Sea temperature plays a critical role in the distribution and diversity of marine species, which means warming oceans are having a wide range of flow-on effects.
Over the past century, the average global temperature warmed by more than 0.85 degrees Celsius, with most of the warming occurring since the 1970s. All of the warmest 20 years on record have occurred since 1990.
In Australia, mean surface air temperature warmed by 0.9°C since 1910. Our sea surface temperatures are increasing too, as 90 per cent of the excess heat in our atmosphere is stored in oceans.
Sea surface temperatures in north-eastern Australia warmed, on average, by 0.12 degrees per decade since 1950. In the Coral Sea over the past century, 15 of the 20 warmest years occurred in the past 20 years.
The sea surface temperature on the Great Barrier Reef, when averaged across the last 30 years, has increased by about 0.4 degrees, compared to records averaged across 30 years in the late 1800s.
In 2016, sea surface temperatures on the Great Barrier Reef were the hottest ever recorded for the months of February, March and April.
Analysis of coral cores in centuries-old corals suggests current temperatures are warmer now than over the past three centuries.
The Intergovernmental Panel on Climate Change predicts that by 2035 the average sea surface temperature will be warmer than any previously recorded, and by 2100 sea temperatures off north-eastern Australia could be about 2.5 degrees Celsius warmer than the present average.
Rising sea temperatures are not expected to be uniform across the Great Barrier Reef. Rather, the number, size and duration of warm pools (or hotspots) are all expected to increase.
Rising sea surface temperatures are affecting every aspect of the Great Barrier Reef, as sea temperature is a key factor in controlling the diversity of marine life, and how far north or south an animal can live.
Like all marine species, corals have adapted over many thousands of years within limited temperature ranges. This makes corals highly vulnerable to the potential effects of higher sea surface temperatures.
For example, water temperature helps determine the north-south limits of reefs, as well as their diversity. Temperature also helps control the rate of coral reef growth, making it critical in reef building.
When temperature limits are exceeded, corals are put under thermal stress, causing them to expel the algae that live within their tissues — this algae gives corals much of their colour and 90 per cent of their energy needs.
This results in coral bleaching, and if conditions don’t ease within weeks, the corals eventually starve and die. Coral bleaching is not always fatal, but has been one of the main causes of coral death around the world in the past two decades.
Severe bleaching is linked to climate phenomena such as El Niño events. These typically warm sea surface temperatures around the Great Barrier Reef, resulting in sustained elevated regional temperatures.
Extreme El Niño occurrences are projected to increase due to climate change.
At least 10 mass bleaching events have affected the world’s reefs since 1979. The Great Barrier Reef was most severely affected by the 2016 event, and a second consecutive year of severe mass bleaching in 2017.
Coral bleaching is expected to occur more often and with greater severity in the future, making it difficult for corals to recover between bleaching events. This is likely to reduce the abundance of living corals, with flow-on effects for other species dependent on reefs.
Some coral types, such as staghorn corals, are especially sensitive to bleaching. Coral types that are more tolerant to temperature stress are likely to increasingly dominate coral communities.
Large, fleshy seaweeds (called macroalgae), which compete with corals for space, will likely also benefit from rising temperatures and coral bleaching. Degrading reefs can be rapidly overgrown by macroalgae, which in turn impede coral recovery.
Water temperature moderates fish body temperature, which means warmer oceans can affect important biological processes of fish, including growth, reproduction, swimming ability and behaviour.
Temperature limits can also affect the distribution and abundance of bait-fish aggregations. Some species are likely to expand their geographic ranges southward (or contract their migrations northward) as waters warm.
Some fish respond well to high sea temperatures, as these temperatures can shorten incubation time, increase growth rates and improve swimming ability in juvenile fish. However, these benefits are limited to relatively minor temperature increase.
Climate change affects turtles, sea snakes and crocodiles because the environmental temperature controls the reptiles’ body temperatures (except for the leatherback turtle).
Of all the marine reptiles on the Reef, turtles are the most vulnerable to climate change. The temperature of the sand, where eggs are laid, determines the sex of turtles. Air temperature and sea temperature increases will alter turtle breeding seasons and patterns, egg hatching success and the sex ratio of the populations.
Temperature is also an important factor for the estuarine crocodile, the only crocodile species on the Reef. Nesting periods, sex determination and the running and swimming speed of a hatchling are environmentally determined and will be influenced by changes in temperature.
While currently estuarine crocodiles are most likely to be found in the northern parts of the Reef, higher air and sea temperatures could see populations move further south.
Seabirds are considered to be some of the most vulnerable species to climate change impacts.
During frequent or intense El Niño/La Niña-Southern Oscillation events in tropical waters, seabirds have fewer breeding cycles, slowed chick development and reduced nesting success. This is because higher sea temperatures during such events affect the availability of food for seabirds.
A steady decline in most seabird species at Raine Island (the biggest seabird nesting colony in the Great Barrier Reef) has been recorded over the last 12 years. There is also evidence that climate change has driven the ranges of Australian seabirds further south.
Water temperature partly determines the photosynthesis rates for seagrass — an important food source for dugongs and marine turtles.
Temperature increases can reduce the efficiency of photosynthesis; however, the extent of this impact may depend on the species' reliance on light.
Temperature also plays a role in seagrass flowering (and thus reproductive) patterns.