Oceans

Sea surface temperature 

Average sea surface temperature in the Australian region has warmed by 1.08 °C since 1900, with 9 of the 10 warmest years on record occurring since 2010. This rate of warming is close to that of the global mean sea surface temperature. The year with the highest average sea surface temperature on record was 2022, which was associated with a strong negative Indian Ocean Dipole event and mass coral bleaching in the Great Barrier Reef, which had never previously occurred in a La Niña event. Extremely high Australian region sea surface temperatures have previously been associated with the end of significant El Niño events.

The greatest ocean warming in the Australian region has occurred in the Coral Sea, and off south-east Australia and Tasmania where more rapid warming trends have occurred over the past 4 decades. The East Australian Current now extends further south, creating an area of more rapid warming in the Tasman Sea, where the warming rate is now twice the global average. There has also been warming across large areas of the Indian Ocean region to the west coast of Australia.

Warming of the ocean has contributed to longer and more frequent marine heatwaves. Marine heatwaves are periods when temperatures are in the upper range of historical baseline conditions for at least 5 days. Heatwaves in the ocean often persist much longer than heatwaves on land, sometimes spanning multiple months or even years.

The increasing frequency of marine heatwaves around Australia in recent years has contributed to permanent impacts on marine ecosystem health, marine habitats, and species. These impacts include depleting kelp forests and seagrasses, a poleward shift in marine species, and increased occurrence of disease.

Ocean heat content 

The world’s oceans are a major component of the Earth’s climate system and have a profound effect on the climate, taking up vast quantities of heat from the atmosphere and redistributing it. Seawater stores about 4 times more heat for every degree of temperature rise than dry air of the same weight. The total weight of water in the ocean is about 280 times greater than the weight of the Earth’s atmosphere, so the capacity for the ocean to store heat is vast. The way the ocean redistributes this heat influences our weather patterns and the climate change signal we see in temperature and rainfall.

While the temperature changes over the whole ocean depth are small compared to those at the land and ocean surface, the ocean has taken up more than 90% of the excess energy in the Earth system arising from enhanced greenhouse gas concentrations. Oceans have therefore slowed the rate of warming near the Earth’s land and ocean surface. Heat absorbed at the surface is redistributed both horizontally and vertically by ocean circulation. As a result, the ocean is warming both near the surface and at depth, with the rate of warming varying between regions and depths.

Ocean warming has accelerated since the early 2000s. In 2023, the global ocean heat content was the highest on record, with an estimated additional 42.8 ±1 x 10²² joules of energy relative to 1960. The Southern Ocean has taken up more than half of that excess heat, as its circulation takes heat from near the surface and transfers it into the deep ocean. A warming ocean affects the global ocean and atmospheric circulation, the cryosphere, global and regional sea levels, oceanic uptake of anthropogenic CO₂, and causes losses in dissolved oxygen and impacts on marine ecosystems.

Regionally, ocean warming can vary substantially from year to year due to climate phenomena such as the El Niño-Southern Oscillation. In areas of strong warming, changes in heat content can be several times larger than the global mean change. This is the case in the oceans around Australia, where strong warming results from a redistribution of heat due to changes in the structure of the East Australian Current, and enhanced heat uptake in the subantarctic region south of Australia.

Year-to-year changes in ocean heat content associated with interannual climate variability are large in the top 300 metres of the ocean but have little impact on the waters below. On long-term timescales, the deep waters below 2,000 metres have also warmed throughout most of the global ocean, but there are far fewer observations of the deep ocean and the magnitude of this warming is less certain. Maintaining the global ocean observing system and expanding its coverage in the deep ocean, the polar oceans, and continental shelves will be critical to prepare for, and adapt to, a changing climate.

Marine heatwaves and coral reefs

Warming oceans, together with an increase in the frequency, intensity and duration of marine heatwaves, pose a significant threat to the long‑term health and resilience of coral reef ecosystems. Mass coral bleaching events have occurred with increasing frequency and extent around the world since the 1970s, including on the Great Barrier Reef. Mass bleaching is a stress response of corals that occurs primarily due to elevated ocean temperature. Recovery is possible, but mortality can occur if the thermal stress is too severe or prolonged. Ocean acidification places further stress on corals.

