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Great Barrier Reef facing catastrophic damage

Leading scientists have found that sea surface temperatures on the Great Barrier Reef have reached a 400-year critical level, with human-induced climate change to blame.

In a collaborative research project, involving The University of Queensland’s Professor Ove Hoegh-Guldberg and led by University of Wollongong and University of Melbourne researcher Dr Benjamin Henley, scientists confirmed human-induced climate change was responsible for rapid ocean warming in recent decades.

The team reconstructed centuries’ sea surface temperatures in the Coral Sea for the January–March period and found the hottest temperature in 400 years was recorded in 2024, followed by 2017 and 2020.

Professor Hoegh-Guldberg said the findings confirmed that extreme ocean warming had led to mass coral bleaching and mortality on the Great Barrier Reef.

“The recent mass coral bleaching events coincide with five of the six hottest years in the new 400 year-long record,” Professor Hoegh-Guldberg said.

“The world’s largest coral reef is under critical pressure, with warming sea temperatures and mass coral bleaching and mortality threatening to destroy its critically important ecology and biodiversity and its value to culture, business and communities.

“This paper provides several new lines of evidence which together demonstrate that mass coral bleaching will likely devastate the ecological function of the Great Barrier Reef in the coming decades.

“This finding shows we must double down on reducing greenhouse gas emissions as an absolute necessity.”

The research team used climate modelling to assess how human influence had impacted ocean temperatures.

“When assessing natural trends without human impacts, the ocean temperature would have warmed by less than 0.01 degree Celsius per decade,” Professor Hoegh-Guldberg said.

“However, when considering human impacts, we found the ocean warmed by more than one degree Celsius.

“This confirms human impacts on the climate are the primary driver of this longer-term warming in the Coral Sea.”

“Without urgent intervention, our iconic Great Barrier Reef is at risk of experiencing temperatures conducive to near-annual coral bleaching, which would have catastrophic consequences for coral reef ecosystems,” Professor Hoegh Guldberg said.

“This research has profound implications for marine ecosystems globally with similar sensitivities to rising sea temperatures.

“Almost every part of the ocean, from kelp forests to the deep sea, is changing in response to thermal stress and mass mortalities, highlighting the serious link between the long-term trajectory of extreme ocean temperatures and the ecological health and biodiversity of the Ocean. 

“We must take action now before it is too late”, he said.

The research  was published in Nature.

Photos are available via Dropbox.

* Normally brown, coral colonies turn white when bleached , as seen at The University of Queensland’s research station on Heron Island, March 2024. Image, Prof. Ove Hoegh-Guldberg.

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Saving the Great Barrier Reef: these recent research breakthroughs give us renewed hope for its survival

CEO, Australian Institute of Marine Science

Executive Director of Strategic Development at Australian Institute of Marine Science, Australian Institute of Marine Science

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Disclosure statement

AIMS receives funding from the Commonwealth Government for its work on reef adaptation, directly and through the Reef Trust Partnership with the Great Barrier Reef Foundation.

Rob Vertessy is an independent chair of the Reef Restoration and Adaptation Program governing board, a role for which he is remunerated.

University of Melbourne provides funding as a founding partner of The Conversation AU.

Australian Institute of Marine Science provides funding as a member of The Conversation AU.

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With yet another coral bleaching event underway on the Great Barrier Reef, we’re reminded of the tragic consequences of climate change.

Even if we manage to stop the planet warming beyond 1.5°C this century, scientists predict up to 90% of tropical coral reefs will be severely damaged.

But we believe there’s a chance the Great Barrier Reef can still survive. What’s needed is ongoing, active management through scientific interventions, alongside rapid, enormous cuts to global greenhouse gas emissions.

In 2020, the federal government announced the Reef Restoration and Adaptation Program , which aims to help coral reefs adapt to the effects of warming oceans. It included research and development funding into 35 cutting-edge technologies that could be deployed at large scale, from cloud brightening to seeding reefs with heat-tolerant corals.

Now, two years into the effort, we’re seeing a number of breakthroughs that bring us renewed hope for the reef’s future.

Bleaching on the reef

Aerial surveys of the entire reef are currently underway to determine the extent and severity of current bleaching. These should be complete before the end of March.

Meanwhile, United Nations’ reef monitoring delegates are visiting the Great Barrier Reef this week to determine whether its World Heritage status should be downgraded.

Early indications suggest bleaching is most severe in areas of greatest accumulated heat stress, particularly in the area around Townsville. In some places, water temperatures have reached 3°C higher than normal.

Read more: Adapt, move, or die: repeated coral bleaching leaves wildlife on the Great Barrier Reef with few options

Researchers involved in the Reef Restoration and Adaptation Program are examining a wide range of interventions to repair coral reefs. Unlike current reef restoration efforts, which are done by hand on a few square metres of reef, these interventions are designed to be applied at tremendous scales – across thousands of square kilometres.

Major scientific, technological, process, communication and management breakthroughs are required to see this become successful. We’re pleased to report that we’re already seeing the first successes, with others becoming more likely as research and development continues.

Early success stories

One key family of possible interventions involves culturing and deploying millions of heat-tolerant corals onto selected reefs.

Over the last two years, the research team has accelerated the natural adaptation of several coral species to warmer temperatures, allowing them to survive up to an additional four weeks of 1°C excess heat stress. We believe a total of eight weeks of 1°C excess heat stress can be achieved.

This level of additional heat tolerance can make a real difference for reef survival if we can limit greenhouse gas emissions driving climate change.

Read more: If we can put a man on the Moon, we can save the Great Barrier Reef

We’ve also developed novel seeding devices , which allow mass delivery of juvenile corals to reefs in a way that enhances their survival, paving the way for larger field trials.

Seeding heat-tolerant corals onto the reef will require significant improvements in coral aquaculture – the process of raising healthy coral in an aquarium before transporting them to the Great Barrier Reef. While current methods are limited to producing and deploying a few thousand corals per year, new advanced methods are designed to produce tens of millions per year – faster and cheaper than ever before.

Another breakthrough relates to the ongoing development of new models and the data to calibrate them.

These are set to vastly improve our ability to predict where interventions are best deployed, and how well they’ll function. Early modelling results suggest even at a modest scale, well-targeted interventions could be enough to shift the state of individual reefs from terminal decline to survival over several decades.

4 conditions for lasting benefit

For these early breakthroughs to bring lasting benefit at such tremendous scales, four key conditions must be met:

interventions will have to be readily scalable and affordable. That means methods and technologies now being trialled in labs and on small patches of reef will have to be automated, mass-produced, up-sized and delivered in ways not previously considered feasible. All of this will take significant investment

interventions must be safe and acceptable to regulators and the public

a range of people, especially Traditional Owners of reef sea-country, must be involved in the effort. This includes through consultation, in decision-making and design

most importantly, global emissions must be brought rapidly under control, ideally to keep warming to under 1.5°C this century.

Over the coming months, the program will be conducting more trials on the reef. Alongside recent advances by other programs, such as approaches to control coral-eating crown of thorns starfish, there’s now real promise that a combined intervention at scale can be successful.

Saving the reef

Imagine a world where coral reefs have largely disappeared from the world. The few remaining reefs are a shadow of what they once were: grey, broken, covered in weeds and devoid of colourful fish.

