Antarctica enters a new ice-free era: the disappearance of sea ice is changing life in the Southern Ocean
In recent years, Antarctica has been changing faster than scientists expected, and one of the most noticeable changes is taking place where seasonal regularity long prevailed – in the extent of sea ice that spreads and retreats around the continent. While the melting of ice in the Arctic has for decades been one of the most recognizable symbols of climate change, it is now becoming increasingly clear that Antarctica, too, is undergoing a profound shift. New European research, funded through European Space Agency programmes, shows that the sharp decline in Antarctic sea ice is not merely a physical change in the polar environment, but a process that is reshaping the entire marine ecosystem, starting from the very base of the food chain.
This is a change that is not visible only on ice maps or satellite images. Its consequences are spreading throughout the Southern Ocean: from microscopic algae that convert solar energy into organic matter, through krill and salps, all the way to fish, seals, penguins and whales. Behind this change lies a transition into what part of the scientific community is increasingly openly calling a new era with less ice. Such a development is causing concern not only because of local biodiversity, but also because of Antarctica’s role in the global climate system, especially in carbon storage and the regulation of heat on the planet.
A turning point that surprised both models and researchers
For years, Antarctic sea ice was considered more complex and less predictable than Arctic sea ice. Unlike the Arctic, where the long-term downward trend was clear and strongly documented, Antarctica long showed considerably greater variability. But around 2016 and 2017, a sudden, almost step-like drop in sea-ice extent occurred. An area of ocean comparable in size to Greenland lost its seasonal ice within just a few years, after a period that had been relatively stable.
In the first reactions, some researchers believed this was a temporary anomaly. However, later observations and a series of new analyses showed that conditions did not return to previous levels. Earlier papers had already warned that the 2016 decline was the most pronounced in the satellite era, and more recent data further confirm that sea ice around Antarctica remains at very low levels. According to the US National Snow and Ice Data Center, the Antarctic annual minimum in March 2025 was 1.98 million square kilometres, placing it among the lowest values recorded since satellite measurements began in 1979. This means that extremely low values are not appearing as an isolated incident, but as part of a worrying sequence.
For climatologists and biologists, the problem was twofold. On the one hand, many computer models did not capture such an abrupt change well, because they are more inclined toward gradual trends than tipping points. On the other hand, field research in Antarctica is logistically demanding, seasonally limited and expensive, so scientists were left without enough direct observations precisely at the moment when the environment began changing rapidly. That is exactly why the new study turned to an approach that is almost irreplaceable in such conditions – long-term observation from space.
What satellites revealed about changes in the sea
The team led by Plymouth Marine Laboratory in the United Kingdom analysed data from ESA’s Climate Change Initiative Ocean Colour project, which combines ocean-colour measurements from multiple satellite missions. Such data do not show only an aesthetic difference in shades on the sea surface. They make it possible to assess biological conditions in the water, because the way the ocean reflects sunlight at certain wavelengths reveals how much phytoplankton is present in the water and which groups dominate.
Based on these optical signals, the researchers divided the Southern Ocean into so-called “seascapes”. Just as forest, mountain or wetland landscapes are distinguished on land, the sea can also be classified according to dominant biological and optical characteristics. In this case, that meant it was possible to track not only the quantity of phytoplankton, but also changes in the structure of the communities that form the foundation of the Antarctic food web.
The results did not point to a simple story of “more food, so everything is better”. On the contrary, it turned out that large and distant parts of the Southern Ocean had shifted from a state of very low productivity to more moderately productive areas. On average, nearly 70 percent of the studied area now records higher phytoplankton concentrations during summer than before the sharp decline in ice began about ten years ago. But that increase is not equally beneficial to all organisms.
Why phytoplankton is more important than it seems at first glance
Phytoplankton are microscopic algae and other tiny photosynthetic organisms that drift freely in the water. Although invisible to the naked eye, they support a huge share of life in the ocean. In the Antarctic system, diatoms, larger silica-rich algae, are especially important because they transfer energy very efficiently to higher levels of the food chain. When conditions are favourable, diatoms feed organisms such as Antarctic krill and, through it, larger animals as well.
In this story, sea ice is not just a frozen surface. It creates shelter, a nursery and specific microhabitat conditions. Dense communities of algae develop on and beneath the ice, and the ice also affects the stability of the upper layer of the sea, the amount of light and the availability of nutrients. When there is less of this ice, it is not only a physical barrier between air and sea that disappears, but the entire regime of food production in the water changes as well.
In a separate review this year of changes in Antarctic plankton, ESA warned that after 2016 a decline in diatoms was recorded on the continental shelf, while smaller phytoplankton groups strengthened sharply. This matters because a change in the “quality” of food can be just as crucial as a change in its quantity. In practice, that means more phytoplankton does not automatically mean better conditions for organisms that depend specifically on certain groups of algae.
Krill, a key species of the Antarctic world
At the centre of this story is Antarctic krill, a small crustacean resembling a shrimp, but with enormous ecological weight. The British Antarctic Survey states that this is a species whose abundance is measured in hundreds of trillions of individuals, with approximately 780 trillion adults often mentioned, not including eggs and larvae. Krill is distributed throughout the Southern Ocean and occurs at different depths, and its role in the diet of other species is hard to overstate.
Penguins, whales, seals, many fish and other organisms depend heavily on krill as a source of energy. In addition, through grazing on phytoplankton, krill participates in the cycling of carbon, nitrogen and other important elements. When it feeds in surface layers and then excretes or dies and sinks, it helps transfer carbon toward the deep ocean. That is why krill is important not only as food for higher species, but also as part of the mechanism by which the ocean mitigates climate change by removing part of the carbon from the atmospheric cycle.