Four mass coral bleaching events have occurred on the Great Barrier Reef over the past 10 years: in 2016, 2017, 2020 and 2022. In 2016, bleaching was associated with record high sea surface temperatures, which in turn led to the largest recorded mass bleaching on the Great Barrier Reef. The impact of the 2020 mass bleaching event appears to be second only to 2016 and was associated with severely bleached coastal reefs along the entire 2300 km length of the Great Barrier Reef. The 2022 event was the first time that mass bleaching has occurred on the Reef during a La Niña year.

These four recent bleaching events are associated with marine heatwaves driven by anthropogenic climate change. Rapidly recurring bleaching events do not give the reef ecosystem time to fully recover.

In 2022, bleaching was also observed on some reefs on Australia’s west coast, including Ningaloo Reef. This was due to warm ocean temperatures, driven by the 2021–22 La Niña. The region’s last severe marine heatwave was driven by the 2010–11 La Niña, which resulted in bleaching being recorded for the first time on Ningaloo and the closure of several Western Australian fisheries.

Climate models project more frequent, extensive, intense and longer‑lasting marine heatwaves in the future. Worsening impacts on coral reefs from marine heatwaves are expected in the future with continued warming. The intensification of marine heatwaves is much greater under high greenhouse gas emission scenarios. This implies more frequent and severe coral bleaching events are likely, leading to the potential loss of many types of coral and impacts on reef fisheries. Along with ocean acidification and nutrient runoff, the increased severity and frequency of marine heatwaves are likely to reduce reef resilience and hinder coral recovery from future bleaching events.

Sea level

Global mean sea level has risen by over 22 cm since 1900, with half of this rise occurring since 1970. Rising sea levels pose a significant threat to coastal communities and coastal ecosystems by amplifying the risks of coastal inundation, storm surge, erosion and saltwater intrusion into groundwater systems. Coastal communities in Australia are already experiencing some of these changes.

Global mean sea level rise is accelerating. Tide gauge and satellite altimetry observations show that the rate of global mean sea level rise increased from 1.5 cm (± 0.2 cm) per decade from 1901 to 2000, and is now approaching 4 cm (± 0.4 cm) per decade from 1993 to 2023. The dominant cause of global mean sea level rise since 1970 is anthropogenic climate change.

As the ocean warms it expands, causing sea levels to rise. This thermal expansion has contributed about one-third of the sea level rise observed globally. Ice loss from glaciers and polar ice sheets, together with changes in the amount of water stored on the land contribute the remaining two-thirds of the observed global sea level rise.

Confidence in assessing changes in global mean sea level has continuously improved because there has been more analysis of satellite altimetry data, and because the data record becomes longer over time. Ongoing research has also resulted in increased confidence in quantifying the various contributions to sea level rise, and a greater understanding of the processes involved.

Australia, like other nations, is already experiencing sea level rise. Sea level varies from year to year and from place to place, partly due to the natural variability of the climate system from the effect of climate drivers such as El Niño and La Niña. Satellite altimetry observations since 1993 show that the rates of sea level rise to the north and south-east of Australia have been significantly higher than the global average, whereas rates of sea level rise along the other coasts of the continent have been closer to, or lower than, the global average. Altimetry data show higher sea level rise near Australia’s south-east coast than near the south coast, which may indicate the emergence of climate change impacts from a poleward shifting and strengthening of the subtropical ocean gyre circulation, of which the East Australian Current is a part.

The long-term satellite altimetry sea level record is typically restricted to the offshore region, beyond 25–50 km from the coast, while changes closer to Australia’s shoreline are estimated from tide gauge measurements at a limited number of locations. Tide gauges with reliable long-term records around Australia show overall changes in sea level rise that are consistent with offshore observations from satellite altimetry. Where local differences exist between coastal and offshore data, they may be influenced by factors such as local coastal processes and the effects of vertical land motion.