Millions of people who’ve depended on reefs must turn to other livelihoods, which may contribute to climate-related migration . Imagine, too, how we’d feel knowing it could have been prevented.

We are hopeful for an alternative vision for the future of the world’s reefs. It’s one in which the amazing beauty and diversity, and the huge global economic benefits, are intact and thriving well into the next century.

The difference between these two possible futures depends on choices we make right now. To save our reefs, we must simultaneously mitigate global warming and adapt to impacts already locked in. Neither alone will be enough.

Read more: Mass starvation, extinctions, disasters: the new IPCC report’s grim predictions, and why adaptation efforts are falling behind

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• Global warming
• Coral bleaching
• UNESCO World Heritage sites

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Heat Raises Fears of ‘Demise’ for Great Barrier Reef Within a Generation

A new study found that temperatures in the Coral Sea have reached their highest levels in at least four centuries.

By Catrin Einhorn

This generation will probably see the demise of the Great Barrier Reef unless humanity acts with far more urgency to rein in climate change, according to scientists in Australia who released new research on heat in the surrounding ocean.

The Great Barrier Reef is the largest coral reef system in the world and is often called the largest living structure on Earth. The study, published on Wednesday in the journal Nature , found that recent extreme temperatures in the Coral Sea are at their highest in at least 400 years, as far back as their analysis could reach.

It included modeling that showed what has been driving those extremes: Greenhouse gas emissions caused by humans burning fossil fuels and destroying natural places that store carbon, like forests.

“The heat extremes are occurring too often for those corals to effectively adapt and evolve,” said Ben Henley, a paleoclimatologist at the University of Melbourne and an author of the new study. “If we don’t divert from our current course, our generation will likely witness the demise of one of Earth’s great natural wonders, the Great Barrier Reef.”

The study’s scientific prose put it this way: “The existential threat to the Great Barrier Reef ecosystem from anthropogenic climate change is now realized.”

Tanya Plibersek, Australia’s environment minister, said in a statement that the government understood its responsibility to act on climate change and safeguard the reef. She pointed to a recent law that calls for a 43 percent reduction of emissions by 2030 and to $1.2 billion in measures to protect the reef.

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Advances in Remote Sensing and Mapping for Integrated Studies of Reef Ecosystems in Oceania (Great Barrier Reef and Beyond)

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A special issue of Remote Sensing (ISSN 2072-4292). This special issue belongs to the section " Coral Reefs Remote Sensing ".

Deadline for manuscript submissions: closed (15 February 2022) | Viewed by 39120

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Dear Colleagues,

The Great Barrier Reef is the largest coral reef system on Earth and a hotspot of marine biodiversity. The recent widespread and recurrent coral bleaching events are a reminder of the vulnerability of this unique ecosystem, and of similar ecosystems in Oceania, to human activities and a warming world. Protection of these reef systems requires a better understanding of environmental and socio-economic pressures, and the development of integrated management strategies. The rapid expansion of Earth Observation technologies and data has greatly advanced our capability to map and monitor such ecosystems, providing essential information to support and evaluate management and conservation strategies. Submissions are thus invited on recent advances in remote sensing of the Great Barrier Reef and other reef ecosystems in Oceania, from novel methodological approaches (sensors, algorithms development and data management) to applications for environmental monitoring and management, including but not limited to:

• Novel data acquisition platforms and sensors (in situ, aerial and from space).
• Advances in remote sensing algorithms for monitoring coastal and marine water quality (from riverine inputs to algal blooms) and marine habitats and associated health (from coral reefs to seagrass meadows, mangroves and macroalgae).
• Uncertainty propagation in remote sensing products.
• Data access, archiving and analysis tools, and information infrastructure and platforms.
• Citizen Science, crowdsourcing and community engagement.
• Assimilation of remote sensing products into hydrodynamic, sediment transport and biogeochemical models.
• Combined role of environmental stress factors (heat, light, pollutants) and changing carbonate chemistry.
• Integrated linkage between changes in catchments land-use, water quality in the coastal zone and marine ecosystems.
• Critical scales of variability (from site to basin-scale and from short-term processes to long-term trends).
• Development of metrics and guidelines (individual, combined and holistic levels) to quantify risks, changes in the receiving environment and ecosystems, and to evaluate results of management strategies.

Dr. Kadija Oubelkheir Dr. Michelle Devlin Dr. Caroline Petus Guest Editors

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website . Once you are registered, click here to go to the submission form . Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Remote Sensing is an international peer-reviewed open access semimonthly journal published by MDPI.

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News from the Columbia Climate School

New 400-Year Record Shows Great Barrier Reef Faces Catastrophic Damage

Kevin Krajick

Australia’s Great Barrier Reef is under unprecedented pressure, with recent record-high sea-surface temperatures threatening to destroy its remarkable ecology, biodiversity and beauty, according to a new study. The study reconstructs 400 years of summer surface temperatures in the surrounding Coral Sea, providing new evidence that repeated recent coral-bleaching events are linked to increasingly warm summer temperatures―a result of human-caused climate change, the authors say. The study was just published in the journal Nature .

The Great Barrier Reef, encompassing some 133,000 square miles off Australia’s northwest coast, is the world’s largest reef system. It contains thousands of species of fish, molluscs, sponges, crustaceans and many other creatures. Hundreds of coral species comprise the base of the ecosystem, but the corals are stressed when water temperatures suddenly rise beyond normal levels in the austral summer. This causes them to expel the colorful symbiotic algae that inhabit their white skeletons, leading to so-called mass bleaching events. Bleaching does not kill corals outright, but it does make them more vulnerable to starvation and disease, especially if events happen frequently enough that algae populations have trouble recovering.

The research team combined sea-surface temperature reconstructions using geochemical data from coral cores previously collected from the region. They also analyzed climate-model simulations of sea-surface temperatures run with and without climate change. They found that six years over the past two decades were the warmest in the entire 400-year record. In order, starting with the warmest, these were 2024, 2017, 2020, 2016, 2004 and 2022. All the years except 2004 coincided with mass bleaching events. The study concluded that human-caused climate change is to blame.

The magnitude of the recent warming astounded the researchers. University of Melbourne lecturer Benjamin Henley , who led the study, said, “When I plotted the 2024 data point, I had to triple check my calculations. It was off the charts, far above the previous record high in 2017. Tragically, mass coral bleaching has occurred yet again this year.”

“At least over the past 400 years, the frequent bleaching going on now year to year seems unprecedented,” said study coauthor Braddock Linsley, a coral specialist at the Columbia Climate School’s Lamont-Doherty Earth Observatory. “It seems to be connected with what is going on with the global climate.”

Helen McGregor of the University of Wollongong, the second author of the study, said urgent action to stem climate change is needed to prevent devastation of the reef system. “There is no ‘if, but or maybe,’” she said. “The ocean temperatures during these bleaching events are unprecedented in the past four centuries.”

The authors say the research has implications for coral reefs throughout the world, highlighting the link between the long-term trajectory of extreme ocean temperatures and the health and biodiversity of marine ecosystems.

Adapted from a press release by the University of Wollongong.

