Antarctic sea ice in crisis
In February 2024, data from the US National Snow and Ice Data Center (NSIDC) revealed a third consecutive summer of exceptionally low sea-ice extent around Antarctica.
Sea-ice extent around the frozen continent was measured at 1.99 million square kilometres, with scientists suggesting a ‘regime shift’ could be underway.
Every year, sea-ice coverage around Antarctica expands between autumn and winter to cover up to 19–20 million square kilometres of the Southern Ocean – some four per cent of the Earth’s surface and one and a half times the area of the Antarctic continent.
By the end of summer, it contracts to 2–4 million square kilometres.
This regular cycle of freezing and thawing keeps our planet functioning.
But in February 2023, summer sea-ice extent reached only 1.77 million square kilometres – a record low since satellite records began in 1979, and 36% less than the long-term summer average.
Worse was to come.
On September 10, 2023, Antarctica’s maximum winter sea-ice extent dipped to a new record low of 16.96 million square kilometres.
Above graph: Daily observations of overall Antarctic sea-ice extent [SIE] reveal 2023 (dark green line) as the lowest winter extent since records began in 1979. The light green line shows 2014 data, with the record high (winter) Antarctic SIE of 20.12 million square kilometres. The dark green line shows the 2023 data, with the current record low summer and winter Antarctic SIE. The black line shows the long-term mean SIE based on 1981–2010, with grey traces showing the remaining observations for each year since 1979. Credit A. Steketee and P. Heil using data from NSIDC (data from 11 February 2024).
Below video: Animation showing the Antarctic sea-ice minimum extent, February 21 2023, to its maximum, September 10 2023. Credit: NASA's Scientific Visualization Studio.
The sudden decline comes after more than 30 years of a small, steady gain in sea-ice coverage, including a record winter high in 2012 and another in 2014.
Then, in the spring of 2016, Antarctic sea-ice cover fell to a (then) record low, and has been below average for most years since.
Research points to ocean warming as playing a key role in the deficit of sea ice around Antarctica since 2016. Scientists say Antarctic sea ice may have been pushed to new state of diminished coverage (similar to that in the Arctic), from which it may not recover.
Australian Antarctic Division sea-ice scientist, Dr Petra Heil, said the decline in sea ice has serious consequences.
“The Antarctic sea-ice deficit has direct impacts on the climate and ecosystems, both nearby and further afield, including at lower latitudes, which are home to the majority of the human population and their economic interests,” she said.
“While satellite-derived products and model simulations have been invaluable to our understanding, we urgently need to intensify our field research to obtain critical observations now.”
Beating heart of the planet
Antarctic sea ice has been described as the beating heart of the planet, as it expands in winter and contracts in summer.
When sea ice forms, at about −1.8°C, salt is expelled into the ocean’s surface waters. This brine drainage produces a pool of very cold, salty, oxygen-rich water that sinks to the seafloor to form ‘Antarctic Bottom Water’.
Large amounts of sea ice and Antarctic Bottom Water form in ‘sea-ice factories’, or ‘polynyas’ – areas of open water on the continental shelf, where strong winds blowing down from the Antarctic Plateau, create the cold surface conditions needed to promote rapid sea-ice formation.
Antarctic Bottom Water spreads around Antarctica and connects with the Atlantic, Pacific and Indian oceans as part of a ‘global overturning circulation’.
Through this circulation, cold water moves from Antarctica towards the Equator, with some making it as far as the sub-Arctic oceans.
Along the way, it warms and mixes vertically between the deep ocean and the surface. The water then recirculates back towards Antarctica.
This process transports and recycles heat, carbon, oxygen and nutrients around the world's oceans.
Recent ocean modelling and physical measurements suggests there will be further deep ocean warming, and a slowing of the global overturning circulation, over the next few decades, with global consequences.
Learn more about the global overturning circulation, and why Antarctic Bottom Water is so important to this circulation and the world’s climate and ecosystems, in this Australian Academy of Science video.
Let’s get physical
There are two major types of sea ice.
The first is ‘landfast ice’ or ‘fast ice’, which is attached to the Antarctic coastline, islands or to grounded icebergs.
The second type of sea ice is ‘pack ice’, which is made up of ice ‘floes’ that move with winds, tides, currents and waves.
Scientists also recognise the importance of the ‘Marginal Ice Zone’ (MIZ), which provides a transition between the open ocean and dense pack ice.
“The MIZ is a dynamic area of interaction between the surface ocean and atmosphere, which drives important global processes,” Dr Heil said
All sea ice acts like an insulating blanket over the ocean, moderating the exchange of heat and gases (including carbon dioxide) between the Southern Ocean and relatively cooler atmosphere in autumn and winter.
Snow that accumulates on top of the sea ice, contributes to this insulation, as well as the reflection of sunlight back into space. Snow-covered sea ice reflects 80 to 90 per cent of solar radiation, compared to about seven percent for the ice-free ocean.
Sea ice also supports a range of biological and chemical processes that release gases and aerosols into the atmosphere. These contribute to cloud formation, which in turn affects the amount of sunlight reaching the Earth’s surface.
