The Great Barrier Reef is part of a larger system of ocean circulation in the Pacific Ocean. This system delivers nutrients and larvae from other regions as well as deep water into the Great Barrier Reef region.

They play an important part in regulating heat, connecting biodiversity and transporting fish and other marine life. Currents vary naturally, but climate change now influences them.

Ocean current trends

There is increasing evidence of strengthening and accelerated warming in the East Australian Current, which runs south along Australia’s east coast. This current is transporting greater volumes of warmer water southward.

There is little information about the Hiri Current travelling north along the coast in northern Great Barrier Reef waters.

Ocean current effects

Changes to ocean currents have the potential to affect entire marine food webs, from microscopic organisms, corals and sponges to top predators such as sharks.

Altered ocean circulation patterns may affect the transport of eggs and larvae within and among coral reefs and other Great Barrier Reef habitats.

Changes to ocean circulation will also affect tiny plants that support the open ocean food web. These microscopic plants (known as phytoplankton) rely on nutrients brought up from deeper in the ocean.

As surface water warms, it becomes less dense and does not mix as readily with the cooler water below. This makes it harder for nutrients to reach the surface, reducing the production of phytoplankton. This, in turn, affects food supply for larger animals, including fish, seabirds, whales and dolphins.

Changes to currents may also result in different species of marine life being more widely distributed or establishing themselves in different areas.

Our sea level is rising at an increased rate, and the primary cause is higher temperatures.

As the temperature rises, the water gets hotter. As it heats up, the water expands, with warmer oceans taking up more space. Higher temperatures also cause melting glaciers and ice caps to melt faster, putting more water into the ocean.

Sea level rise trends

The global average sea level rose by 0.18 centimetres per year from 1961 to 2003. The total rise from 1901 to 2010 was 19 centimetres, which is larger than the average rate during the previous 2000 years.

Around Australia and in the Great Barrier Reef, the fastest rates of sea level rise are in the north.

Extreme sea level events (storm-driven waves and surges) also became about three times more frequent during the 20th century.

The Intergovernmental Panel on Climate Change projects that global sea levels will rise by around 26 to 29 centimetres by 2030 and by around 47 to 62 centimetres by 2080.

Sea level rise effects

Sea level is important in determining the distribution of species and habitats and affects many species' foraging and breeding activities.

Over the past 100,000 years, sea levels have risen and fallen many times, shifting where reefs grow on the continental shelf.  Sea levels on the Great Barrier Reef have already risen by approximately 3mm per year since 1991.

Since 1959, records of sea levels for Townsville, in north Queensland, show an average increase of 1.2mm per year. However, the rate of increase may be accelerating, with records of sea levels at Cape Ferguson near Townsville showing an average increase of 2.9mm every year between 1991 and 2006.

This sea level change is considered small in the context of the Great Barrier Reef’s geological history; however, it’s believed sea levels have been very constant for the past 6000 years.

Most reefs in the region will probably be able to accommodate the current rate of sea level increase as the maximum rate of reef growth is about twice this.

However, sea level rise is predicted to increase at a higher rate, and coral reef growth may not be able to keep pace. The shape and existence of some coastlines, cays and islands may also be affected.

Because much of the land adjacent to the Great Barrier Reef is low-lying, small changes in sea level will mean greater erosion and land inundation. This will cause significant changes in tidal habitats, such as mangroves, and move saltwater into low-lying freshwater habitats. This will have flow-on effects for juvenile fish that use these habitats for protection and food resources.

Turtle and seabird nesting beaches are particularly vulnerable to rising sea levels, as this would exacerbate beach erosion and flood nests.

Ocean acidification significantly impacts a changing climate on the Great Barrier Reef ecosystem.

Acidification occurs because the ocean acts as a carbon sink, absorbing carbon dioxide from the atmosphere. This changes the ocean's chemistry by reducing the ocean's pH — which measures acidity or alkalinity — over an extended period.

When seawater absorbs carbon dioxide, chemical reactions occur, resulting in a greater concentration of hydrogen ions. This causes the seawater to become more acidic and carbonate ions to be relatively less abundant.

Carbonate ions are the building blocks for many marine animals such as corals, oysters, clams, sea urchins, molluscs, crustaceans and echinoderms, helping them to produce shells and skeletons.