Ocean acidification

Rising atmospheric CO₂ levels increase the uptake of CO₂ by the oceans, which absorb 26% of annual global emissions. This affects the oceans’ carbonate chemistry and decreases their pH, a process known as ocean acidification. The pH changes in surface waters are primarily driven by increasing CO₂ in the atmosphere, causing the uptake of CO₂ which reacts with water producing hydrogen ions and a pH decrease. Impacts of ocean acidification on marine ecosystems include changes in reproduction, organism growth and physiology, species composition and distributions, food web structure, nutrient availability, and reduced calcification rate. The latter is particularly important for species that produce shells or skeletons of calcium carbonate, such as corals and shellfish. Ocean acidification is occurring along with changes in ocean warming and deoxygenation, resulting in compounding pressures on the marine environment.

Since the decade of 1880−1889 the average pH of surface waters around Australia and globally is estimated to have decreased by about 0.12, corresponding to about a 30% increase in acidity. There are regional variations in acidity increases; between 1982 and 2022 the greatest acidity increases have occurred in the Southern Ocean (21%) and in the Coral Sea (19%), with the smallest increases to the north-west of Australia (15%). The pH changes tend to be greater at higher latitudes where there is more total dissolved CO₂ in the surface waters, which reduces their capacity to buffer against pH change. The major boundary currents that transport surface waters poleward along the Australian coast also influence patterns of pH changes along with regional temperature and precipitation trends.

The current rate of change of pH in open ocean surface waters is about 10 times faster than at any time in the past 65 million years, and the rate of acidification has grown in recent decades. Some ecosystems are now exposed to conditions outside the pH ranges experienced in the pre-industrial era before 1850. The changes are expected to reduce the capacity of coral reefs, including those of the Great Barrier Reef, to survive and grow. The growth of many carbonate producing organisms along the southern Australian shelf including commercially important shellfish are also likely to be impacted in future as acidification continues with rising atmospheric CO₂.

Cryosphere

The cryosphere is the part of Earth’s surface characterised by frozen water. The cryosphere includes the ice sheets (glacial ice that has accumulated from precipitation over land) and ice shelves (floating sheets of ice formed from glacial ice sheets). Together the ice sheets of Antarctica and Greenland contain about 99% of the Earth’s fresh water, which is the equivalent of over 60 metres of sea level rise.

Ice shelves around Antarctica help stabilise the ice sheet there by restricting the flow of glacial ice from the continent to the ocean. Warm ocean water penetrating below the ice shelves of the West Antarctic ice sheet, along with increased iceberg calving, is now destabilising several glaciers, increasing the Antarctic contribution to sea level rise. Surface melt, particularly over Greenland and the Antarctic Peninsula, is also contributing to sea level rise. Partially offsetting this is increased precipitation (snowfall) over Antarctica, due to increased evaporation of moisture from nearby oceans as a result of reduced sea-ice extent. Over the last few decades, the Amundsen Sea sector contributed most to the net mass loss of the Antarctic ice sheet. Between January and November 2023, the net mass loss over the Antarctic ice sheet is estimated to be about 170 Gt. This is in contrast to the net mass gain observed in 2022 which was the highest on record (since 1980), driven by enhanced snowfall.

Unlike the continental ice sheets, changes in sea ice shelves have a negligible direct impact on sea level, though sea ice influences the rate of regional climate warming and ocean/atmosphere moisture fluxes. Since the commencement of satellite monitoring of sea ice in the late 1970s, Arctic sea-ice cover has consistently decreased, whereas the Antarctic has shown a more complex pattern of changes. Overall, Antarctic sea-ice extent increased slightly from 1979 to 2014, but with substantial regional and seasonal variations. The largest daily recorded wintertime sea ice extent since satellite monitoring began, of approximately 20.2 million km², was in September 2014. Since 2014, there has been a marked, abrupt and relatively persistent decrease in net sea ice extent, which in early 2022 dropped below 2.0 million km² for the first time since satellite observations began. Extraordinarily low net Antarctic sea-ice extents occurred throughout 2023, with new record low observations in 7 months. Unusually, negative anomalies in sea-ice extent almost surrounded the continent, with only the Bellingshausen and Amundsen Seas showing positive ice extent anomalies. Regional negative anomalies were coincident with above average upper ocean and surface temperatures.

