Oceans
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 1.08 °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 have taken up more than 90% 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 effective way to monitor global warming.
- The ocean does not warm evenly. Some regions, including some around Australia, show increases in ocean heat several times faster than the global mean.
- The rate at which the oceans are taking up heat has increased over recent decades.
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.
Five mass coral bleaching events have occurred on the Great Barrier Reef over the past 10 years: in 2016, 2017, 2020, 2022 and 2024. In 2016, bleaching was associated with then record high sea surface temperatures, which in turn led to the largest recorded mass bleaching to date on the Great Barrier Reef. The impact of the 2020 mass bleaching event was associated with severely bleached coastal reefs along the entire 2,300 km length of the Great Barrier Reef. The 2022 event was the first time that mass bleaching occurred on the Reef during a La Niña year. Accumulated thermal stress during the 2024 event was higher than in 2016, although the full impact in terms of bleaching is still being assessed.
These 5 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 2021–2022 La Niña. The region’s previous severe marine heatwave was driven by the 2010–2011 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 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. More frequent and severe coral bleaching events are likely, potentially leading to the 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. 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.
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
- 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 acidity occurring south of Australia.
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.
