3.2.6 Sea temperature

Sea temperatures within the Region are affected by global-, regional- and local-scale processes. Global anthropogenic warming is the main driver of increasing ocean temperatures 536 both at the sea surface and at depth. Large-scale climate drivers, such as the El Niño–Southern Oscillation (Box 6.1), influence sea temperatures on an annual basis: sea temperatures are warmer during the El Niño phase.540,632,633 A positive Indian Ocean Dipole may also be associated with warmer-than-average sea temperatures in the Region.634 Seasonal and regional patterns in solar energy, cloud cover, currents 252,635 and storm events 163,636 affect sea temperature and the circulation of ocean heat within the Region. Regional oceanographic processes, such as upwellings, river plumes and seasonal current flows lead to variation in surface and subsurface temperatures within the Region.635,637,638 During winter, dense shelf water cascades occur in some locations where cool water (which forms near the coast) flows offshore along the seabed.639 At fine scales, tides, wind and waves affect the flow of water over shallow reef structures, leading to thermal microclimates within individual reefs.635,640,641

Oceans play a significant role in regulating global climate systems and in slowing the rate of warming on land and at the sea surface.540 Since the 1970s, the world’s oceans have absorbed 91 per cent of the extra heat stored by the planet due to increased greenhouse gas emissions.536,540 This increasing ocean heat content has consequences for oceanic and atmospheric circulation patterns, sea levels (Section 3.2.5), decreasing solubility of oxygen 540,642,643 (Box 6.2) and the frequency and intensity of cyclones. Ocean warming gives rise to longer and more frequent marine heatwaves within the Region.644 Marine heatwaves, defined as periods of at least 5 days where temperatures are in the upper range of historical conditions for that area,645,646 have major impacts on reef ecosystems.647

Ocean warming and increasing marine heatwaves affect the health and resilience of marine ecosystems (Section 6.3.2).246,333,647 Most marine animals are ectotherms, so changes in sea temperature can alter their distribution, survival, metabolism, respiration and behaviour.330,648,649,650 Thermal stress, as a result of high temperatures, is a primary cause of bleaching in reef-building corals and can lead to mortality. While corals can recover from bleaching, their capacity to recover is reduced with increasing intensity, duration and frequency of thermal stress.163 Thermal stress, particularly when associated with marine heatwaves, has significant impacts for many habitats and species, including corals,163,651,652,653,654 seagrasses 185,655 and fishes.252,366,656 Additionally, warmer sea temperatures reduce the solubility and mixing of oxygen within the ocean,657 reducing the availability of oxygen for respiration by marine organisms.

Global sea temperatures are continuing to increase,  leading to record monthly global sea surface temperatures during 2023 166 and high daily means recorded so far in early 2024 (Figure 3.8). Since 1950, each decade has been warmer than the previous. Ocean heat content reached its highest level in 2023 the latest available full year on record,166 superseding previous highs in 2022, 2021 and 2020.540,658,659 Average sea surface temperatures around Australia have risen by 1.05 degrees Celsius since 1900,540 higher than the global average.536

An image of a coral reef scene. There are a number of table corals in the foreground that show the different stages of bleaching, including healthy colonies, and colonies with areas that are bleached white, fluorescing pink and some recently dead areas with algae growing on top.
Coral colonies in varying stages of bleaching © Matt Curnock 2022
Figure 3.8
Warming of sea surface temperature at global and Reef scales since the 1980s

Graph (top) shows daily mean global (60°S to 60°N) sea surface temperatures between 1981 and early 2024. Sea surface temperatures from the past 5 years (since 2019) are highlighted in orange; 2023 and 2024 are shown in bold. The dark grey dotted line is the average daily sea surface temperature from 1982 to 2011, and the dark grey dashed lines above and below it show 2 standard deviations of the mean (±2σ) for the same period. Map (bottom) shows average warming of annual sea surface temperature between 1985 and 2023 for the Great Barrier Reef. Since 1985, average sea surface temperatures in the Region have increased by between 0.1 and 0.3 degrees Celsius per decade. Rates of increase are higher in northern and offshore areas. Graph source: ClimateReanalyzer.org, Climate Change Institute, University of Maine, using data from NOAA Optimum Interpolation SST (OISST) version 2.1.668,669 Map source: NOAA Thermal history annual sea surface temperature trend product (1985 to 2023), Version 3.5, using CoralTemp daily 5 km satellite sea surface temperature dataset, Version 3.1.670,671

Composite figure with a line graph on top, and a map of the Great Barrier Reef Region and World Heritage boundary below. The x-axis on the graph shows a calendar year from January to December and the y-axis shows temperature and ranges from 19.5 to 21.5 degrees Celsius.

In the Great Barrier Reef Region, sea surface temperatures have risen by 0.94 degrees Celsius since 1900 660 and warmed by 0.08 degrees Celsius on average per decade from 1950 to 2021.540 Record high sea surface temperatures were recorded in February 2020.635,661 Sea temperatures over the 2021–22 summer were again above the long-term average despite the occurrence of La Niña, a climatic phase usually associated with more cloud cover and cooler ocean temperatures 185,662 (Figure 3.9).

Figure 3.9
Sea surface temperature anomalies for Great Barrier Reef waters, 1910 to 2023

Anomalies are relative to a 30-year climatology (1961 to 1990). Source: Bureau of Meteorology.660

Bar graph showing annual difference in sea surface temperatures from the long-term average. Y-axis ranges from -1.5 to 1 degree Celsius.

Thermal stress affects many habitats and species, especially reef-building corals

Rising temperatures and marine heatwaves are associated with mass coral bleaching events in the Region, most recently in 2016, 2017, 2020, and 2022 (and 2024, Section 2.3.5).663 Bleaching in 2020 was associated with a strong marine heatwave,636 while the event in early 2022 was the first mass bleaching event to occur under La Niña conditions.163 Potential areas of thermal refugia for coral reefs exist where oceanographic features such as upwellings help to reduce the accumulation of heat.158,180,664 With climate models predicting increasingly frequent and intense marine heatwaves,665 the persistence of such refugia under future warming conditions is uncertain.666,667

Average sea surface temperatures within the Region have continued to increase over the past 5 years, and record high temperatures were recorded in February 2020. Marine heatwaves occurred in 2020 and 2022, triggering mass coral bleaching in both years, although relatively limited coral mortality. Another mass coral bleaching event unfolded in early 2024, after the end of the 2019 to 2023 assessment period.

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