State of the Climate 2018


Sea surface temperature

The ocean surface around Australia has warmed over recent decades at a similar rate to the air temperature. Sea surface temperature in the Australian region has warmed by around 1 °C since 1910, with eight of the ten warmest years on record occurring since 2010. Part of the East Australian Current now extends further south, creating an area of more rapid warming in the Tasman Sea. This extension is having numerous impacts on marine ecosystems, including many marine species extending their habitat range further south.


Trends in sea surface temperatures in the Australian region from 1950 to 2017 (data source: ERSST v5,


Anomalies in annual sea surface temperature in the Australian region. Anomalies are the departures from the 1961–1990 standard averaging period. Sea surface temperatures are for the Australian region (4–46° S and 94–174° E).

Warming of the ocean has contributed to longer and more frequent periods when the sea surface temperature is in the upper range of historical baseline conditions for five days or more, known as marine heatwaves. There were long and intense marine heatwaves in the Tasman Sea and around southeast Australia and Tasmania from September 2015 to May 2016 and from November 2017 to March 2018. Scientific analysis shows that the severity of both events can be attributed to anthropogenic climate change. Recent marine heatwaves are linked to coral bleaching in the Great Barrier Reef (see Ocean heat and coral reefs box).

Key points

  • The ocean surface around Australia has warmed, contributing to longer and more frequent marine heatwaves.

Ocean heat content

The world’s oceans play a critical role in the climate system. More than 90 per cent of the additional energy arising from the enhanced greenhouse effect is taken up by the ocean, slowing down the rate of warming at the Earth’s surface. Heat is absorbed at the surface and then moves to the deep ocean in those regions where currents move water vertically. As a result, the ocean is warming both near the surface and at depth, with the rate varying between regions and depths.


Estimated change in ocean heat content over the full ocean depth, from 1960 to 2017. Shading provides an indication of the confidence range of the estimate. Note that data contributing to the early part of the record are sparse and trends estimated over this period are small compared to the error bars, hence considered unreliable.

The upper ocean is more extensively measured than the deep ocean. Between 1960 and 2017 the global ocean between the surface and a depth of 700 m gained 24 x 1022 joules of additional heat. This is more than 60 per cent of the heat accumulated over the full depth of the ocean. Due to the location of currents that move water vertically, the southern hemisphere oceans have taken up the majority of the heat, including the Southern Ocean to the south of Australia.


Estimated linear trend in ocean heat content between 1970 and 2017 in the top 700 m of the ocean, showing the highest uptake of heat in regions where ocean currents move heat to the deep ocean such as the Southern Ocean south of Australia.

The rate at which the deep ocean below 700 m is warming is slow but steady, compared to the more variable rate of change in the upper ocean. Long-term trends in the deep ocean show a clear warming, however there are far fewer observations below 2000 m than near the surface, so the magnitude of this warming is less certain. The observation coverage in the deep ocean will dramatically increase in coming years as the newest generation of observing technology becomes operational, including the automated samplers known as ARGO floats with an extended depth range down to 6000 m.

Key points

  • The world’s oceans are taking up more than 90 per cent of the extra energy stored by the planet as a result of enhanced greenhouse gas concentrations, and the southern hemisphere oceans have taken up the majority of this heat.

Ocean heat and coral reefs

Warming ocean temperatures and an increase in the frequency and intensity of marine heatwaves pose a major threat to the long-term health and resilience of coral reef ecosystems. Globally, large-scale mass coral bleaching events have occurred with increasing frequency and extent since the latter decades of the 20th Century. Bleaching is a stress response of corals, as the water warms the symbiotic relationship between the coral and its zooxanthellae breaks down, turning corals pale. Without these zooxanthellae, most corals struggle to survive, and can ultimately die if the thermal stress is too severe or prolonged.

Bleaching on the Great Barrier Reef has occurred in the past, but with increased frequency and extent in recent decades. Widespread bleaching was observed in 1998, driven by higher summer temperatures associated with a strong El Niño combined with long-term warming trends. The last two years (2016 and 2017) have seen mass bleaching over parts of the Reef in consecutive years for the first time, with the northern Great Barrier Reef experiencing bleaching in both summers.

In February to May 2016, the bleaching was associated with some of the warmest sea surface temperatures ever recorded, with temperatures well above the long-term monthly averages in February, March and April. As a result, 30 per cent of all coral cover across the entire Great Barrier Reef was lost, and 50 per cent in the northern third was lost between March and November 2016. This was four times greater than previous mass-bleaching events in 2002 and 1998.

