Marine Blueprint

Climate change and the fate of the Great Barrier Reef

The Great Barrier Reef (GBR) faces a greater trial of its resilience and ability to survive in the next 100 years than it has in the past 10,000, mostly because of global climate change. This fact has caused some scientists to predict its loss or such a radical change in its community structure that it will look nothing like the reef we know today.

The costs to Australia’s economy and reputation of losing all or a significant part of one of the Earth’s greatest natural wonders make this an issue of pressing national urgency and a major research effort at the AIMS.

That corals can become extinct we know from fossil evidence of the “Great Death”, 250 million years ago, when 96 per cent of all corals died out.

The truth is we do not know how resilient today’s Reef is, its chances of surviving with its species and ecosystems relatively intact, or exactly what must be done to give it its best chance. And while corals are remarkably tough and versatile organisms, the question of whether they can adapt fast enough to cope with the speed of climate change is unanswered.

AIMS and its sister scientific institutions are working hard to find out.

The main issues of global climate change affecting coral reefs are:

• Heat stress caused by rising sea temperatures which trigger large-scale bleaching, in which the corals lose their symbiotic algae. Even if they survive the initial overheating, they may still experience a lingering death from starvation or as their tissues are damaged by chemical changes. In 1998 a severe event killed 16 per cent of the world’s corals and in 2002 an event in our region bleached half of the GBR, although most did recover.
• Ocean acidification caused by CO2 emissions from human activities dissolving into the oceans. Over time, this could prevent corals and other organisms from forming their chalky skeletons. Recent AIMS research has shown some worrying evidence of a decline in coral growth on the GBR and to a possible effect on fish ear bone growth.
• An increase in frequency and intensity of extreme weather events such as tropical cyclones and storm surges, causing local destruction of reefs
• Food chain effects, in which coral loss leads to loss of associated fauna including fish, birds, turtles and mammals (all the many organisms that rely on the reef structure created by corals for food and habitat) as well as impacting on fishing catches
• Changes in ocean currents delivering food, larvae and warm or cool water to reef communities
• Changes in El Niño/La Niña climate patterns affecting the weather of the Reef.
• Changes in the volume, frequency and timing of floods of fresh water off the land onto the reef, affecting salinity, sediment levels and coral health
• Increasing microbial infections of coral, thought to be due to the stress it is already undergoing from other causes
• ‘Phase shifts’ in which damaged reefs fail to recover quickly and are replaced by weedy growth, changing the whole ecosystem
• Loss of fish populations which help maintain the health of the reef
• The interplay of all these effects with declining water quality and pollution due to human activities, mainly on land
• An increase in coral destruction by predators such as Crown of Thorns Starfish, now also thought to be triggered by nutrients from human activity.

Rising sea levels are unlikely to have a direct impact on corals, but could cause increased erosion on land leading to poorer water quality on coasts and loss of habitat for nesting turtles and seabirds on coral sand cays.

Even if CO₂ emissions are reduced now, it is likely to take several centuries to reverse climate change. The issue is what can be done in the short term to ensure the reef’s survival and minimise the main impacts. This is a central focus of AIMS’ research, not only for the GBR but for all coral systems in Australia’s northern tropical waters that make up over a third of our sovereign territory.

The heat is on

During most summers, corals live in water that is only than 1-2^oC below the temperature at which bleaching occurs. A 2^oC increase kills the coral. Observations indicate that the waters of the GBR are today 0.4^oC warmer, on average, than they were 100 years ago, but that is only part of the story. It is how long the warm water lingers over areas of the Reef that decrees how much coral bleaches or dies. Bleaching also varies with coral species (e.g. branching corals are more susceptible that massive corals, which will result in a change in community structure) and in space. Some reefs and parts of reef do not warm as much as others due to strong water movements that prevent the water column warming under “bleaching weather” conditions. These sites where the bleaching risk is lower need to be mapped and given increased protection as they are likely to provide areas of refuge for affected species as global warming continues.