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Interventions to help coral reefs under global change—A complex decision challenge

* E-mail: [email protected]

Affiliations Australian Institute of Marine Science, QLD, Australia, School of Biological Sciences, The University of Queensland, St. Lucia, QLD, Australia

Affiliation ARC Centre of Excellence in Mathematical and Statistical Frontiers, School of Mathematical Sciences, Queensland University of Technology, QLD, Australia

Affiliation Australian Institute of Marine Science, QLD, Australia

Affiliation Centre for Policy Futures, The University of Queensland, QLD, Australia

Affiliation Great Barrier Reef Foundation, QLD, Australia

Affiliation TropWATER, James Cook University, QLD, Australia

Affiliation School of Biological Sciences, The University of Queensland, St. Lucia, QLD, Australia

Affiliation ARC Centre of Excellence for Environmental Decisions, The University of Queensland, QLD, Australia

• Kenneth R. N. Anthony, 
• Kate J. Helmstedt, 
• Line K. Bay, 
• Pedro Fidelman, 
• Karen E. Hussey, 
• Petra Lundgren, 
• David Mead, 
• Ian M. McLeod, 
• Peter J. Mumby, 

Published: August 26, 2020

• https://doi.org/10.1371/journal.pone.0236399
• Reader Comments

Climate change is impacting coral reefs now. Recent pan-tropical bleaching events driven by unprecedented global heat waves have shifted the playing field for coral reef management and policy. While best-practice conventional management remains essential, it may no longer be enough to sustain coral reefs under continued climate change. Nor will climate change mitigation be sufficient on its own. Committed warming and projected reef decline means solutions must involve a portfolio of mitigation, best-practice conventional management and coordinated restoration and adaptation measures involving new and perhaps radical interventions, including local and regional cooling and shading, assisted coral evolution, assisted gene flow, and measures to support and enhance coral recruitment. We propose that proactive research and development to expand the reef management toolbox fast but safely, combined with expedient trialling of promising interventions is now urgently needed, whatever emissions trajectory the world follows. We discuss the challenges and opportunities of embracing new interventions in a race against time, including their risks and uncertainties. Ultimately, solutions to the climate challenge for coral reefs will require consideration of what society wants, what can be achieved technically and economically, and what opportunities we have for action in a rapidly closing window. Finding solutions that work for coral reefs and people will require exceptional levels of coordination of science, management and policy, and open engagement with society. It will also require compromise, because reefs will change under climate change despite our best interventions. We argue that being clear about society’s priorities, and understanding both the opportunities and risks that come with an expanded toolset, can help us make the most of a challenging situation. We offer a conceptual model to help reef managers frame decision problems and objectives, and to guide effective strategy choices in the face of complexity and uncertainty.

Citation: Anthony KRN, Helmstedt KJ, Bay LK, Fidelman P, Hussey KE, Lundgren P, et al. (2020) Interventions to help coral reefs under global change—A complex decision challenge. PLoS ONE 15(8): e0236399. https://doi.org/10.1371/journal.pone.0236399

Editor: Chaolun Allen Chen, Academia Sinica, TAIWAN

Copyright: © 2020 Anthony et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All data referred to in this paper are available via a web portal associated with the Reef Restoration and Adaptation Program: https://www.gbrrestoration.org/reports#technical-reports .

Funding: The work is supported by a grant from the Australian Government to the Reef Restoration and Adaptation Program ( https://www.gbrrestoration.org ). The funding body played no role in the writing of this paper. Opinions expressed in the paper is that of the authors in their individual capacity.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Climate change is impacting tropical coral reefs globally. Solutions are needed urgently to help reefs cope—and for three reasons. First, coral reefs are biologically the richest ecosystem in the world’s oceans [ 1 , 2 ]. Second, they provide ecosystem services that support livelihoods, recreation and economic activities worth hundreds of billions of dollars annually [ 3 – 6 ]. Third, coral reefs are among the most climate-sensitive ecosystems on Earth [ 7 , 8 ].

The recent marine heat wave exacerbated by the 2015/16 El Niño event led to extensive episodes of coral bleaching [ 9 , 10 ]. On Australia’s Great Barrier Reef, back-to-back bleaching in 2016 and 2017 led to unprecedented loss of coral cover [ 11 , 12 ]. While corals, the reef ecosystem engineers, can recover from severe disturbances [ 13 ], the projected shortening of interludes between increasingly severe bleaching events under even optimistic climate futures [ 14 , 15 ] will diminish the scope for net reef recovery. Growing pressure from ocean acidification, a chemical consequence of carbon emissions, will further diminish this scope [ 16 ].

Reducing greenhouse gas emissions will be necessary to sustain coral reefs in the long term. However, global emissions increased in 2017, 2018 and 2019 [ 17 , 18 ]. Current unconditional climate-mitigation pledges would see the world warm by 2.9 to 3.4°C above pre-industrial levels this century [ 19 ]. Even if global warming could be kept below 1.5°C–currently with less than 1% chance given pledges [ 20 ]–the surface waters of tropical oceans would warm another 0.3°C in coming decades [ 16 ]. Even such minimal continued warming would damage the sensitive coral species [ 21 ] that drive reef recovery [ 22 ] and form critical habitats [ 23 ]. Thus, as it currently stands, the Paris Accord will not protect coral reefs.

Another avenue is to build ecosystem resilience by further improving conventional management interventions and their governance [ 6 ]. Reducing nutrient pollution [ 24 , 25 ], limiting herbivore overfishing [ 26 ] and removing coral predators [ 27 ] can support resilience by enhancing coral growth and survival. This is so because (i) sediments have direct negative effects on coral recruitment and growth [ 28 , 29 ], and (ii) nutrient run-off in combination with herbivore overfishing reduce coral resilience by favouring the growth and survival of algae which prevent coral recruitment [ 30 , 31 ]. Reducing nutrient run-off may also reduce bleaching risks [ 32 – 34 ] and dampen outbreak risks of coral-eating crown-of-thorns starfish [ 35 ]. A problem, however, is that climate change—in addition to causing increased mortality via bleaching events [ 11 ] and storms [ 36 ]—erodes two key biological processes that underpin coral resilience: growth rate [ 37 , 38 ] and recruitment rate [ 39 , 40 ]. Thus, increasing conventional management action cannot compensate for the climate-driven decline in coral survival, growth and recruitment of many coral species in many places [ 16 , 22 , 41 , 42 ]. The situation is analogous to that of a cancer patient: good care helps, but it is only a solution when combined with a cure.

Both climate mitigation and intensified conventional management are indispensible to sustaining healthy coral reefs into the future. But more is needed. While natural processes of physiological acclimation may improve coral heat tolerance [ 43 , 44 ] genetic adaptation generally acts on longer timescales [ 45 ]. Warm-adapted traits may not spread fast enough in most coral species to keep up with the rate of global warming, even under strong carbon mitigation [ 14 , 46 – 48 ].