When there is less sea ice, more heat escapes from the ocean into the cooler atmosphere, potentially increasing storminess and precipitation (primarily snowfall) over the Southern Ocean and Antarctica – the flow-on effects of which need further study.
Less sea-ice cover means the darker ocean absorbs more solar energy, which warms surface waters and exacerbates the cycle of sea-ice loss and the release of heat into the atmosphere.
The health of the Southern Ocean’s sea-ice cover is crucial to the stability of ice shelves that skirt much of the Antarctic continent.
Ice shelves play a critical role in slowing the gravity-driven flow of the ice sheet into the ocean, helping to moderate the ice sheet’s contribution to sea level rise.
By forming a narrow, consolidated band around much of the Antarctic coast, fast ice helps to minimize ice-shelf loss by iceberg calving.
At the same time, loss of pack ice and fast ice increases the exposure of Antarctica’s coastline to potentially damaging ocean swells.
This increases the susceptibility of ice shelves to calving, and can trigger large-scale ice-shelf disintegration and sea-level rise.
Changes in sea-ice coverage and processes can also influence the entry of warm ocean waters underneath the ice shelves, which melts them from below.
Wildlife haven
Sea ice provides critical habitat and ecosystem functions for a range of marine mammals, birds and smaller marine organisms that are highly adapted to its presence and seasonal rhythms.
These include a variety of penguins, seals and whales, as well as fish, krill and phytoplankton species.
Two species that are especially vulnerable to changes in sea ice are emperor penguins and Weddell seals, which both rely on stable fast ice to raise their young.
Reports of huge mortalities amongst emperor penguin chicks in 2022, because of a record low sea-ice extent, highlight its importance.
AAD seabird ecologist Dr Barbara Wienecke, wrote in The Guardian: “It is important to understand that it does not take the complete loss of sea ice, but a shortening of the sea-ice season, to threaten the emperors’ continued existence.
“Unfortunately, since the rate at which their environment is changing is faster than the rate at which they can adapt, we are likely to witness the demise of an extraordinary species that has graced Earth for millions of years.”
Sea ice is also critical to prey species, including Antarctic krill, a ‘keystone’ species in the Southern Ocean food chain.
Krill relies on sea ice during key phases of its life cycle for food (sea-ice algae) and as a refuge from predators.
AAD krill scientist, Dr So Kawaguchi, found some populations of Antarctic krill are shifting south, closer to Antarctica, as ocean warming and sea-ice changes alter their habitat.
This shift is associated with a reduced occurrence of krill swarms and a decline in krill density.
“Large changes in krill density in space and/or time has major implications for their food, their predators and for carbon and nutrient cycling,” Dr Kawaguchi said.
Carbon sink
Sea ice provides an environment for the growth of sea-ice algae and phytoplankton blooms. These tiny marine plants provide food for krill, other small grazers, and their larvae, at different times of the year.
When the sea ice melts in spring and summer, it releases the ice algae, freshwater and nutrients, including iron, into the water. This fuels the formation of intense phytoplankton blooms.
These phytoplankton blooms use sunlight and atmospheric carbon dioxide to grow and reproduce – a process called ‘primary production’. When the blooms are eaten by grazers that then defecate or die, much of this carbon sinks to the deep ocean, where it is stored.
On the Antarctic Peninsula, scientists have observed a shift to smaller phytoplankton species as a result of changes in sea-ice extent and duration. These smaller phytoplankton are a less suitable food source for krill, which only graze on phytoplankton bigger than 0.006 mm.
Dr Heil said further research, in the form of long-term physical-biological-biogeochemical observing and research programs, would help scientists understand the response of different components of the marine ecosystem to sea-ice changes.
Sea-ice research
The Australian Antarctic Program has a range of long-term observing and other research activities to understand the nature and impacts of sea-ice change.
The data collected from this work will help validate and calibrate satellite measurements of sea ice, and improve satellite-derived products.
These products allow scientists to scale up more localised measurements, and provide an Antarctic-wide picture of the sea-ice environment.
Sea-ice concentration, extent and seasonality are well characterised from satellite data, and form the basis of major assessments of sea-ice change and variability since the late 1970s.
However, this is not the case with sea-ice thickness and its snow cover. As a result, there is no accurate baseline information on these variables and whether they are changing.
Dr Heil said physical sea-ice measurements were essential to building an accurate picture of how ocean, ice and atmospheric processes contribute to sea-ice changes, and providing baseline data to ensure satellite-derived products and model forecasts and predictions are accurate.
These field measurements are made using a variety of instruments (see the video below for some examples).
These include ice corers to study ice-crystal structure, salinity and density.
Snow radars and electromagnetometers are used to derive snow and sea-ice thickness.
Autonomous underwater instruments measure the sea-ice profile from below, as well as its temperature and biological properties.
Snow pits and snow microprobes provide profiles of snow density, structure and temperature.
Together, satellite and field measurements, and experimental research, contribute to better models of the Antarctic ice-ocean ecosystem, as well as weather and climate models.
