarctica’s heartbeat. In this short clip, she shares a summary from her second episode of Voice of the Sea Ice podcast series.
Select here to view the video transcript and copyright information.
The sea ice cycle
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.
This is the annual sea ice cycle in Antarctica, a vital regulator of the planet’s climate.
Pressure ridges at Scott Base
Pressure ridges form when sea ice pushes up against the land. They are dynamic spaces that continue to form mounds of ice that then crack and break apart. Weddell seals use pressure ridges to access the ocean when they’re on land caring for young pups.
Photo by Claire Concannon.
The global impacts of Antarctic sea ice and the ice albedo effect
Ice and snow reflect a large portion of incoming solar radiation (heat, UV and light from the Sun) back into space, preventing it from being absorbed by the Earth’s surface. This is the basic principle of the ice albedo effect.
In winter, Antarctic sea ice covers around 18 million square kilometres. Its white surface reflects most of the solar radiation back into space. In summer, when there’s less ice and more dark ocean water, more solar heat is absorbed instead of being reflected. The dark water absorbs solar heat, warming the ocean and making it harder for new ice to form. It also melts nearby ice faster. As a result of climate change, the Earth is getting warmer, causing sea ice to melt more quickly and leading to more open oceans and land mass being exposed – creating further warming of the Earth. This is called the ice-albedo feedback.
For an understanding of the different measurements of sea ice go to the end of this article.
Ice-albedo feedback
The ice albedo effect refers to the general tendency of ice and snow to reflect solar radiation due to their high albedo (reflectivity).
Ice-albedo feedback is the process where changes in ice cover, due to warming, affect the Earth’s albedo, which in turn amplifies the initial warming or cooling.
Also in winter, 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 known as the Global Ocean Conveyor. You can learn more about the ocean conveyor belt in Ocean motion.
Signs the sea ice 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, with similar low trends seen in 2022 and 2024.
With a minimum sea ice extent close to the historic low minimum of 2023, 2025 looks on course to be yet another poor year for Antarctic sea ice. In February 2025, global sea ice (both the Arctic and Antarctic combined) hit a record low since satellite monitoring began.
Antarctic sea ice cycle
A time-lapse video showing land fast sea ice 'break out' in the Antarctica summer.
Select here to view the discussion points and copyright information.
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 where researchers can camp, live and work.
Because different things can impact sea ice freezing and melting – like the frequency of storms in the winter that flush out new sea ice or a big iceberg hanging out in a sound meaning more ice stays 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.
Monitoring and measuring
New Zealand scientists have been monitoring the sea ice in McMurdo Sound for more than three decades.
Icefish
The most abundant fish of the Auckland Islands are the icefish of the family Nototheniidae. This family is known for its adaptation to its Antarctic origin – having ‘antifreeze proteins’ in their blood. These proteins ‘attach’ to the surface of ingested ice crystals, halting ice formation and allowing bodily fluids to stay fluid. The icefish at the Auckland Islands are non-Antarctic members of the genus Notothenia – N. angustata and N. microlepidota. These fish have extremely low antifreeze levels in their blood. However, as the antifreeze proteins are present, an Antarctic evolutionary origin of the icefish found at the Auckland Islands can be concluded.
Meet some related Antarctic fish species here.
Why do you think biotechnologists are interested in antifreeze proteins?
Image acknowledgement: Photo by Dave Allan, NIWA.
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 track 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.
The ice and the trackers start drifting northwards out of the region. And yeah, that drift is what we’re interested in. So, when does it start? Where does it go? And what might be driving it? Is it the ocean? Is it the wind? Does it matter how much other ice is around or not?
Sea ice trackers are placed on the ice, ready to report 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 of the physics of the land-fast sea ice.
Sea ice tracker
University of Otago PhD candidate Antonia Radlwimmer prepares a sea ice tracker to collect data on sea ice as it breaks apart and moves away from the ice shelf.
A GPS tracker inside the plastic buoy-like container communicates in real time with the Iridium network of satellites in a low-Earth orbit. Learn more in this explainer from the first year when the trackers were deployed.
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 last few years. McMurdo sea ice has formed much later than normal, which is making scientists increasingly concerned that this is the beginning of a negative trend.
In the podcast Antarctica heartbeat, University of Otago Associate Professor Inga Smith and PhD candidate Antonia Radlwimmer explain their research to better understand the physics of the sea ice annual cycle.
Antarctica’s heartbeat – podcast
This is the second episode of six in the Voice of the Sea Ice podcast series from RNZ science communicator Dr Claire Concannon. It delves into the sea ice cycle in Antarctica, the role it plays in global oceans and climate and the research by scientists to better understand the cycle.
Select here to view the audio transcript, jargon definitions and copyright information.
Measuring Antarctic sea ice
To better understand climate impacts, scientists want to know the volume of sea ice, not just the sea ice extent. How do scientists measure the volume of sea ice across 18 million square kilometres in the Antarctic region?
We know the area of sea ice very well since several decades, so there are satellites out there to do that. But if we want to know how much ice is out there, we need to also know the thickness, because we need to multiply the area with the thickness, and then we know the volume or the mass of the ices.