To build the biological resilience required to tolerate and recover from the projected escalation of marine heat waves [ 49 ] and increasing pressure from ocean acidification [ 50 ], high rates of coral adaptation will be needed. Active interventions to assist adaptation include ways to enhance coral performance including thermal tolerance [ 51 – 53 ] and/or lowering the exposure of corals to bleaching conditions–i.e. dampening heat waves locally and shading against strong solar radiation. A recent review by the National Academy of Sciences, Engineering and Medicine identified 23 candidate interventions with varying scope to become effective, feasible and safe [ 54 ]. While such measures are often referred to as restoration, they go beyond classical restoration techniques by altering biological and ecological resilience or stress exposure, or both. A similar review completed for the Australian Government’s Reef Restoration and Adaptation program (RRAP) examined 160 such interventions across a range of scales (from a few square metres to hundreds of reefs), concluding that 43 warranted more research and development (Box 1 ) and that the possibilities for positive impact overall were promising enough to warrant further investment [ 55 ].

Box 1. Categories of intervention based on functional objective as used in the Reef Restoration and Adaptation Program (RRAP) on the Great Barrier Reef [ 55 ]

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https://doi.org/10.1371/journal.pone.0236399.t001

The questions are then: what new interventions should be developed and added to the management toolbox for coral reefs? And once developed, when and where should they be deployed? How should performance expectations, risks and uncertainties be managed? We argue that an expanded intervention toolbox, as an adaptation strategy, presents at least three core challenges for reef managers, policy-makers and regulators: (1) framing the problem and setting the right objectives, (2) managing risks and uncertainties given the urgency, and (3) assessing and making necessary trade-offs ( Fig 1 ). Here we address each of these challenges. We close with a discussion of how fast and effective research and development (R&D) strategies provide options in a time of crisis and how the governance of on-reef intervention will face unprecendented challenges of coordination and integration. We conclude that the sooner we step up to this challenge, the closer we will be to producing solutions.

The framework is centred on an adaptive management cycle of intervention research and development (R&D), stakeholder and regulatory consultation, and governance. Two-way arrows indicate that steps in the structured decision-making framework form adaptive links with R&D, consultation and the governance of how resources are allocated and actions implemented given updated information. Adapted from: [ 56 – 58 ].

https://doi.org/10.1371/journal.pone.0236399.g001

Challenge 1: Setting the right objectives to solve the right problem

Pristine coral reefs are no more [ 59 , 60 ]. Even under best-case emissions trajectories, coral reefs will likely be transformed by climate change [ 11 ], so striving to retain or recreate historical levels of biodiversity and richness in a warming world may be futile. The most a conservation program may hope to deliver are sustained, yet altered, ecosystem services and priority values. And the results of any program will ultimately depend on how successful emission reductions become. These considerations affect our problem framing and the objectives we can achieve ( Fig 1A ). For example, is the objective to stem the decline in reef biodiversity, is it to sustain ecosystem services, or perhaps to create new ones? Is the objective to stem the decline of key (prized) species, or to sustain the key ecological functions they underpin? Perhaps provocatively, is it really coral reefs we seek to sustain, or is it the benefits they provide for society? We can’t have one without the other, but asking the question helps clarify objectives, and ultimately what we are willing to trade off. Different answers to these questions would lead to very different reef conservation programs.

Defining multiple, and often conflicting, objectives for complex social-ecological systems such as coral reefs is challenging, but critical. Within objectives, which values can be sustained with the capabilities and resources available? Coral reefs produce numerous value streams to society [ 61 , 62 ]. Bona fide adaptation solutions would be those that strike a balance across such value streams–monetary and otherwise. Altered, but functionally resilient, ecosystems are increasingly being embraced in terrestrial and freshwater conservation programs [ 63 – 66 ]. The time may now be right to explore such options for coral reefs also. We revisit this challenge under Prioritisation and trade-offs . With a clear understanding of objectives and values, the decision-making process around developing and applying new and potentially contentious intervention options, in combination with mitigation and conventional management ( Fig 1 , step B), can become informed and transparent [ 57 ].

Challenge 2: Balancing benefits and risks in the face of uncertainty

Developing new technologies for environmental management and conservation is risky: it is expensive, takes a long time, and success is not guaranteed. Risks associated with emerging technologies, whether perceived or real, and their potential side effects, costs, and uncertainties trigger precaution [ 67 ]. There is good reason for this as history is replete with examples of how interventionist management can result in destructive outcomes [ 68 ]. The managers who introduced cane toads to Australia in 1935 to manage the cane beetle did neither have experience nor foresight to consider the catastrophic invasive potential of the toads. Today, the scientific and regulatory communities are much more informed about the biological, ecological, ethical, legal and social implications of new and emerging technologies [ 69 ]. Examples of advancement in the management of risk and uncertainty across a diversity of fields include the protection of nature reserves against invasive species [ 70 ], managed readiness levels of new technologies that enter aviation and space programs [ 71 ], risk assessments of new drugs prior to approval [ 72 , 73 ] and the adoption of driverless cars [ 74 ]. Applied coral reef research and development can and should learn from these and other fields. Doing so can help identify options that, when implemented in a coordinated approach after rigorous development and consultation ( Fig 1 , step C), are effective, safe, acceptable to the public and regulators, and economically rational.

Critically, in a time of rapid climate change, being risk averse can be risky [ 75 ]. Delaying new interventions because of uncertainty around side effects could mean losing key species and functions. However, the risk associated with status quo under different climate futures must be balanced against the risk of premature intervention, especially with technologies that are not yet ready for deployment [ 76 , 77 ]. Premature deployment of untested interventions (e.g. genetic engineering, assisted migration, solar radiation management) may cause ecosystem disruptions [ 54 , 78 , 79 ]. The sooner research and development programs evaluate the potential risks and benefits of interventions, the more informed policy decisions can be about whether to deploy, delay, or dismiss an intervention. This approach is the basis for NASA’s assessment of readiness levels of new technologies entering space programs [ 80 ], for expanding the number of options for medical treatments [ 81 ], and most recently for Australia’s Reef Restoration and Adaptation Program for the Great Barrier Reef [ 55 ]. Unfortunately, the motivation and social license to start conservation programs typically come when ecosystems or species are already in advanced decline [ 75 ]. Such delayed action represents a lost opportunity as interventions take time to develop, and because damage-prevention and restoration are now both needed to sustain ecosystems [ 82 , 83 ]. For example, coral populations in the northern Great Barrier Reef (GBR) are adapted to 1–2°C higher temperatures than populations in the central section [ 84 ], but the North-to- South larval spread is limited by diverging currents [ 47 , 85 ]. Under expectations of escalated GBR-wide warming [ 86 ], building resilience in the central and south using warm-adapted coral stock from the north will be a race against time as both donor reefs and receiving reefs are at risk. While classical reef-restoration approaches using local coral stock or larvae may enhance reef recovery following disturbances [ 87 , 88 ], enhanced climate tolerance is needed to support coral resilience under climate change [ 54 ].

Precaution is central to policy and regulation, but social science research indicates the need to interpret and understand risks more broadly [ 68 ]. Risk assessments of new interventions need to consider views that go beyond those of scientists and regulatory experts. Thus, decision makers and management agencies need to consult reef stakeholders (e.g. tourism operators, commercial and recreational fisheries, conservation groups), Traditional Owners and the wider community. Risk assessments in this context need to be tackled at three levels (1) the risk regime of future climatic conditions, (2) whether interventions will really produce the intended benefits, and (3) risks and costs versus benefits of early vs delayed implementation ( Fig 1 , step D). Such assessments are complicated by the fact that different future conditions will require different solutions, timing and risk tolerance [ 53 ]. What would constitute premature intervention deployment under the expectation of 1.5°C warming this century could be too-little-too-late under the expectation of 3°C warming. Further, picking intervention solutions that are robust to climate change could be a blunt strategy because both timing and intervention type could be misaligned with the conditions that eventually unfold. The most effective solution from a risk-management perspective could be a combination of intervention hedging and improved forecasting, not unlike an investment portfolio strategy [ 89 ].