Underway observatory
With the commissioning of Australia’s icebreaker, RSV Nuyina, scientists from around the world have access to one of the most advanced sea-ice research platforms.
Facilities include the Ocean–Sea Ice–Atmosphere (OSIA) underway observatory – a permanent, automated suite of instruments that take continuous measurements of the ocean, sea ice, snow cover and atmosphere, as the vessel undertakes its operational or marine-science missions.
Information from additional systems on the ship that monitor ocean biology, deep-ocean properties, and seafloor structure, also contribute to a complete picture of the environment.
Among the instruments in the OSIA observatory are seven cameras that allow scientists to measure important ocean-state and sea-ice variables.
These variables include wave heights, ice-floe size and distribution, ice thickness, chlorophyll content (from algae) in the ice, surface melt, temperature and brightness, and wave penetration into the sea-ice zone.
Atmospheric instruments include a ‘lidar’ to measure aerosols that contribute to cloud formation, a ‘ceilometer’ to measure cloud height and a ‘disdrometer’ and ‘micro rain radar’ to measure rain and snowfall.
The ship can also deploy instruments from its bow, including a snow radar, an electromagnetic instrument and a point laser, that together measure the thickness of snow and sea ice layers above and below the ocean surface.
The OSIA observatory and other ship-based measurements add to almost 30 years of ocean, sea-ice, snow and atmosphere observations work, undertaken from other ships and through field work and remote sensing (satellites), by the international Expert Group on Antarctic Sea-ice Processes and Climate (ASPeCt).
Fast-ice network
Scientists and other personnel at Australia’s Antarctic stations also conduct regular, long-term sea-ice measurements, using dedicated autonomous instruments placed on top of fast ice, or through the ice into the underlying water column.
These instruments form part of the ‘Antarctic Fast-Ice Network’ (AFIN).
“Some of these instruments measure the physical strength and cohesion of the ice, others measure the ice temperature, thickness, crystal structure, light penetration and biological properties, as well as the underlying ocean temperature and chemistry,” Dr Heil said.
“These measurements all contribute to a better understanding of sea-ice processes and its interaction with the ocean and atmosphere and any biological or geophysical components.”
Recently, the AFIN site at Davis Station was accepted as a World Meteorological Organisation Global Cryosphere Watch CryoNet site.
“The idea behind this is to have sustained and high quality observations from representative sites across the world’s cryosphere, including glacial ice sheets and shelves, snow, permafrost and sea ice,” Dr Heil said.
“These data feed into weather prediction and climate models and are used to derive products from satellite observations.
"Information derived from these long-term sites is really important for assessments such as the climate and state of the environment reports. And of course it’s important for ongoing scientific research.”
Sea-ice mapping and reporting
Sea-ice researchers are also benefitting from a new interactive sea-ice mapping tool.
‘Nilas’ displays multiple layers of historical ice, ocean and atmospheric data and allows scientists to overlay ship movements and animal or instrument tracks, for planning and research.
The tool includes sea-ice data dating back to 1979, chlorophyll data from 1998 onwards, and sea surface temperature from 1981 onwards.
“We can use this tool to plan marine-science voyages, based on past sea ice conditions, and pinpoint deployment locations for autonomous instruments to study ice-edge processes, such as wave-induced ice breakup,” Dr Heil said.
“There are many ways to look at sea ice in Antarctica, but our tool brings together a diverse set of observations to explore Earth system characteristics and processes that are relevant to the Australian Antarctic Division and the Australian Southern Ocean science community.”
Nilas is also being used to generate monthly sea-ice report cards that provide decision makers and the general public with the latest sea-ice information.
East Antarctic Monitoring Program
An East Antarctic Monitoring Program (EAMP) is currently being developed by the Australian Antarctic Division to provide situational awareness of East Antarctica and the Southern Ocean.
It will do this by making long-term scientific observations of essential climate, ocean, and biodiversity variables in the region.
“A better observational capability, across Antarctica and across different scientific disciplines, will improve our ability to predict future climate and Earth-system change,” Dr Heil said.
“It will also provide a foundation to assess and measure impacts for ecosystems in Antarctica, the Southern Ocean and further afield, in mid- and low-latitude regions.”
What’s next?
With the magnitude of recent change in and around Antarctica exceeding previously observed rates of change, there are many questions to be answered.
Dr Heil said a program of sustained observations was essential to addressing them.
“Studies have reported that the Earth climate system is out of energy balance, due to greenhouse gas emissions, and 2023 has officially been confirmed as the warmest year on record,” she said.
“The window for choosing a future with limited warming is closing.
“Given Antarctica’s global reach, if we want to understand the risks to Australia from climate tipping points, we need to improve our knowledge of processes and change in Antarctica and the Southern Ocean, including the sea-ice zone.”
Sustained observations will improve satellite-product development and climate and ecosystems models, and their predictive capability, allowing scientists to explore what different emissions scenarios will mean for the future of the Antarctic region.
Further reading
Information papers
- Antarctic sea ice: Physical role and function
- Antarctic sea ice: Biological importance
- Antarctic sea ice: Trends and future projections
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