How to work out which coral reefs will bleach, and which might be spared


Clothilde Emilie Langlais, CSIRO; Andrew Lenton, CSIRO, and Scott Heron, National Oceanic and Atmospheric Administration

Regional variations in sea surface temperature, related to seasons and El Niño, could be crucial for the survival of coral reefs, according to our new research. This suggests that we should be able to identify the reefs most at risk of mass bleaching, and those that are more likely to survive unscathed.

Healthy coral reefs support diverse ecosystems, hosting 25% of all marine fish species. They provide food, coastal protection and livelihoods for at least 500 million people.

But global warming, coupled with other pressures such as nutrient and sediment input, changes in sea level, waves, storms, ventilation, hydrodynamics, and ocean acidification, could lead to the end of the world’s coral reefs in a couple of decades.


Read more: How much coral has died in the Great Barrier Reef’s worst bleaching event?


Climate warming is the major cause of stress for corals. The world just witnessed an event described as the “longest global coral die-off on record”, and scientists have been raising the alarm about coral bleaching for decades.

The first global-scale mass bleaching event happened in 1998, destroying 16% of the world coral reefs. Unless greenhouse emissions are drastically reduced, the question is no longer if coral bleaching will happen again, but when and how often?

To help protect coral reefs and their ecosystems, effective management and conservation strategies are crucial. Our research shows that understanding the relationship between natural variations of sea temperature and human-driven ocean warming will help us identify the areas that are most at risk, and also those that are best placed to provide safe haven.

A recurrent threat

Bleaching happens when sea temperatures are unusually high, causing the corals to expel the coloured algae that live within their tissues. Without these algae, corals are unable to reproduce or to build their skeletons properly, and can ultimately die.

The two most devastating global mass bleaching events on record – in 1998 and 2016 – were both triggered by El Niño. But when water temperatures drop back to normal, corals can often recover.

Certain types of coral can also acclimatise to rising sea temperatures. But as our planet warms, periods of bleaching risk will become more frequent and more severe. As a consequence, corals will have less and less time to recover between bleaching events.

We are already witnessing a decline in coral reefs. Global populations have declined by 1-2% per year in response to repeated bleaching events. Closer to home, the Great Barrier Reef lost 50% of its coral cover between 1985 and 2012.

A non-uniform response to warming

While the future of worldwide coral reefs looks dim, not all reefs will be at risk of recurrent bleaching at the same time. In particular, reefs located south of 15ºS (including the Great Barrier Reef, as well as islands in south Polynesia and Melanesia) are likely to be the last regions to be affected by harmful recurrent bleaching.

We used to think that Micronesia’s reefs would be among the first to die off, because the climate is warming faster there than in many other places. But our research, published today in Nature Climate Change, shows that the overall increase in temperature is not the only factor that affects coral bleaching response.

In fact, the key determinant of recurrent bleaching is the natural variability of ocean temperature. Under warming, temperature variations associated with seasons and climate processes like El Niño influence the pace of recurrent bleaching, and explain why some reefs will experience bleaching risk sooner than others in the future.

Different zones of the Pacific are likely to experience differing amounts of climate variability.
Author provided
Degrees of future bleaching risk for corals in the three main Pacific zones.
Author provided

Our results suggest that El Niño events will continue to be the major drivers of mass bleaching events in the central Pacific. As average ocean temperatures rise, even mild El Niño events will have the potential to trigger widespread bleaching, meaning that these regions could face severe bleaching every three to five years within just a few decades. In contrast, only the strongest El Niño events will cause mass bleaching in the South Pacific.

In the future, the risk of recurrent bleaching will be more seasonally driven in the South Pacific. Once the global warming signal pushes summer temperatures to dangerously warm levels, the coral reefs will experience bleaching events every summers. In the western Pacific, the absence of natural variations of temperatures initially protects the coral reefs, but only a small warming increase can rapidly transition the coral reefs from a safe haven to a permanent bleaching situation.


Read more: Feeling helpless about the Great Barrier Reef? Here’s one way you can help


One consequence is that, for future projections of coral bleaching risk, the global warming rate is important but the details of the regional warming are not so much. The absence of consensus about regional patterns of warming across climate models is therefore less of an obstacle than previously thought, because globally averaged warming provided by climate models combined with locally observed sea temperature variations will give us better projections anyway.

The ConversationUnderstanding the regional differences can help reef managers identify the reef areas that are at high risk of recurring bleaching events, and which ones are potential temporary safe havens. This can buy us valuable time in the battle to protect the world’s corals.