Challenge 3: Prioritisation and tradeoffs–we can’t save everything

The gap between resources available and resources needed for conservation is widening [ 90 , 91 ]. Consequently, investment prioritisation is necessary [ 92 , 93 ]. How this is done needs to be anchored in the problem framing and by clearly defined ecological, economic and social objectives. Further, prioritisation needs to have line of sight to outcomes that can be achieved given climate uncertainty and funding contraints ( Fig 1 , step E). As an example, consider two extreme yet realistic prioritisation alternatives for a large reef system such as the Great Barrier Reef. Should we aim to sustain a minimum of 5% coral cover over a 1000 km 2 area of reef, or a minimum of 25% coral cover over a 200 km 2 area? Logistics will differ, but the net result is the same in terms of coral area sustained: 50 km 2 . However, depending on the spatial configuration of the saved corals, these alternatives would produce very different ecological outcomes and values for society. Spreading efforts across a large area would speak to system integrity and perhaps the Outstanding Universal Value of the Great Barrier Reef World Heritage Area [ 94 ]. Downsides of spreading efforts thinly include reduced capacity to sustain critical ecological functions such as net reef accretion [ 95 ], and reduced fitness via a reduced demographic Allee effect [ 96 , 97 ]. Conversely, concentrating efforts on a selection of just a few but glorious reefs could sustain parts or all of the GBR’s tourism industry, which is spatially concentrated [ 98 ]. It would enable managers to support ecological functions and services on those focal reefs more easily, and perhaps create spill-over effects to other reefs [ 99 ]. Taken to the extreme under severe climate change, spatial prioritisation under resource constraints could reduce the Great Barrier Reef to a fragmented (and therefore vulnerable) network of coral oases in an otherwise desolate seascape.

Other options might involve targeting reefs that are gateway nodes in the spatial reef network–in other words, investing in well-connected reefs located in the least thermally stressed environments [ 100 ]. Here, efforts to support population growth of climate-hardy corals on source reefs (larval donors) may allow export of their beneficial traits to reefs downcurrent through paths of natural dispersal [ 99 , 101 ]. But risks are that disease agents and potentially invasive species arising from either translocation or assisted gene flow may also spread via similar routes [ 47 ]. Selection criteria should thus favour the dispersal of desirable species only [ 99 ]. The decision challenge associated with spatial prioritisation is therefore one of maximising the spread of genes or traits that produce benefits and minimising those that represent risks. Another option may be to assemble a portfolio of reefs that have less risk of being exposed to the most damaging climate stressors [ 48 , 102 ]. Combining these options may both enhance resilience and reduce stress on priority reefs.

Prioritisation of species adds to the decision challenge for reef restoration and adaptation. Without significant climate mitigation, sensitive coral species will give way to naturally hardier ones [ 11 ], or to species that can adapt faster [ 45 , 103 ]. Picking who should be winners, and ultimately who will be losers, under continued but uncertain climate change is perhaps the biggest challenge facing R&D programs tasked with developing reef rescue interventions. Unfortunately, sensitive coral species tend to be the ones underpinning high-value ecosystem services, including habitat provision for a rich biodiversity [ 23 ] that in part underpin tourism [ 5 ]. Should we invest in making sensitive species hardier but risk failing by not making them hardy enough, thereby wasting resources? Or should we pursue a potentially less risky pathway and support the more climate-hardy species and help them adapt to the consequently altered ecosystems and the different goods and services they provide? Importantly, our best efforts to build coral resilience under severe climate change will not prevent reefs from transitioning to altered ecosystems [ 6 , 60 ]. Strategies to help humans adapt to a changed ecosystem need to combine with strategies that help reefs [ 104 ]. Lastly, can robust keystone species be found that can give climate protection to many other dependent species [ 105 ], thereby sustaining ecosystem services? The latter may ultimately be the most effective choice if species compositions allow the ecosystem to remain functionally resilient [ 64 ]. How these priorities are set ultimately depends on what society wants (objectives and values), what options can be achieved technically, institutionally and socially, and what compromises and risks we are willing to accept. The preferred strategy would be the one that delivers the most positive outcomes to priority objectives (and the values they encompass) with low or manageable risks and within resource constraints [ 57 , 58 , 106 ].

R&D provides options, but choose carefully

The likelihood that the world will warm more than 2°C (air) since preindustrial levels this century was recently 95 percent [ 17 , 20 ]. With this outlook, new intervention options for coral reefs will be in growing demand. Importantly, however, no new intervention can be added to the operational management toolset without significant R&D; it is the prerequisite for intervention effectiveness, safety and cost efficiency [ 67 ]. How interventions are chosen for R&D and progressed through to deployment is both complex and critical because it will determine what options will ultimately be available for managers and when ( Fig 1 , steps B-E). Three questions are at the centre of reef intervention R&D. First, which interventions should be prioritised for development? Second, how should they be queued in time? Third, should intervention strategies be robust or targeted?

Limited resources for R&D means not all interventions can be assessed nor progressed. Complicating this problem is that the more the world warms, and the more ecosystems become affected, the greater the overall demand for intervention resources will be. Misguided investment choices can lock up vital resources in inferior solutions, hampering or preventing the development of superior ones [ 107 ]. Prioritising no-regrets options because they are inexpensive or less challenging technologically [ 77 ] could lead to regrets downstream by preventing or delaying the development of more effective solutions. Prioritisation of interventions for R&D should ideally be a fast adaptive process (indicated by multiple adaptive cycles in Fig 1 ) whereby combinations of interventions are continuously assessed for their combined benefits and risks against environmental, social and economic objectives [ 53 ]. In general terms, the right time to implement an intervention strategy following R&D would be when the cumulative (time-integrated) benefit-to-risk ratio of deployment exceeds the cumulative benefit-to-risk ratio risk of not deploying. Here, benefits are defined as positive outcomes (as likelihood and consequence) for ecosystem services and values for society, and risks as negative outcomes. The benefit-to-risk ratio of these contrasting strategies, however, will depend on the climate future ( Fig 1 , step D).

Robust strategies work across a range of climate change scenarios. While investing in a robust R&D strategy will give some return regardless of climate future, the strategy may eventually underperform because it trades off effectiveness for reduced risk. In contrast, targeted strategies are tuned to different climate scenarios. This involves betting on, and planning for, a specific climate trajectory. This represents high risk, but potentially also high reward. For example, a strategy that buys 1°C thermal tolerance for sensitive and valued coral species (on top of today’s 1°C global warming) may give high ecological, social and economic returns if global warming is kept below 2°C relative to preindustrial. If the world warms much more than 2°C, however, the strategy will be ineffective unless these species continue to adapt. Conversely, a strategy that bets on severe climate change and focuses on helping the hardiest species only (or develops artificial reefs) will miss the opportunity to protect biodiversity if a milder climate scemario unfolds in reality. Developing a portfolio of interventions that allows hedging and a staged roll-out of interventions as climate change unfolds may be ideal, but may again be constrained by resource availability for R&D, and the demands of urgency—real or perceived.