A second mass bleaching occurred in 2017 linked to another marine heatwave, with temperatures again well above the long-term mean.

Both events could have been more extensive in the Southern Great Barrier Reef if it were not for cooling winds associated with distant tropical cyclones Winston and Tatiana in 2016 and tropical cyclone Debbie in 2017.

The primary cause of both marine heatwaves and mass bleaching events was very likely due to warming oceans as a result of anthropogenic climate change, compounded in early 2016 by a very strong El Niño event.


Great Barrier Reef coral bleaching risk map, shown as Degree Heating Days: the accumulated (sum) of positive sea surface temperature anomalies with respect to the long-term average of 2002–2011 each day over the reef for December 2016 to March 2017. Green = OK. Yellow = watch. Orange = coral bleaching risk. Red = coral mortality risk.

Sea level

As the ocean warms it expands and sea level rises. This has contributed about a third of the observed global sea level rise of over 20 cm since the late 19th Century. The remainder comes from the loss of ice from glaciers and polar ice sheets, and changes in the amount of water stored on the land. The confidence range of global sea level change has continuously improved because there has been more analysis of satellite altimetry, the time series has lengthened, and the various contributions to sea level have now all been reliably quantified and accounted for. Since 1993 sea level has been rising at 3.2 cm per decade.


High-quality global sea level measurements showing annual sea level change from 1880 in tide gauge data (1880–2014 blue line, light blue shading indicates confidence range), and annual sea level change in satellite altimetry (1993–2017, red line). The pull out figure shows monthly sea level change from 1880 in satellite altimetry from 1993 to July 2018 (updated from Church and White 2011).

Sea level has been rising around Australia. Sea level rise varies from year to year and from place to place. This is partly due to the natural variability of the climate system from influences such as El Niño, La Niña and the Pacific Decadal Oscillation. Based on the satellite altimetry observations since 1993, the rates of sea level rise to the northwest, north and southeast of Australia have been higher than the global average, while rates of sea level rise along the south and northeast coasts of the continent have been close to or slightly less than the global average

The longer-term satellite record is restricted to offshore (at least 25 km off the coast) and does not include estimates of sea level rise along Australia’s coasts, where changes are instead measured from tide gauge data. Changes in sea level measured by tide gauges may be different from those measured by satellites due to coastal processes, vertical land motion or changes to the surveyed reference level of the tide records (e.g. a site change). These factors introduce some uncertainty to rates of change in sea level experienced at the coast. Nevertheless, tide gauges with good long-term records around Australia show consistent sea level rise over time.


The rate of sea level rise around Australia by satellite observations from 1993 to 2017. Source: CSIRO, update from White et al. (2014).

Key points

  • Global sea level has risen by over 20 cm since 1880, and the rate has been accelerating in recent decades.
  • Rates of sea level rise vary around Australia.

Ocean acidification

The uptake of atmospheric CO2 by the oceans affects the carbonate chemistry and decreases pH, a process known as ocean acidification. Ocean acidification is the consequence of rising atmospheric CO2 levels and impacts the entire marine ecosystem—from plankton at the base through to the top of the food chain. Impacts include changes in reproduction, organism growth and physiology, species composition and distributions, food web structure, nutrient availability and calcification. The latter is particularly important for species that produce shells, or skeletons of calcium carbonate, such as corals and shellfish. The average pH of surface waters around Australia is estimated to have decreased since the 1880s by about 0.1, corresponding to a more than 30 per cent increase in acidity (the waters have become less alkaline). These changes have led to a reduction in coral calcification and growth rates on the Great Barrier Reef, with implications for recovery from coral bleaching events. The current rate of change is ten times faster than at any time in the past 300 million years. Due to the differences in ocean chemistry by latitude, the oceans to the south of Australia are acidifying faster than those to the north.


The pH of surface waters around Australia, top: change between 1880–1889 and 2003–2012, bottom: the average pH of water surrounding Australia. Calculations are based on present-day data on the carbonate chemistry of surface seawater around Australia from the Integrated Marine Observing System and other programs, and extrapolation of atmospheric carbon dioxide concentration changes since the 1880s. Source: CSIRO, Lenton et al. (2016).

Key points

  • The oceans around Australia are acidifying (the pH is decreasing).
  • The changes in ocean acidification have led to detectable impacts in areas such as the Great Barrier Reef.