That is precisely why any longer-term change in habitat suitable for krill goes beyond the biology of a single species. It can mean a different distribution of food for animals at the top of the food web, a different seasonal breeding dynamic and a different efficiency of the ocean in long-term carbon storage.
Salps as unexpected winners of change
Alongside krill, the researchers also analysed salps in particular – transparent, gelatinous filter feeders that feed on plankton. Although at first glance they may seem less important than krill, salps have long been known as organisms that often thrive in conditions that are not ideal for krill. They can live individually or form long chains of individuals, and under favourable circumstances their numbers can explode rapidly.
The new study linked satellite-defined seascapes with the KRILLBASE database, one of the most important historical collections of data on krill and salps in the Southern Ocean. This database includes decades of field samples and makes it possible to compare today’s conditions with a multi-decade picture of the distribution of these two groups. The analysis suggests that the new era with less ice has become more favourable דווקא for salps. They were strongly associated with the types of seascapes that expanded after the decline in sea ice, especially in parts of the Indo-Pacific sector of the Southern Ocean.
At first glance, someone might conclude that it makes no difference whether krill or salps dominate more, as long as there is an abundance of planktonic organisms. But the difference is large. Salps have different nutritional value, fit differently into food chains and participate differently in the carbon cycle. Scientists point out that they contain less carbon than krill and contribute less efficiently to the transfer of carbon into the deep ocean. In other words, a change that on the surface looks like one group replacing another can have consequences for the entire ecosystem, but also for the climate function of the Southern Ocean.
More food does not necessarily mean a healthier ecosystem
One of the most complex conclusions of the research is that an increase in summer phytoplankton concentration should not automatically be interpreted as good news. In many ecosystems, greater primary production would be considered a positive signal. But the Antarctic system depends on very specific relationships between ice, light, water mixing, plankton composition and the seasonal availability of food. When those relationships are disrupted, an increase in one component can conceal the loss of another, perhaps more important component.
That is why scientists emphasize the difference between the quantity and the quality of food. If conditions increasingly favour smaller phytoplankton groups, and favour less the diatoms that are especially important for krill, then the ones that will benefit most from “more food” will be those organisms that adapt more easily to such a change. In this case, those are salps. This increases the possibility of a long-term reshuffling of dominance within the pelagic ecosystem, with consequences that are only beginning to be understood.
Such a change is especially important because of its economic dimension as well. Krill is subject to commercial harvesting, whereas salps are not. If the environment becomes less suitable for krill, this will affect not only wild species that feed on it, but also discussions about fisheries management, habitat protection and the international policy of Southern Ocean conservation.
Antarctica as a signal of global climate instability
Changes in sea ice around Antarctica do not happen in isolation. Earlier scientific studies pointed to a combination of causes: long-term ocean warming, the inflow of warmer air southward, changes in winds, and links with climate patterns in the tropics. This means Antarctic ice responds to a complex network of processes in the atmosphere and the ocean, and not to just one factor. That is precisely why its behaviour was long difficult to predict.
But what is especially important now is the fact that the biological consequences can no longer be viewed as hypothetical. Satellite data, combined with field databases and new methods of analysis, show that the reorganization of the system is already under way. Antarctica is changing not only as an icy landscape, but as a living ocean in which different species gain an advantage and previous balances weaken.
For the wider public, this is important also because the Southern Ocean plays an exceptional role in regulating Earth’s climate. It absorbs heat and carbon and influences global ocean currents. If the relationships between phytoplankton, krill and salps change in that area over the long term, the consequences will not remain confined to a distant polar belt. They can affect the speed and manner in which the ocean binds carbon, and thus the overall resilience of the climate system.
Space as a key tool for tracking change
In this context, the value of satellite observation stands out in particular. Antarctica is vast, difficult to access and extreme, so there is no realistic possibility that all key changes can be monitored only by ships and seasonal expeditions. Satellites provide a continuous, broad and long-term view of the ocean, which is crucial when abrupt changes occur or when patterns stretching across the entire continent are being sought.
This is precisely one of the main messages of the new paper: without space-based measurements, it would have been significantly more difficult to detect how declining ice is changing the feeding habitats of two key plankton groups. Such data do not serve only the academic understanding of the problem. They are also becoming important for future conservation strategies, for marine resource management and for shaping climate policies that must take into account that tipping points in the polar environment can occur faster than models predict.
As Antarctica moves ever more convincingly toward a longer-lasting period with less ice, it is becoming clear that this is a change affecting the entire system – from microscopic algae to the largest marine mammals, from local feeding relationships to the global carbon cycle. In this new Antarctic landscape, the question is no longer whether change is happening, but how deeply it will reshape one of the most important marine ecosystems on Earth.
Sources:- European Space Agency (ESA) – overview of research on changes in Antarctic plankton and the connection with sea-ice loss (link)
- Plymouth Marine Laboratory – summary of the study on changes in “seascapes”, phytoplankton, krill and salps in the Southern Ocean (link)
- Marine Ecology Progress Series / PlyMSEA – original scientific paper “Implications of the recent loss of Antarctic sea ice for phytoplankton and summer feeding habitats of salps and krill” (link)
- National Snow and Ice Data Center (NSIDC) – official data on the very low annual Antarctic sea-ice minimum in 2025 (link)
- Nature Geoscience – review paper on the sharp decline in Antarctic sea ice after 2016 and possible causes (link)
- British Antarctic Survey – description of the KRILLBASE database and data on the ecological importance of Antarctic krill and salps (link)
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