Get people on board

Environmental problems are social problems [ 108 ]. Climate change, mass coral bleaching events and consequent coral reef decline are human-induced and require solutions from science and society. The dynamics of the current coral disease outbreak in the Caribbean are also consistent with ocean warming patterns [ 109 – 111 ]. While interventions that can build resistance to coral disease will differ from those that can build resistance to coral bleaching, a similar approach to solutions is needed. Solutions require innovative thinking and coordination between science, management and policy, and public engagement. There are concerns that restoration and adaptation are distractions from tackling global climate change, the main driver of coral reef decline [ 112 ]. Communication and engagement strategies must reinforce the message that restoration and adaptation are a health-care strategy that can only work in tandem with a cure: urgent global action to address climate change.

Any new interventions on coral reefs, in particular radical ones, will be up against hurdles to achieve social acceptance and to overcome regulatory constraints [ 104 , 105 ], leading to uncertainties that become barriers for solutions ( Fig 1 , step D). Existing regulations operate under a retrospective model that crowbars coral restoration and adaptation into existing policy and legislation. However, future policy development should accommodate risks of future climatic conditions (see challenge two above) whilst simultaneously adapting to the emerging opportunities and challenges of coral restoration and adaptation.

A handful of countries are currently developing or revising existing policy and regulatory processes to assist coral restoration and adaptation in the face of climate change: Australia, USA, Netherlands, France, Costa Rica, Japan, Columbia, and Thailand. The United Nations Environment Program have declared 2021–2030 as the UN Decade of Ecosystem Restoration. The aim is to “support and scale up efforts to prevent, halt and reverse the degradation of ecosystems worldwide and raise awareness of the importance of successful ecosystem restoration” [ 87 ].

To get people on board will require coordinated consulation and transparent decision making that considers all risks, benefits and value consequences of reef intervention in a structured way [ 106 ]. Open communication and engagement around objectives, options and trade-offs will be key.

Strong and coordinated governance needed

Applying a coordinated and well-conceived coral reef intervention program which sets the right objectives, identifies and balances risks, and aims to make optimal tradeoffs, in the face of uncertainty while getting community buy-in and support, will depend on robust and appropriate governance [ 113 , 114 ]. First, at the R&D stage, researchers will need to be provided with the resources to do the job, and the mandate to take risks. For something as new and potentially controversial as large-scale coral reef intervention, consultation and co-development mechanisms must involve regulators, reef stakeholders, Traditional Owners and the public. Internal processes must be agreed on at the outset, to allow ongoing, effective prioritisation while maintaining flexibility in the face of changing conditions and unexpected setbacks. Some of the tested interventions to be examined will simply not work. Strong governance will be particularly important if political pressure for quick action (just do something) mounts in the face of worsening climate conditions. Next, as R&D results yield prospective options for at-scale intervention, governance must adapt to a situation where the costs, profile, and risk associated with failure of the effort have grown substantially. Again, at this stage, costs, benefits, risks and community desires will have to be balanced and trade-offs made, for at-scale deployment to occur.

Conclusions

An expanded toolbox of interventions will provide opportunities to build reef resilience against continued climate change. Without carbon mitigation, no intervention strategy will be successful in the long run. And no single intervention can produce adaptation solutions. A portfolio of new and existing interventions must be combined with mitigation.

New interventions come with risks, but so does the status quo. If the potential of new interventions can be unlocked and their benefits exceed risks for reef, people and economies, they should be developed and deployed. The challenge for science, management and policy, in consultation with communities, is to develop and adopt technologies that will be both safe and effective—and within years rather than decades.

What climate trajectory will unfold is uncertain. But what is certain is that we need an expanded set of options to safeguard coral reefs and dependent people and industries. Research and development can help, but only if efforts are focused, coordinated and highly integrated. To do this will require a level of organisation, collaboration and integration across disciplines never seen before in natural sciences, conservation and policy.

Acknowledgments

The work was conducted as part of the Reef Restoration and Adaptation Program (RRAP, https://www.gbrrestoration.org ). RRAP data referred to in this paper can be accessed via: https://www.gbrrestoration.org/reports#technical-reports . We thank Prof Allen Chen and Prof Bob Steneck for insightful reviews and comments that improved the paper. Opinions expressed in the paper are those of the authors in their individual capacity.

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Great Barrier Reef waters were hottest in 400 years over the past decade, study finds

A school of blue-green chromid fish swim above corals on Moore Reef in Gunggandji Sea Country off coast of Queensland in eastern Australia on Nov. 13, 2022. (AP Photo/Sam McNeil)

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The Remoora pontoon, owned by Reef Magic, sits above a section of the Great Barrier Reef above Moore Reef in Gunggandji Sea Country off coast of Queensland in eastern Australia on Nov. 14, 2022. (AP Photo/Sam McNeil)

WASHINGTON (AP) — Ocean temperatures in the Great Barrier Reef hit their highest level in 400 years over the past decade, according to researchers who warned that the reef likely won’t survive if planetary warming isn’t stopped.

During that time, between 2016 and 2024, the Great Barrier Reef, the world’s largest coral reef ecosystem and one of the most biodiverse, suffered mass coral bleaching events. That’s when water temperatures get too hot and coral expel the algae that provide them with color and food, and sometimes die. Earlier this year, aerial surveys of over 300 reefs in the system off Australia’s northeast coast found bleaching in shallow water areas spanning two-thirds of the reef, according to Great Barrier Reef Marine Park Authority.

Researchers from Melbourne University and other universities in Australia, in a paper published Wednesday in the journal Nature, were able to compare recent ocean temperatures to historical ones by using coral skeleton samples from the Coral Sea to reconstruct sea surface temperature data from 1618 to 1995. They coupled that with sea surface temperature data from 1900 to 2024.

They observed largely stable temperatures before 1900, and steady warming from January to March from 1960 to 2024. And during five years of coral bleaching in the past decade — during 2016, 2017, 2020, 2022 and 2024 — temperatures in January and March were significantly higher than anything dating back to 1618, researchers found. They used climate models to attribute the warming rate after 1900 to human-caused climate change. The only other year nearly as warm as the mass bleaching years of the past decade was 2004.

“The reef is in danger and if we don’t divert from our current course, our generation will likely witness the demise of one of those great natural wonders,” said Benjamin Henley, the study’s lead author and a lecturer of sustainable urban management at the University of Melbourne. “If you put all of the evidence together ... heat extremes are occurring too often for those corals to effectively adapt and evolve.”

Across the world, reefs are key to seafood production and tourism. Scientists have long said additional loss of coral is likely to be a casualty of future warming as the world approaches the 1.5 degrees Celsius (2.7 degrees Fahrenheit) threshold that countries agreed to try and keep warming under in the 2015 Paris climate agreement.

Even if global warming is kept under the Paris Agreement’s goal, which scientists say Earth is almost guaranteed to cross, 70% to 90% of corals across the globe could be threatened, the study’s authors said. As a result, future coral reefs would likely have less diversity in coral species — which has already been happening as the oceans have grown hotter.