Collating accurate measurements of sea ice volume is a research interest of Professor Wolfgang Rack. His work includes validating satellite measurements of sea ice. Unlike freshwater ice, sea ice can’t be measured with standard radar technology because salt in the sea ice absorbs the radar wave. Wolfgang and his team undertake manual measurements by drilling into the sea ice. They also use a special device mounted under a helicopter – the EM bird – which uses electromagnetic induction. Laser measurements made at the same time from the helicopter alongside the EM bird data are used to make sea ice measurements.
Validating satellite measurements in Antarctica
Professor Wolfgang Rack from Gateway Antarctica explains how physical and aerial measurements of sea ice are made and then used to validate measurements received from satellites.
Alongside snow and platelet ice measurements, Wolfgang’s research is contributing data for sea ice growth models. These models will aid our understanding about what’s going on with sea ice and the habitat it provides for phytoplankton growth, which underpins the Antarctic ecosystem.
Understanding sea ice concentration, volume, area and extent
Although 'concentration', 'area' and 'extent' may sound the same, they are different measurements, resulting in different numbers, even from the same satellite observations.
Sea ice concentration is how much sea ice exists in the area covered by each grid cell. Concentration is generally expressed as a percentage
Measuring sea ice extent and area
Grid cells are used to calculate sea ice area and sea ice extent. Each grid cell in this simplified representation, represents a square kilometre (in actuality, the area covered is far greater). The numbers show the cell’s estimated percentage of ice cover. This approach excludes cells below the 15 % threshold (blue) but also treats cells above that threshold as ice-filled (white).
Sea ice area and sea ice extent are calculated variables. The calculations require a concentration threshold of 15%, and grid cells with concentrations below that threshold are dropped from the total. Scientists use this threshold because it provides consistent agreement between satellite and surface observations.
Sea ice extent: Sea ice extent is a measure of the total area of ocean that has at least 15% sea ice. This is calculated by adding up all the areas of the grid cells with 15% or more concentration.
Sea ice area: Sea ice area measures the actual amount of ice within that extent. It is calculated by multiplying the area covered by each grid cell by the percentage of sea ice it contains, provided it has at least 15% concentration.
Sea ice volume measures the total amount of ice, taking into account the sea ice area and also the thickness of the sea ice. So, sea ice volume = Concentration × Area × Thickness.
Measurement information and descriptions are courtesy of Michon Scott, the National Snow and Data Center, University of Colorado. Learn more in the article: What is the difference between sea ice area and extent?
Student activities
These activities visually demonstrate the implications of melting:
Simulate the work of Antarctic scientists by making ice cores in Pringles® tubes to investigate the relationship between the brightness of light and length of ice cores.
Related content
Antarctic sea ice decline and modelling looks at sea ice around Antarctica, and Climate change, melting ice and sea level rise explores ice sheets and ice shelves.
Meet Professor Wolfgang Rack, one of the scientists also involved in sea ice monitoring for the Antarctic Science Platform’s Sea Ice and Carbon Cycle Feedbacks project.
Voice of the Sea Ice is a six-part RNZ podcast series that delves into science research and adventure in Antarctica. Explore the other episodes:
A land of ice and ambition: Learn about different types of ice, crevasses and the work to map safe routes across Antarctica for scientists and their equipment.
Antarctic life – microalgae: Introduces microalgae (zooplankton) that live on the bottom of the ice and among the platelet ice layer just below it.
Antarctic life – penguins, seals and fish: Scientists are investigating the Antarctic food web to better understand the interconnections.
Changing times in Antarctica: Scientists talk about what sea ice decline means for the world and how they feel about it.
Climate change – where to? Human-induced climate change is impacting Earth’s global systems, including ice melt in Antarctica. What is the world doing to combat it? Where does New Zealand fit in, and are we doing our bit as a nation?
For resources that provide a deeper exploration of other ice forms in Antarctica, go to Icebergs and Glaciers.
Satellites are an important tool in many different types of research. Take a closer look at some research in Antarctica that has used satellites:
Use these articles below to explore how rising global temperature is causing both land ice and sea ice to melt:
Rising seas – a Connected article
Antarctica tipping points looks at the irreversible changes we could be facing if we fail to keep global warming below 2°C.
Useful links
Learn more about platelet ice in this well-illustrated article by NIWA marine physicist and polar oceanographer Dr Natalie Robinson.
Read about the role Antarctica plays in global climate and in turn how the global climate impacts Antarctica in this NIWA article.
Learn about work to understand sea ice decline in The Conversation article, ‘Completely unexpected’: Antarctic sea ice may be in terminal decline due to rising Southern Ocean salinity. The expert reaction to this research, Sea ice loss linked to salty Southern Ocean surface – Expert reaction was published by the Science Media Centre. It includes reactions from Dr Inga Smith, Professor Wolfgang Rack and Dr Natalie Robinson amongst others.
RNZ has a number of podcasts that explore physics and sea ice, including Physics on ice, Why is Antarctic sea ice vanishing? and The sea ice factories of Antarctica.
For sea ice statistics, look at these resources and articles:
Acknowledgement
This resource is adapted from the work of Dr Claire Concannon for the RNZ podcast series Voice of the Sea Ice. The series was made with travel support from the Antarctica New Zealand Community Engagement Programme.