A 1-3^oC increase in tropical sea temperatures – which the International Panel on Climate Change (IPCC) regards as likely by the end of this century – is estimated to affect 80-100 per cent of the Reef’s corals, depending on where the water lingers and for how long.

A huge unknown is whether corals can adapt or can employ defensive strategies that enabling them to withstand this rapidly changing ocean climate. AIMS research suggests that some corals have the ability to change the main symbiotic algae inhabiting their tissues to kinds that tolerate warmer water, and that these corals will endure ocean temperatures 1-1.5^oC higher. However even this scenario implies loss of corals that lack this ability and a substantial reduction in biodiversity on the Reef. Furthermore, the change to more heat tolerant algae comes at a cost of reduced carbon fixation, growth rate and reduced reproductive output, possibly affecting their ability to compete for space and maintain the balance their natural environment. Whatever the strategy, corals will have to increase their temperature tolerance by 0.1 to 1^oC every decade to keep pace with global warming, a big ecological ask.

Bleached corals, even if they recover, maybe weaker and more susceptible to impacts such as tropical cyclones, coral diseases and nutrients, chemicals or pesticides released by human activity on land. This, however, is an area where the science gives confidence that there is much we can do to protect the Reef.

These management actions include developing marine protected areas, reducing erosion and contaminated runoff from the land to improve water quality, protecting marine nursery systems like mangroves and seagrass meadows, reducing fishing pressure to preserve the balance of life on the Reef, identifying and protecting coral communities that are naturally resilient to bleaching (and can act as seed stock to help nearby reefs recover) and encouraging sensitive coastal development.

AIMS is keeping a keen eye on developments along the Reef through satellites and one of the world’s most sophisticated networks of sensor stations which measure information about weather, water movement, salinity and sea temperatures, most in real time. This is part of a national and global ocean observing system and allows scientists to track changes in the physical environment of the reef and link these to biological responses.

The Acid Bath

The world’s oceans are gradually becoming more acid, as CO₂ released into the atmosphere by human activity dissolves into them. This has profound implications for the marine food chain, for our food supply from the sea and, indeed, for all life on Earth.

The oceans are naturally alkaline, and many of the plankton and corals in them rely on this to form their chalky skeletons. These make up more than a third of all sea life. The pH of the oceans has already fallen by 0.1 to 8.2. At the current rate at which humans are releasing CO₂, it will drop to 7.7-7.8 by the latter part of this century.

Research shows that at pH 7.9, corals and other chalky animals cease to form their skeletons. Whether they then simply die out, leaving a huge hole in the planetary food web, evolve to suit the new circumstances or are replaced with other organisms is not yet clear, and needs much research to resolve.

Research is the way forward

To prevent the worst-case scenarios we have to drastically and rapidly reduce greenhouse gas emissions and, at the same time, respond positively to unavoidable changes. All is not lost: there are things that can be done and opportunities that will present themselves with further research. Biologists know that the most extreme circumstances force living organisms, including humans, to evolve, innovate and adapt more rapidly.

By being at the forefront in understanding the impact of climate change on the oceans and their life – which underpins all life on the planet – AIMS is helping Australia to position itself in a strategic leadership role in how humanity meets the challenge, and what we can do about it.

The expertise, technology and services in how best to manage and reduce the impact of climate change on marine ecosystems could become a major 21st century export, just as farming, mining and medical technology were in the 20th century.

In any crisis there is opportunity, and the opportunity presented by climate change is to be first in understanding what is going on in the oceans which make up 70 per cent of our planet and drive its systems, whether or not it can be managed and how we can best adapt to changes. AIMS is recognised as a world leader in this field.

Learning how to safeguard our precious ecosystems, even in the face of such vast and rapid changes, will give us ways to secure our own future.

For further information, please contact:
Dr Janice Lough
Team Leader, Climate Change
07-4753 4248
j.lough@aims.gov.au 

March 11, 2008