Clothilde Emilie Langlais, research scientist at CSIRO Oceans and Atmosphere, CSIRO; Andrew Lenton, Senior Research Scientist, Oceans and Atmosphere, CSIRO, and Scott Heron, Physical Scientist, National Oceanic and Atmospheric Administration

This article was originally published on The Conversation. Read the original article.

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Huge restored reef aims to bring South Australia’s oysters back from the brink



File 20170607 29563 agdn8h
Mud oysters played a largely unappreciated part in Australia’s history.
Cayne Layton, Author provided

Dominic McAfee, University of Adelaide and Sean Connell, University of Adelaide

The largest oyster reef restoration project outside the United States is underway in the coastal waters of Gulf St Vincent, near Ardrossan in South Australia. Construction began earlier this month. Some 18,000 tonnes of limestone and 7 million baby oysters are set to provide the initial foundations for a 20-hectare reef.

The A$4.2-million project will be built in two phases and should be complete by December 2018. The first phase is the 4-hectare trial currently being built by Primary Industries and Regions South Australia; the second phase will see the reef expand to 20 hectares, led by The Nature Conservancy.

Some of the 18,000 tonnes of limestone destined for the seafloor.
D. McAfee

Just 200 years ago the native mud oyster, Ostrea angasi, formed extensive reefs in the Gulf, along more than 1,500km of South Australia’s coastline. Today there are no substantial accumulations of mud oysters anywhere around mainland Australia, with just one healthy reef remaining in Tasmania.

This restoration project aims to pull our native mud oyster back from the brink of extinction in the wild, and restore a forgotten ecosystem that once teemed with marine life.

More than just seafood

Oysters played a large role in Australia’s colonial history. When European settlers first arrived they had to navigate a patchwork of oyster reefs (also called shellfish reefs) that filled the shallow waters of our temperate bays. These enormous structures could cover 10 hectares in a single patch, providing an easily exploited food resource for the struggling early settlers. Oyster shell was burned to produce lime, and the colony’s first buildings were built with the help of oyster cement.

Collectively, these pre-colonial oyster reefs would have rivalled the geographic extent of the Great Barrier Reef, covering thousands of kilometres of Australia’s eastern and southern coastlines.

The history goes back much further too. For thousands of years oyster reefs fed and fuelled trade among Aboriginal communities. Shell middens dating back 2,000 years attest to the cultural importance of oysters for coastal communities, who ate them in abundance and used their shells to fashion fishhooks and cutting tools.

Health oyster reef in Tasmania.
C. Gillies

The insatiable appetite of the newly settled Europeans for this bountiful resource was devastating. Not only were live oysters harvested for food, but the dead shell foundations that are critical for the settlement of new oysters were scraped from the seabed for lime burning. Armed with bottom-dredges a wave of exploitation spread across the coast, first overexploiting oyster reefs close to major urban centres and then further afield. The combination of the lost hard shell bed and increased sediment runoff from the rapidly altered coastal landscape saw oyster populations crash within a century of colonisation.

Today oyster populations are at less than 1% of their pre-colonial extent in Australia. This is not a unique story – globally it is estimated that 85% of oyster habitat has been lost in the past few centuries, making it one of the most exploited marine habitats in the world.

Today, across much of Australia’s east coast you will see Sydney rock oysters encrusting rocky shores, creating a thin veneer around the edge of our bays and estuaries. On the south coast you occasionally see a solitary mud oyster clinging to a jetty pylon. Many Australians don’t realise that this familiar sight represents a mere shadow of the incredible and largely forgotten ecosystems that oysters once supported.

Oysters are an unsung ecological superhero, with the capacity to increase marine biodiversity, clean coastal waters, enhance neighbouring seagrass, reduce coastal erosion, and even slow the rate of climate change. When oysters cement together, their aggregations form habitat for a great diversity of other invertebrates. A 25cm-square patch of oysters can host more than 1,000 individual invertebrates from a range of different biological groups, in turn providing a smorgasbord for fish.

Restoration site, formerly covered with dense oyster habitat.
D. McAfee

A solitary oyster can filter about 100 litres of water a day, which means that en masse they can function as the “kidneys” of our bays, filtering excess nutrients from the water and depositing them on the seafloor. In doing so, they encourage seagrass growth, while their physical structures help to dissipate wave energy and thus reduce the impact of storm surges.

As if all that weren’t enough, oysters are also a carbon sink, building calcium carbonate shells that are buried in the seafloor after death and eventually compacted to rock, thus helping to prevent carbon dioxide from cycling back into the atmosphere.