Regionally the trend in sea-ice cover has been variable. Statistically significant trends over the 1979−2023 period show reduced sea-ice duration, by as much as 4 days per year, to the west of the Antarctic Peninsula, offshore of West Antarctica and within the Bellingshausen Sea. These are in contrast to increased sea-ice duration within the western Ross Sea and the southern region of the Weddell Sea, although these increases are smaller, only locally reaching 1 to 2 days per year.

The overall increase in Antarctic sea-ice extent from 1979 to 2014 has largely been attributed to changes in westerly wind strength, whereas the marked decrease since 2015 has been attributed to a combination of atmospheric and oceanic anomalies. The primary influence on low sea-ice growth in recent years, particularly in 2023 and 2024, has been abnormally warm subsurface temperatures in the Southern Ocean, with additional impacts from anomalies caused by large-scale weather patterns across the region.

Reduced Antarctic sea-ice coverage and growth can have significant impact on the global climate, including changes in the ocean circulation. Increased glacial melt has been shown to slow the sinking of dense cold water around the Antarctic margin, causing warming and deoxygenation of the deep ocean. Slowing of the ocean overturning circulation (the network of ocean currents that circles the globe and connects the upper and lower layers of the ocean) would impact climate by reducing how much heat and carbon the ocean can absorb from the atmosphere and transfer to the deep ocean. Slowing of the overturning would also reduce oxygen levels in the deep ocean and the cycling of nutrients and carbon between the upper and lower layers of the ocean. 

Further, the reduced presence of sea ice around the continental edge removes the barrier between ocean swell and waves and the ice shelves, potentially destabilising ice shelves and inducing sudden collapse. Since 2022 there has been anomalously high Antarctic coastal exposure (regions of coastline not protected by a sea-ice buffer), with 154 days of record high coastal exposure during 2023.

Sea surface temperature 

Average sea surface temperature in the Australian region has warmed by 1.08 °C since 1900, with 9 of the 10 warmest years on record occurring since 2010. This rate of warming is close to that of the global mean sea surface temperature. The year with the highest average sea surface temperature on record was 2022, which was associated with a strong negative Indian Ocean Dipole event and mass coral bleaching in the Great Barrier Reef, which had never previously occurred in a La Niña event. Extremely high Australian region sea surface temperatures have previously been associated with the end of significant El Niño events.

The greatest ocean warming in the Australian region has occurred in the Coral Sea, and off south-east Australia and Tasmania where more rapid warming trends have occurred over the past 4 decades. The East Australian Current now extends further south, creating an area of more rapid warming in the Tasman Sea, where the warming rate is now twice the global average. There has also been warming across large areas of the Indian Ocean region to the west coast of Australia.

Warming of the ocean has contributed to longer and more frequent marine heatwaves. Marine heatwaves are periods when temperatures are in the upper range of historical baseline conditions for at least 5 days. Heatwaves in the ocean often persist much longer than heatwaves on land, sometimes spanning multiple months or even years.

The increasing frequency of marine heatwaves around Australia in recent years has contributed to permanent impacts on marine ecosystem health, marine habitats, and species. These impacts include depleting kelp forests and seagrasses, a poleward shift in marine species, and increased occurrence of disease.

Ocean heat content 

The world’s oceans are a major component of the Earth’s climate system and have a profound effect on the climate, taking up vast quantities of heat from the atmosphere and redistributing it. Seawater stores about 4 times more heat for every degree of temperature rise than dry air of the same weight. The total weight of water in the ocean is about 280 times greater than the weight of the Earth’s atmosphere, so the capacity for the ocean to store heat is vast. The way the ocean redistributes this heat influences our weather patterns and the climate change signal we see in temperature and rainfall.