Coral reefs have been evolving over the past quarter century in response to bleaching events like the ones the study’s authors highlighted, said Michael McPhaden, a senior climate scientist at the National Oceanic and Atmospheric Administration who was not involved with the study. But even the most robust coral may soon not be able to withstand the elevated temperatures expected under a warming climate with “the relentless rise in greenhouse gas concentrations in the atmosphere,” he said.

The Great Barrier Reef serves as an economic resource for the region and protects against severe tropical storms.

As more heat-tolerant coral replaces the less heat-tolerant species in the colorful underwater rainbow jungle, McPhaden said there’s “real concern” about the expected extreme loss in the number of species and reduction in area that the world’s largest reef covers.

“It’s the canary in the coal mine in terms of climate change,” McPhaden said.

This story has corrected attribution for the aerial surveys in the 2nd paragraph to the Great Barrier Reef Marine Park Authority, rather than NASA.

The Associated Press’ climate and environmental coverage receives financial support from multiple private foundations. AP is solely responsible for all content. Find AP’s standards for working with philanthropies, a list of supporters and funded coverage areas at AP.org .

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• Published: 16 March 2017

Global warming and recurrent mass bleaching of corals

• Terry P. Hughes 1 ,
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• Mariana Álvarez-Noriega 1 , 2 ,
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• Kristen D. Anderson 1 ,
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Nature volume  543 ,  pages 373–377 ( 2017 ) Cite this article

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During 2015–2016, record temperatures triggered a pan-tropical episode of coral bleaching, the third global-scale event since mass bleaching was first documented in the 1980s. Here we examine how and why the severity of recurrent major bleaching events has varied at multiple scales, using aerial and underwater surveys of Australian reefs combined with satellite-derived sea surface temperatures. The distinctive geographic footprints of recurrent bleaching on the Great Barrier Reef in 1998, 2002 and 2016 were determined by the spatial pattern of sea temperatures in each year. Water quality and fishing pressure had minimal effect on the unprecedented bleaching in 2016, suggesting that local protection of reefs affords little or no resistance to extreme heat. Similarly, past exposure to bleaching in 1998 and 2002 did not lessen the severity of bleaching in 2016. Consequently, immediate global action to curb future warming is essential to secure a future for coral reefs.

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Acknowledgements

The authors acknowledge the 21 institutions that supported this research, in Australia, the UK, and the USA. Twenty-six of the authors are supported by funding from the Australian Research Council’s Centre of Excellence Program. Other funding support includes the Australian Commonwealth Government, the European Union, the USA National Oceanographic & Atmospheric Administration, and USA National Science Foundation. GlobColour data ( http://globcolour.info ) used in this study has been developed, validated, and distributed by ACRI-ST, France. The contents in this manuscript are solely the opinions of the authors and do not constitute a statement of policy, decision or position on behalf of NOAA or the US Government. We thank the many student volunteers who participated in field studies.

Author information

24 Hanwood Court, Gilston, Queensland 4211, Australia

Authors and Affiliations

Australian Research Council Centre of Excellence for Coral Reef Studies, James Cook University, Townsville, 4811, Queensland, Australia

Terry P. Hughes, James T. Kerry, Mariana Álvarez-Noriega, Jorge G. Álvarez-Romero, Kristen D. Anderson, Andrew H. Baird, David R. Bellwood, Tom C. Bridge, Sean R. Connolly, Graeme S. Cumming, Hugo B. Harrison, Andrew S. Hoey, Mia O. Hoogenboom, Chao-yang Kuo, Janice M. Lough, Michael J. McWilliam, Morgan S. Pratchett, Gergely Torda & Bette L. Willis

College of Science and Engineering, James Cook University, Townsville, 4811, Queensland, Australia

Mariana Álvarez-Noriega, David R. Bellwood, Ray Berkelmans, Sean R. Connolly, Mia O. Hoogenboom & Bette L. Willis

Commonwealth Science and Industry Research Organization, GPO Box 2583, Brisbane, 4001, Queensland, Australia

Russell C. Babcock

School of Biology, University of Leeds, Leeds, LS2 9JT, UK

Maria Beger

Queensland Museum, 70-102 Flinders St, Townsville, 4810, Queensland, Australia

Tom C. Bridge

Australian Research Council, Centre of Excellence for Coral Reef Studies, School of Biological Sciences, University of Queensland, Brisbane, 4072, Queensland, Australia

Ian R. Butler, John M. Pandolfi & Brigitte Sommer

School of Medical Sciences, University of Sydney, Sydney, New South Wales, 2006, Australia

Maria Byrne

Australian Institute of Marine Science, PMB 3, Townsville, 4810, Queensland, Australia

Neal E. Cantin, Janice M. Lough & Gergely Torda

Australian Research Council Centre of Excellence in Coral Reef Studies, Oceans Institute and School of Earth and Environment, University of Western Australia, Crawley, Western Australia, 6009, Australia

Steeve Comeau, Ryan J. Lowe, Malcolm T. McCulloch & Verena Schoepf

Department of Primary Industries, Fisheries Research, PO Box 4291, Coffs Harbour, 2450, New South Wales, Australia

Steven J. Dalton & Hamish A. Malcolm

School of Environment, and Australian Rivers Institute, Griffith University, Brisbane, 4111, Queensland, Australia

Guillermo Diaz-Pulido & Emma V. Kennedy

Coral Reef Watch, US National Oceanic and Atmospheric Administration, College Park, Maryland, 20740, USA

C. Mark Eakin, Scott F. Heron, Gang Liu & William J. Skirving

School of Biological Sciences, University of Sydney, Sydney, 2006, New South Wales, Australia

Will F. Figueira

Australian Institute of Marine Science, Indian Oceans Marine Research Centre, University of Western Australia, Crawley, 6009, Western Australia, Australia

James P. Gilmour

Global Science & Technology, Inc., Greenbelt, 20770, Maryland, USA

Scott F. Heron, Gang Liu & William J. Skirving

Marine Geophysical Laboratory, College of Science, Technology and Engineering, James Cook University, Townsville, 4811, Queensland, Australia

Scott F. Heron

Department of Environment and Agriculture, Curtin University, Perth, 6845, Western Australia, Australia

Jean-Paul A. Hobbs

Great Barrier Reef Marine Park Authority, PO Box 1379, Townsville, 4810, Queensland, Australia

Rachel J. Pears & David R. Wachenfeld

Torres Strait Regional Authority, PO Box 261, Thursday Island, 4875, Queensland, Australia

Tristan Simpson

Department of Parks and Wildlife, Kensington, Perth, 6151, Western Australia, Australia

Shaun K. Wilson

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Contributions

The study was conceptualized by T.P.H. who wrote the first draft of the paper. All authors contributed to writing subsequent drafts. J.T.K. coordinated data compilation, analysis and graphics. Aerial bleaching surveys in 2016 of the Great Barrier Reef and Torres Strait were executed by J.T.K., T.P.H. and T.S., and in 1998 and 2002 by R.B. and D.R.W. Underwater bleaching censuses in 2016 were undertaken on the Great Barrier Reef by M.A.-N., A.H.B., D.R.B., M.B., N.E.C., C.Y.K., G.D.-P., A.S.H., M.O.H., E.V.K., M.J.M., R.J.P., M.S.P., G.T. and B.L.W., in the Coral Sea by T.C.B. and H.B.H., in subtropical Queensland and New South Wales by M.B., I.R.B., R.C.B., S.J.D., W.F.F., H.A.M., J.M.P. and B.S., off western Australia by R.C.B., S.C., J.P.G., J.-P.A.H., M.T.M., V.S. and S.K.W. J.G.A.-R., S.R.C., C.M.E., S.F.H., G.L., J.M.L. and W.J.S. undertook the analysis matching satellite data to the bleaching footprints on the Great Barrier Reef.