Ocean acidification trends

The pH of seawater has remained steady for millions of years, and marine life has evolved based on the ocean's delicate chemical balance. However, the oceans are estimated to have absorbed about 30 per cent of the emitted carbon dioxide from human activities since pre-industrial times.

The carbon dioxide is contained in the upper 10 per cent of oceans (less than 1000 metres in depth) because of slow ocean mixing processes.

A decline of 0.1 from pre-industrial times has already been recorded in the pH of the ocean's surface, taking it to 8.1. This corresponds to a 26 per cent increase in acidity. Reef development is thought to cease at pH 7.8.

The Intergovernmental Panel on Climate Change expects this decline to continue, with average reductions of between 0.06 and 0.32 units over the 21st century.

The effects of global warming and ocean acidification may magnify each other but may not occur uniformly from place to place and over time.

Ocean acidification effects

Even relatively small increases in ocean acidity reduce the capacity of corals to build skeletons, reducing their capacity to create protective habitat for the Reef's marine life.

The rate of skeleton formation, known as calcification, is already likely to have been affected, resulting in slower growth rates and weaker coral structures.

However, the impact of acidification is likely to vary between coral species and organisms. The predicted warming of the oceans speeds up the calcification process, potentially counteracting to some extent the negative effects of decreasing ocean pH at some reefs.

Ocean acidification is also expected to make it more difficult for many planktons — which form the basis of the entire marine food chain — to build calcium carbonate (limestone) shells, plates and skeletons.

It will also likely affect fish reproduction, as fish eggs are more sensitive to pH changes than fish adults, thus potentially reducing populations. Ocean acidification has also been shown to reduce the ability of fish larvae to find suitable habitats and find their way home.

Coral reefs, seagrass meadows and islands have a natural resilience to physical disturbances from weather events such as storms and cyclones.

However, climate change-induced shifts in weather patterns that affect the frequency, intensity or distribution of these disturbances will have important implications.

Storms and cyclones trends

The Intergovernmental Panel on Climate Change (IPCC) found a warming climate is increasing the frequency and severity of many extreme weather events and is changing rainfall patterns. The IPCC predicts it is very likely that extreme rain events will become more intense and frequent in many regions.

According to the State of the Climate Report 2014, fewer tropical cyclones are projected for the Australian region, on average. However, it expects an increased proportion of intense cyclones.

Cyclones were responsible for 48 per cent of coral loss recorded by the Australian Institute of Marine Science’s Long-term Monitoring Program between 1985 and 2012.

Between 2004 and 2018, 10 cyclones of category three or greater crossed the Great Barrier Reef Marine Park. Impacts were most severe in the southern half of the region, causing significant damage to coral reef habitats.

Cyclone Hamish in March 2009 affected more than 50 per cent of the coral reefs in the region. Cyclone Yasi crossed the Queensland coast in February 2011 and was one of the most powerful cyclones to have affected Queensland since records commenced.

Cyclone Debbie in 2017 caused serious impacts to the central reef region.

Storms and cyclones' effects

Cyclones

Cyclones are low-pressure systems that derive their energy from warm tropical oceans — they form when the sea-surface temperature is above 26.5°C.

Tropical cyclones can cause extensive damage to individual corals and the Reef's structure and can affect large areas. The impacts can last for decades, if not centuries.

Powerful waves generated during cyclones can seriously damage habitats and landforms, particularly coral reefs and shorelines.

Cyclonic winds can also cause substantial changes in the shape of islands and coastlines, affect ocean currents and increase inshore ocean turbidity through the suspension of sediments.

Intense rainfall

Intense rainfall and damaging storm surges can cause low-lying coastal areas to be inundated.

Large river foods that affect the central Great Barrier Reef have become more frequent since the late 19th century and are now occurring on average every six years (1948–2011), compared with every 20 years between 1748 and 1847.

Floodwaters running off the land and into the Great Barrier Reef lagoon can form plumes laden with sediments (which can block light needed for seagrass and coral growth) and nutrients (which are linked to outbreaks of the coral predator, the crown-of-thorns starfish).

Extreme flood events are also resulting in more frequent freshwater impacts. The freshwater inflow makes ocean waters less saline and can stress or kill many animals and plants living on coral reefs.