While the temperature changes over the whole ocean depth are small compared to those at the land and ocean surface, the ocean has taken up more than 90% of the excess energy in the Earth system arising from enhanced greenhouse gas concentrations. Oceans have therefore slowed the rate of warming near the Earth’s land and ocean surface. Heat absorbed at the surface is redistributed both horizontally and vertically by ocean circulation. As a result, the ocean is warming both near the surface and at depth, with the rate of warming varying between regions and depths.

Ocean warming has accelerated since the early 2000s. In 2023, the global ocean heat content was the highest on record, with an estimated additional 42.8 ±1 x 10²² joules of energy relative to 1960. The Southern Ocean has taken up more than half of that excess heat, as its circulation takes heat from near the surface and transfers it into the deep ocean. A warming ocean affects the global ocean and atmospheric circulation, the cryosphere, global and regional sea levels, oceanic uptake of anthropogenic CO₂, and causes losses in dissolved oxygen and impacts on marine ecosystems.

Regionally, ocean warming can vary substantially from year to year due to climate phenomena such as the El Niño-Southern Oscillation. In areas of strong warming, changes in heat content can be several times larger than the global mean change. This is the case in the oceans around Australia, where strong warming results from a redistribution of heat due to changes in the structure of the East Australian Current, and enhanced heat uptake in the subantarctic region south of Australia.

Year-to-year changes in ocean heat content associated with interannual climate variability are large in the top 300 metres of the ocean but have little impact on the waters below. On long-term timescales, the deep waters below 2,000 metres have also warmed throughout most of the global ocean, but there are far fewer observations of the deep ocean and the magnitude of this warming is less certain. Maintaining the global ocean observing system and expanding its coverage in the deep ocean, the polar oceans, and continental shelves will be critical to prepare for, and adapt to, a changing climate.

Marine heatwaves and coral reefs

Warming oceans, together with an increase in the frequency, intensity and duration of marine heatwaves, pose a significant threat to the long‑term health and resilience of coral reef ecosystems. Mass coral bleaching events have occurred with increasing frequency and extent around the world since the 1970s, including on the Great Barrier Reef. Mass bleaching is a stress response of corals that occurs primarily due to elevated ocean temperature. Recovery is possible, but mortality can occur if the thermal stress is too severe or prolonged. Ocean acidification places further stress on corals.

Four mass coral bleaching events have occurred on the Great Barrier Reef over the past 10 years: in 2016, 2017, 2020 and 2022. In 2016, bleaching was associated with record high sea surface temperatures, which in turn led to the largest recorded mass bleaching on the Great Barrier Reef. The impact of the 2020 mass bleaching event appears to be second only to 2016 and was associated with severely bleached coastal reefs along the entire 2300 km length of the Great Barrier Reef. The 2022 event was the first time that mass bleaching has occurred on the Reef during a La Niña year.

These four recent bleaching events are associated with marine heatwaves driven by anthropogenic climate change. Rapidly recurring bleaching events do not give the reef ecosystem time to fully recover.

In 2022, bleaching was also observed on some reefs on Australia’s west coast, including Ningaloo Reef. This was due to warm ocean temperatures, driven by the 2021–22 La Niña. The region’s last severe marine heatwave was driven by the 2010–11 La Niña, which resulted in bleaching being recorded for the first time on Ningaloo and the closure of several Western Australian fisheries.

Climate models project more frequent, extensive, intense and longer‑lasting marine heatwaves in the future. Worsening impacts on coral reefs from marine heatwaves are expected in the future with continued warming. The intensification of marine heatwaves is much greater under high greenhouse gas emission scenarios. This implies more frequent and severe coral bleaching events are likely, leading to the potential loss of many types of coral and impacts on reef fisheries. Along with ocean acidification and nutrient runoff, the increased severity and frequency of marine heatwaves are likely to reduce reef resilience and hinder coral recovery from future bleaching events.

Sea level

Global mean sea level has risen by over 22 cm since 1900, with half of this rise occurring since 1970. Rising sea levels pose a significant threat to coastal communities and coastal ecosystems by amplifying the risks of coastal inundation, storm surge, erosion and saltwater intrusion into groundwater systems. Coastal communities in Australia are already experiencing some of these changes.