Building it back

Restoring oyster reefs has the potential to return these ecosystem services and increase the productivity of our coastal ecosystems. The Gulf of St Vincent project came about through an election promise by the South Australian Government to boost recreational fishing. A collaboration between The Nature Conservancy, Yorke Penninsula Council and the South Australian Government will deliver the reef’s foundations, while my colleagues and I at the University of Adelaide are working to ensure that the restored oysters survive and thrive, and that the reef continues to grow.

The ConversationHopefully this is just the beginning for large-scale oyster restoration in Australia, and the lessons learned from this project will guide more restoration projects to improve the health of our oceans. With other restoration projects also underway in Victoria and Western Australia, the tide is hopefully turning for our once numerous oysters.

Dominic McAfee, Postdoctoral researcher, marine ecology, University of Adelaide and Sean Connell, Professor, Ecology, University of Adelaide

This article was originally published on The Conversation. Read the original article.

The world’s coral reefs are in trouble, but don’t give up on them yet


Terry Hughes, James Cook University and Joshua Cinner, James Cook University

The world’s coral reefs are undoubtedly in deep trouble. But as we and our colleagues argue in a review published today in Nature, we shouldn’t give up hope for coral reefs, despite the pervasive doom and gloom.

Instead, we have to accept that coral reefs around the world are transforming rapidly into a newly emerging ecosystem unlike anything humans have experienced before. Realistically, we can no longer expect to conserve, maintain, preserve or restore coral reefs as they used to be.

This is a confronting message. But it also focuses attention on what we need to do to secure a realistic future for reefs, and to retain the food security and other benefits they provide to society.

The past three years have been the warmest on record, and many coral reefs throughout the tropics have suffered one or more bouts of bleaching during prolonged underwater heatwaves.

A bleached coral doesn’t necessarily die. But in 2016, two-thirds of corals on the northern Great Barrier Reef did die in just six months, as a result of unprecedented heat stress. This year the bleaching happened again, this time mainly on the middle section of the reef.

Reefs are being degraded by global pressures, not just local ones.
Terry Hughes, Author provided

In both years, the southern third of the reef escaped with little or no bleaching, because it was cooler. So bleaching is patchy and it varies in severity, depending partly on where the water is hottest each summer, and on regional differences in the rate of warming. Consequently some regions, reefs, or even local sites within reefs, can escape damage even during a global heatwave.

Moderate bleaching events are also highly selective, affecting some coral species and individual colonies more than others, creating winners and losers. Coral species also differ in their capacity to reproduce, disperse as larvae, and to rebound afterwards.

This natural variability offers hope for the future, and represents different sources of resilience. Surviving corals will continue to produce billions of larvae each year, and their genetic makeup will evolve under intense natural selection.

In response to fishing, coastal development, pollution and four bouts of bleaching in 1998, 2002, 2016 and 2017, the Great Barrier Reef is already a highly altered ecosystem, and it will change even more in the coming decades. Although reefs will be different in future, they could still be perfectly functional in centuries to come – capable of sustaining ecological processes and regenerating themselves. But this will only be possible if we act quickly to curb climate change.

The Paris climate agreement provides the key framework for avoiding very dangerous levels of global warming. Its 1.5℃ and 2℃ targets refer to increases in global average land and sea temperatures, relative to pre-industrial times. For most shallow tropical oceans, where temperatures are rising more slowly than the global average, that translates to 0.5℃ of further warming by the end of this century – slightly less than the amount of warming that coral reefs have already experienced since industrialisation began.

If we can improve the management of reefs to help them run this climate gauntlet, then reefs should survive. Reefs of the future will have a different mix of species, but they should nonetheless retain their aesthetic values, and support tourism and fishing. However, this cautious optimism is entirely contingent on steering global greenhouse emissions away from their current trajectory, which could see annual bleaching of corals occurring in most tropical locations by 2050. There is no time to lose before this narrowing window of opportunity closes.

A crisis of governance

Reef governance is failing because it is largely set up to manage local threats, such as overfishing and pollution. In Australia, when the Great Barrier Reef Marine Park Authority was set up in 1976, the objective of managing threats at the scale of (almost) the entire Great Barrier Reef was revolutionary. But today, the scale of threats is global: market pressures for Australian reef fish now come from overseas; port dredging and shipping across the reef are spurred on by fossil fuel exports to Asia; a housing crisis in the United States can batter reef tourism half a world away; and record breaking marine heatwaves due to global warming can kill even the most highly protected and remote corals.

