Sea surface temperature
- Sea surface temperatures around Australia have warmed by over 1 °C since 1900.
Average sea surface temperature in the Australian region has warmed by 1.05 °C since 1900, with eight of the 10 warmest years on record occurring since 2010. The year with the highest sea surface temperature on record was 2016, which was associated with one of the strongest negative Indian Ocean Dipole events on record and the 2015–16 extreme El Niño event.
The greatest ocean warming in the Australian region since 1970 has occurred off south-east Australia and Tasmania. 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 five days. Heatwaves in the ocean often persist much longer than heatwaves on land, sometimes spanning multiple months or even years.
As discussed in the 2021 Australian State of the Environment report, the increasing frequency of marine heatwaves around Australia in recent years has permanently impacted marine ecosystem health, marine habitats and species.
These impacts include depleting kelp forests and seagrasses, a poleward shift in some marine species, and increased occurrence of disease.
Ocean heat content
- The world’s oceans have taken up 91 per cent of the extra energy stored by the planet as a result of enhanced greenhouse gas concentrations. Measuring changes in ocean heat content is therefore an accurate way to monitor global warming.
- The ocean does not warm evenly. Some regions, including around Australia, are warming several times faster than the global mean.
- The rate at which the oceans are taking up heat has increased over recent decades.
Seawater stores about four 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.
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 91 per cent of the excess energy in the Earth system 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.
Warming has accelerated since the early 2000s. In 2021, the global ocean was the warmest on record, with an estimated additional 39 x 1022 joules of energy relative to 1960. The Southern Ocean takes up more than half of that warming. This is because the Southern Ocean 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, 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 ENSO. 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 a southward extension of the East Australian Current, and enhanced heat uptake in the subantarctic region south & of Australia.
Changes in ocean heat content associated with climate variability are large in the top 300 m of the ocean but have little impact on the waters below. The deep waters below 2000 m have also warmed throughout most of the global ocean. However, there are far fewer observations of the deep ocean, and the magnitude of this warming is less certain.
Monitoring heat content globally over time is the most effective way to track climate change and the impacts of a warming ocean. Maintaining the ocean observing system and expanding 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.
- Global mean sea level has risen by around 25 cm since 1880; half of this rise has occurred since 1970.
- Rates of sea level rise since 1993 vary across the Australian region, with the largest increases to the north and south-east of the Australian continent.
As the ocean warms, it expands and sea level rises. This thermal expansion has contributed about one-third of the sea level rise observed globally—around 25 cm since the late 19th century. 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. Ice loss from Greenland, Antarctica and glaciers has been the dominant contributor to global sea level rise from 1993 to the present.
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 ± 0.2 cm per decade (1901–2000) to 3.5 ± 0.4 cm per decade (1993–2021). The dominant cause of global mean sea level rise since 1970 is anthropogenic climate change.
Confidence in assessing changes in mean global sea level has continuously improved because there has been more analysis of satellite altimetry and longer records. Efforts to reliably quantify the various contributions to sea level rise have also led to greater confidence and process understanding.
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. Based on satellite altimetry observations since 1993, 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 the global average.
The long-term altimetry sea level record is typically restricted to the offshore region, beyond 25–50 km, while changes closer to Australia’s shoreline are estimated from tide gauge measurements at a limited number of locations. Local coastal processes, the effects of vertical land motion, and changes in site and/or reference levels affect local estimates of sea level change. For example, estimates from nearshore tide gauge measurements may differ from estimates derived from satellite altimetry tens of kilometres offshore.
Nevertheless, tide gauges with good long-term records around Australia show overall changes in sea level rise consistent with offshore observations from satellite altimetry.
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. As emphasised in the 2021 State of the Environment report, coastal communities in Australia are already experiencing some of these changes.
- The acidification of the oceans around Australia continues (pH is decreasing), with changes happening faster in recent decades.
- Increasing CO2 in the atmosphere will continue to drive ocean acidification, with the greatest changes in temperate and cooler waters in the south.
Rising atmospheric CO2 the uptake of CO2 by the oceans, which take up 26 per cent of annual global emissions. This affects the oceans’ carbonate chemistry and decreases their pH, a process known as ocean acidification. In conjunction with other environmental changes, such as ocean warming and deoxygenation, ocean acidification brings additional pressures to the marine environment.
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.
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.
The rate of increase has grown in recent decades and updates indicate an increase in acidity of between 12 per cent and 18 per cent between 1982 and 2020. The changes are linked to increased concentrations of CO2 in the atmosphere. Due to latitudinal differences in ocean chemistry, the oceans to the south of Australia are acidifying faster than those to the north.
The current rate of change for pH in open ocean surface waters is about 10 times faster than at any time in the past 300 million years. Some ecosystems are now exposed to conditions outside the pH ranges experienced in the pre‑industrial era before 1850.
The impacts will be compounded by other stressors, including ocean warming and pollution. This reduces the capacity of coral reefs, including those of the Great Barrier Reef, to survive and grow.
- The ice sheets and ice shelves of Antarctica and Greenland are losing ice and contributing to global sea level rise due to a warmer climate.
- There has been an abrupt decrease in Antarctic sea-ice extent since 2015, after a small increase over the period from 1979–2014.
The Earth’s ice sheets—glacial ice that has accumulated from precipitation over land—and ice shelves, which are floating sheets of ice formed from glacial ice sheets, play crucial roles in our global climate system. Ice shelves help stabilise the Antarctic ice sheet 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 is destabilising several glaciers, increasing the Antarctic contribution to sea level rise. Atmospheric warming is also causing surface melting of ice sheets and ice shelves, particularly in Greenland and on the Antarctic Peninsula. From 1993–2018, melt from Greenland and Antarctica combined contributed around 1.7 cm of global sea level rise.
Changes in sea ice have little direct impact on sea level because sea ice is frozen seawater that floats. When it melts, it returns the original volume of water to the sea. However, the presence or absence of sea ice influences the climate, including the rate of regional climate warming.
Antarctic sea ice also acts as a protective buffer for ice shelves against destructive ocean swells. Changes in Antarctic sea-ice cover can also be an indicator of wider changes in climate because it is an integrator of ocean, atmosphere and cryosphere components, from local change to the tropics.
Satellite monitoring of sea ice began in the late 1970s. Since then, Arctic sea‑ice cover has consistently decreased, whereas net sea-ice cover changes within the Antarctic have been mixed.
Overall, Antarctic sea-ice extent showed a small increase from 1979–2014, but with substantial regional variations. The largest daily recorded wintertime extent of approximately 20.2 million km2 was in September 2014. Since 2014, there has been a marked, abrupt and relatively persistent decrease in sea-ice extent, which in early 2022 dropped below 2.0 million km2 for the first time since observations began.
The overall increase in Antarctic sea-ice extent during 1979–2014 has mostly 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.
Observed changes in Antarctic sea-ice cover are also regionally variable, as depicted in the trends of yearly sea‑ice duration. Statistically significant increases of up to two days per year in sea-ice duration have occurred in the Ross Sea, between 160° E and 150° W. Decreases in sea-ice duration of as much as four days per year are seen west of the Antarctic Peninsula and the Bellingshausen Sea offshore of West Antarctica. Recent sea-ice seasons (since 2015) have shown opposite regional responses to the long-term trend.
The rate of mass loss from the Antarctic ice sheet has increased over recent decades, with a total loss of 2670 ± 870 Gt of ice over the period 1992-2022.