Global mean sea level rise is accelerating. Tide gauge and satellite altimetry observations show that the rate of global mean sea level rise increased from 1.5 cm (± 0.2 cm) per decade from 1901 to 2000, and is now approaching 4 cm (± 0.4 cm) per decade from 1993 to 2023. The dominant cause of global mean sea level rise since 1970 is anthropogenic climate change.

As the ocean warms it expands, causing sea levels to rise. This thermal expansion has contributed about one-third of the sea level rise observed globally. Ice loss from glaciers and polar ice sheets, together with changes in the amount of water stored on the land contribute the remaining two-thirds of the observed global sea level rise.

Confidence in assessing changes in global mean sea level has continuously improved because there has been more analysis of satellite altimetry data, and because the data record becomes longer over time. Ongoing research has also resulted in increased confidence in quantifying the various contributions to sea level rise, and a greater understanding of the processes involved.

Australia, like other nations, is already experiencing sea level rise. Sea level varies from year to year and from place to place, partly due to the natural variability of the climate system from the effect of climate drivers such as El Niño and La Niña. Satellite altimetry observations since 1993 show that the rates of sea level rise to the north and south-east of Australia have been significantly higher than the global average, whereas rates of sea level rise along the other coasts of the continent have been closer to, or lower than, the global average. Altimetry data show higher sea level rise near Australia’s south-east coast than near the south coast, which may indicate the emergence of climate change impacts from a poleward shifting and strengthening of the subtropical ocean gyre circulation, of which the East Australian Current is a part.

The long-term satellite altimetry sea level record is typically restricted to the offshore region, beyond 25–50 km from the coast, while changes closer to Australia’s shoreline are estimated from tide gauge measurements at a limited number of locations. Tide gauges with reliable long-term records around Australia show overall changes in sea level rise that are consistent with offshore observations from satellite altimetry. Where local differences exist between coastal and offshore data, they may be influenced by factors such as local coastal processes and the effects of vertical land motion.

Ocean acidification

Rising atmospheric CO₂ levels increase the uptake of CO₂ by the oceans, which absorb 26% of annual global emissions. This affects the oceans’ carbonate chemistry and decreases their pH, a process known as ocean acidification. The pH changes in surface waters are primarily driven by increasing CO₂ in the atmosphere, causing the uptake of CO₂ which reacts with water producing hydrogen ions and a pH decrease. Impacts of ocean acidification on marine ecosystems include changes in reproduction, organism growth and physiology, species composition and distributions, food web structure, nutrient availability, and reduced calcification rate. The latter is particularly important for species that produce shells or skeletons of calcium carbonate, such as corals and shellfish. Ocean acidification is occurring along with changes in ocean warming and deoxygenation, resulting in compounding pressures on the marine environment.

Since the decade of 1880−1889 the average pH of surface waters around Australia and globally is estimated to have decreased by about 0.12, corresponding to about a 30% increase in acidity. There are regional variations in acidity increases; between 1982 and 2022 the greatest acidity increases have occurred in the Southern Ocean (21%) and in the Coral Sea (19%), with the smallest increases to the north-west of Australia (15%). The pH changes tend to be greater at higher latitudes where there is more total dissolved CO₂ in the surface waters, which reduces their capacity to buffer against pH change. The major boundary currents that transport surface waters poleward along the Australian coast also influence patterns of pH changes along with regional temperature and precipitation trends.

The current rate of change of pH in open ocean surface waters is about 10 times faster than at any time in the past 65 million years, and the rate of acidification has grown in recent decades. Some ecosystems are now exposed to conditions outside the pH ranges experienced in the pre-industrial era before 1850. The changes are expected to reduce the capacity of coral reefs, including those of the Great Barrier Reef, to survive and grow. The growth of many carbonate producing organisms along the southern Australian shelf including commercially important shellfish are also likely to be impacted in future as acidification continues with rising atmospheric CO₂.