Increasingly, coral reef researchers are turning to the social sciences, not just biology, in search of solutions. We need better governance that addresses both local and larger-scale threats to coral reef degradation, rather than band-aid measures such as culling starfish that eat corals.

In many tropical countries, the root causes of reef degradation include poverty, increasing market pressures from globalisation, and of course the extra impacts of global warming. Yet these global issues desperately need more attention at just the time when some governments are reducing foreign aid, failing to address global climate change, and in the case of Australia and the US, trying to resuscitate the dying fossil fuel industry with subsidies for economically unviable projects.

Effective reef governance will not only require increased cooperation among nations to tackle global issues, as in the case of the Paris climate deal, but will also require policy coordination at the national level to ensure that domestic action matches and supports these larger-scale goals.

The ConversationQuite simply, we can’t expect to have thriving coral reefs in the future as well as new coal mines – policies to promote both are incompatible.

Terry Hughes, Distinguished Professor, James Cook University, James Cook University and Joshua Cinner, Professor & ARC Future Fellow, ARC Centre of Excellence, Coral Reef Studies, James Cook University

This article was originally published on The Conversation. Read the original article.

Back-to-back bleaching has now hit two-thirds of the Great Barrier Reef


Terry Hughes, James Cook University and James Kerry, James Cook University

Corals on the Great Barrier Reef have bleached again in 2017 as a result of extreme summer temperatures. It’s the fourth such event and the second in as many years, following earlier mass bleachings in 1998, 2002 and 2016. The Conversation

The consecutive bleaching in 2016 and 2017 is concerning for two reasons. First, the 12-month gap between the two events is far too short for any meaningful recovery on reefs that were affected in 2016.

Second, last year’s bleaching was most severe in the northern section of the reef, from the Torres Strait to Port Douglas, whereas this year the most intense bleaching has occurred further south, between Cooktown and Townsville. The combined footprint of this unprecedented back-to-back bleaching now stretches along two-thirds of the length of the Great Barrier Reef.

Last year, after the peak temperatures in March, 67% of the corals died along a 700km northern section of the reef – the single greatest loss of corals ever recorded on the reef.

Further offshore and to the south, most of the bleached corals regained their colour after the 2016 bleaching, and survived. The patchiness of the bleaching means that there are still sections of the Great Barrier Reef that remain in good condition.

It is still too early to tell how many corals will survive or die over the next few months in the central section as a result of this year’s bleaching.

Four major events

Each of the four bleaching events has a distinctive geographic pattern that can be explained by where the water was hottest for sustained periods during each summer.

For example, the southern Great Barrier Reef escaped bleaching in both 2016 and 2017 because the summer sea temperatures there remained close to normal. Similarly, the earlier mass bleaching events in 1998 and 2002 were relatively moderate, because the elevated water temperatures experienced then were lower than those in 2017 and especially 2016.

The marine heatwaves in 1998 and 2016 coincided with El Niño periods, but this was not the case in 2002 or this year, when water temperatures were also abnormally high. Increasingly around the tropics, we are seeing more and more bleaching events, regardless of the timing relative to the El Niño-La Niña cycle. This reflects the growing impact of global warming on these events.

The local weather also plays an important role in determining where and when bleaching occurs. For example, in 2016, ex-Tropical Cyclone Winston came from Fiji to Australia at the end of February as a rain depression, and cooled the southern region of the Great Barrier Reef, saving it from bleaching.

This year, the category 4 Tropical Cyclone Debbie tracked across the reef in late March, close to the southern boundary of the latest bleaching.

But TC Debbie was too far south to prevent the bleaching that was already under way in the reef’s central and northern sections. Instead of helping to ameliorate the bleaching, this powerful cyclone has added to the pressures on some southern reefs by smashing corals and exacerbating coastal runoff.

Prospects for the future

The fallout from this and last year’s events will continue to unfold in the coming months and years. It takes several months for severely bleached corals to regain their colour, or to die. On some reefs in the Great Barrier Reef’s central region, underwater surveys in 2017 are already documenting substantial loss of corals.

The recovery times for northern and now central reefs that have lost many corals will be at least 10-15 years, assuming that conditions remain favourable for corals during that period.

We have a narrowing window of opportunity to tackle global warming, and no time to lose in moving to zero net carbon emissions. We have already seen four major bleaching events on the Great Barrier Reef with just 1℃ of global average warming.