Cryosphere

The cryosphere is the part of Earth’s surface characterised by frozen water. The cryosphere includes the ice sheets (glacial ice that has accumulated from precipitation over land) and ice shelves (floating sheets of ice formed from glacial ice sheets). Together the ice sheets of Antarctica and Greenland contain about 99% of the Earth’s fresh water, which is the equivalent of over 60 metres of sea level rise.

Ice shelves around Antarctica help stabilise the ice sheet there by restricting the flow of glacial ice from the continent to the ocean. Warm ocean water penetrating below the ice shelves of the West Antarctic ice sheet, along with increased iceberg calving, is now destabilising several glaciers, increasing the Antarctic contribution to sea level rise. Surface melt, particularly over Greenland and the Antarctic Peninsula, is also contributing to sea level rise. Partially offsetting this is increased precipitation (snowfall) over Antarctica, due to increased evaporation of moisture from nearby oceans as a result of reduced sea-ice extent. Over the last few decades, the Amundsen Sea sector contributed most to the net mass loss of the Antarctic ice sheet. Between January and November 2023, the net mass loss over the Antarctic ice sheet is estimated to be about 170 Gt. This is in contrast to the net mass gain observed in 2022 which was the highest on record (since 1980), driven by enhanced snowfall.

Unlike the continental ice sheets, changes in sea ice shelves have a negligible direct impact on sea level, though sea ice influences the rate of regional climate warming and ocean/atmosphere moisture fluxes. Since the commencement of satellite monitoring of sea ice in the late 1970s, Arctic sea-ice cover has consistently decreased, whereas the Antarctic has shown a more complex pattern of changes. Overall, Antarctic sea-ice extent increased slightly from 1979 to 2014, but with substantial regional and seasonal variations. The largest daily recorded wintertime sea ice extent since satellite monitoring began, of approximately 20.2 million km², was in September 2014. Since 2014, there has been a marked, abrupt and relatively persistent decrease in net sea ice extent, which in early 2022 dropped below 2.0 million km² for the first time since satellite observations began. Extraordinarily low net Antarctic sea-ice extents occurred throughout 2023, with new record low observations in 7 months. Unusually, negative anomalies in sea-ice extent almost surrounded the continent, with only the Bellingshausen and Amundsen Seas showing positive ice extent anomalies. Regional negative anomalies were coincident with above average upper ocean and surface temperatures.

Regionally the trend in sea-ice cover has been variable. Statistically significant trends over the 1979−2023 period show reduced sea-ice duration, by as much as 4 days per year, to the west of the Antarctic Peninsula, offshore of West Antarctica and within the Bellingshausen Sea. These are in contrast to increased sea-ice duration within the western Ross Sea and the southern region of the Weddell Sea, although these increases are smaller, only locally reaching 1 to 2 days per year.

The overall increase in Antarctic sea-ice extent from 1979 to 2014 has largely been attributed to changes in westerly wind strength, whereas the marked decrease since 2015 has been attributed to a combination of atmospheric and oceanic anomalies. The primary influence on low sea-ice growth in recent years, particularly in 2023 and 2024, has been abnormally warm subsurface temperatures in the Southern Ocean, with additional impacts from anomalies caused by large-scale weather patterns across the region.

Reduced Antarctic sea-ice coverage and growth can have significant impact on the global climate, including changes in the ocean circulation. Increased glacial melt has been shown to slow the sinking of dense cold water around the Antarctic margin, causing warming and deoxygenation of the deep ocean. Slowing of the ocean overturning circulation (the network of ocean currents that circles the globe and connects the upper and lower layers of the ocean) would impact climate by reducing how much heat and carbon the ocean can absorb from the atmosphere and transfer to the deep ocean. Slowing of the overturning would also reduce oxygen levels in the deep ocean and the cycling of nutrients and carbon between the upper and lower layers of the ocean. 

Further, the reduced presence of sea ice around the continental edge removes the barrier between ocean swell and waves and the ice shelves, potentially destabilising ice shelves and inducing sudden collapse. Since 2022 there has been anomalously high Antarctic coastal exposure (regions of coastline not protected by a sea-ice buffer), with 154 days of record high coastal exposure during 2023.