The goals enshrined in the Paris climate agreement, which aims to hold global warming well below 2℃ and as close as possible to 1.5℃, will not be sufficient to restore the Great Barrier Reef to its former glory. But they should at least ensure that we continue to have a functioning coral reef system.

In contrast, if the world continues its business-as-usual greenhouse emissions for several more decades, it will almost certainly spell the end of the Great Barrier Reef as we now know it.

Terry Hughes, Distinguished Professor, James Cook University, James Cook University and James Kerry, Senior Research Officer, ARC Centre of Excellence for Coral Reef Studies, James Cook University

This article was originally published on The Conversation. Read the original article.

Year-on-year bleaching threatens Great Barrier Reef’s World Heritage status


Terry Hughes, James Cook University; Barry Hart, Monash University, and Karen Hussey, The University of Queensland

The Great Barrier Reef has already been badly damaged by global warming during three extreme heatwaves, in 1998, 2002 and 2016. A new bleaching event is under way now. The Conversation

As shown in a study published in Nature today, climate change is not some distant future threat. It has already degraded large tracts of the Great Barrier Reef over the past two decades.

The extreme marine heatwave in 2016 killed two-thirds of the corals along a 700km stretch of the northern Great Barrier Reef, from Port Douglas to Papua New Guinea. It was a game-changer for the reef and for how we manage it.

Our study shows that we cannot climate-proof coral reefs by improving water quality or reducing fishing pressure. Reefs in clear water were damaged as much as muddy ones, and the hot water didn’t stop at the boundaries of no-fishing zones. There is nowhere to hide from global warming. The process of replacement of dead corals in the northern third of the reef will take at least 10-15 years for the fastest-growing species.

The Great Barrier Reef is internationally recognised as a World Heritage Area. In 2015 UNESCO, the world body with oversight of World Heritage Areas, considered listing the reef as a site “in danger” in light of declines in its health.

Australia’s response falling short

In response to concerns from UNESCO, Australia devised a plan, called the Reef 2050 Long-term Sustainability Plan. Its ultimate goal is to improve the “Outstanding Universal Value” of the reef: the attributes of the Great Barrier Reef that led to its inscription as a World Heritage Area in 1981.

We have written an independent analysis, delivered to UNESCO, which concludes that to date the implementation of the plan is far too slow and has not been adequately funded to prevent further degradation and loss of the reef’s values. A major shortcoming of the plan is that it virtually ignores the greatest current impact on the Great Barrier Reef: human-caused climate change.

The unprecedented loss of corals in 2016 has substantially diminished the condition of the World Heritage Area, reducing its biodiversity and aesthetic values. Key ecological processes are under threat, such as providing habitat, calcification (the formation of corals’ reef-building stony skeletons) and predation (creatures eating and being eaten by corals). Global warming means that Australia’s aim of ensuring the Great Barrier Reef’s values improve every decade between now and 2050 is no longer attainable for at least the next two decades.

What needs to change

Our report makes 27 recommendations for getting the Reef 2050 Plan back on track. The following are critical:

  • Address climate change and reduce emissions, both nationally and globally. The current lack of action on climate is a major policy failure for the Great Barrier Reef. Local action on water quality (the focus of the Reef 2050 Plan) does not prevent bleaching, or “buy time” for future action on emissions. Importantly, though, it does contribute to the recovery of coral reefs after major bleaching.

  • Reduce run-off of sediment, nutrients and pollutants from our towns and farms. To date the progress towards achieving the water quality targets and uptake of best management practice by farmers is very poor. Improving water quality can help recovery of corals, even if it doesn’t prevent mortality during extreme heatwaves.

  • Provide adequate funding for reaching net zero carbon emissions, for achieving the Reef 2050 Plan targets for improved water quality, and limiting other direct pressures on the reef.

At this stage, we do not recommend that the reef be listed as “in danger”. But if we see more die-backs of corals in the next few years, little if any action on emissions and inadequate progress on water quality, then an “in danger” listing in 2020, when UNESCO will reconsider the Great Barrier Reef’s status, seems inevitable.


This article was co-authored by Diane Tarte, co-director of Marine Ecosystem Policy Advisors Pty Ltd. She was a co-author of the independent report to UNESCO on the Great Barrier Reef.

Terry Hughes, Distinguished Professor, James Cook University, James Cook University; Barry Hart, Emeritus Professor Water Science, Monash University, and Karen Hussey, Deputy Director, Global Change Institute, The University of Queensland

This article was originally published on The Conversation. Read the original article.