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Correspondence to Terry P. Hughes .

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Extended data figures and tables

Extended data figure 1 a generalized linear model to explain the severity of coral bleaching..

Curves show the estimated relationships between probability of severe bleaching (>30%) on individual reefs of the Great Barrier Reef in 2016 and three explanatory variables (DHWs, chlorophyll a , and reef zoning, see Extended Data Table 1 ). The DHW-only model is shown in black. For the DHW plus chlorophyll a model, the blue threshold shows the estimated relationship between probability of severe bleaching and DHW for the 25th percentile of chlorophyll a , and the brown threshold shows the same for the 75th percentile of chlorophyll a . For the DHW plus reef zoning model, the red threshold shows the relationship for fished reefs, and the green for unfished reefs. Water-quality metrics and level of reef protection make little, if any, difference.

Extended Data Figure 2 Difference in daily sea surface temperatures between the northern and southern Great Barrier Reef, before and after ex-tropical cyclone Winston.

The disparity between Lizard Island (14.67° S) and Heron Island (23.44° S) increased from 1 °C in late February to 4 °C in early March 2016.

Extended Data Figure 3 A test for the effect of past bleaching experience on the severity of bleaching in 2016.

The relationship between previous bleaching scores (in 1998 or 2002, whichever was higher) and the residuals from the DHW generalized linear model ( Extended Data Table 1 ). Each data point represents an individual reef that was scored repeatedly. There is no negative relationship to support acclimation or adaptation.

Extended Data Figure 4 Flight tracks of aerial surveys of coral bleaching, conducted along and across the Great Barrier Reef and Torres Strait in March and April 2016.

Blue colour represents land, white colour represents open water.

Extended Data Figure 5 Ground-truthing comparisons of aerial and underwater bleaching scores.

Aerial scores are: 0 (<1% of colonies bleached), 1 (1–10%), 2 (10–30%), 3 (30–60%) and 4 (60–100%) on the Great Barrier Reef in 2016 ( Fig. 1a ). Continuous (0–100%) underwater scores are based on in situ observations from 259 sites (104 reefs). Error bars indicate two standard errors both above and below the median underwater score, separately for each aerial category.

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Hughes, T., Kerry, J., Álvarez-Noriega, M. et al. Global warming and recurrent mass bleaching of corals. Nature 543 , 373–377 (2017). https://doi.org/10.1038/nature21707

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Received : 07 October 2016

Accepted : 16 February 2017

Published : 16 March 2017

Issue Date : 16 March 2017

DOI : https://doi.org/10.1038/nature21707

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Great Barrier Reef waters were hottest in 400 years over the past decade, study finds

A school of blue-green chromid fish swim above corals on Moore Reef in Gunggandji Sea Country off coast of Queensland in eastern Australia on Nov. 13, 2022.

Sam McNeil / AP

Ocean temperatures in the Great Barrier Reef hit their highest level in 400 years over the past decade, according to researchers who warned that the reef likely won’t survive if planetary warming isn’t stopped.

During that time, between 2016 and 2024, the Great Barrier Reef, the world’s largest coral reef ecosystem and one of the most biodiverse, suffered mass coral bleaching events. That’s when water temperatures get too hot and coral expel the algae that provide them with color and food, and sometimes die. Earlier this year, aerial surveys of over 300 reefs in the system off Australia’s northeast coast found bleaching in shallow water areas spanning two-thirds of the reef, according to Great Barrier Reef Marine Park Authority.

Researchers from Melbourne University and other universities in Australia, in a paper published Wednesday in the journal Nature, were able to compare recent ocean temperatures to historical ones by using coral skeleton samples from the Coral Sea to reconstruct sea surface temperature data from 1618 to 1995. They coupled that with sea surface temperature data from 1900 to 2024.

They observed largely stable temperatures before 1900, and steady warming from January to March from 1960 to 2024. And during five years of coral bleaching in the past decade — during 2016, 2017, 2020, 2022 and 2024 — temperatures in January and March were significantly higher than anything dating back to 1618, researchers found. They used climate models to attribute the warming rate after 1900 to human-caused climate change. The only other year nearly as warm as the mass bleaching years of the past decade was 2004.

“The reef is in danger and if we don’t divert from our current course, our generation will likely witness the demise of one of those great natural wonders,” said Benjamin Henley, the study’s lead author and a lecturer of sustainable urban management at the University of Melbourne. “If you put all of the evidence together ... heat extremes are occurring too often for those corals to effectively adapt and evolve.”

Across the world, reefs are key to seafood production and tourism. Scientists have long said additional loss of coral is likely to be a casualty of future warming as the world approaches the 1.5 degrees Celsius (2.7 degrees Fahrenheit) threshold that countries agreed to try and keep warming under in the 2015 Paris climate agreement.

Even if global warming is kept under the Paris Agreement’s goal, which scientists say Earth is almost guaranteed to cross, 70% to 90% of corals across the globe could be threatened, the study’s authors said. As a result, future coral reefs would likely have less diversity in coral species — which has already been happening as the oceans have grown hotter.

Coral reefs have been evolving over the past quarter century in response to bleaching events like the ones the study’s authors highlighted, said Michael McPhaden, a senior climate scientist at the National Oceanic and Atmospheric Administration who was not involved with the study. But even the most robust coral may soon not be able to withstand the elevated temperatures expected under a warming climate with “the relentless rise in greenhouse gas concentrations in the atmosphere,” he said.

The Great Barrier Reef serves as an economic resource for the region and protects against severe tropical storms.

As more heat-tolerant coral replaces the less heat-tolerant species in the colorful underwater rainbow jungle, McPhaden said there’s “real concern” about the expected extreme loss in the number of species and reduction in area that the world’s largest reef covers.

“It’s the canary in the coal mine in terms of climate change,” McPhaden said.

The Associated Press’ climate and environmental coverage receives financial support from multiple private foundations. AP is solely responsible for all content. Find AP’s standards for working with philanthropies, a list of supporters and funded coverage areas at AP.org .

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New 400-year temperature record shows Great Barrier Reef is facing catastrophic damage, researchers warn

by University of Wollongong

The Great Barrier Reef is under critical pressure, with warming sea temperatures and mass coral bleaching events threatening to destroy the remarkable ecology, biodiversity, and beauty of the world's largest coral reef, according to new research.

"Highest ocean heat in four centuries places Great Barrier Reef in danger," published in Nature 8 August, led by University of Wollongong (UOW) Honorary Fellow and University of Melbourne Lecturer Dr. Benjamin Henley, provides evidence of the impact that rising sea surface temperatures have had, and will continue to have, on Australia's ecological jewel.

The research reconstructs 400 years of summer sea surface temperatures in the Coral Sea. The results chronicle extreme recent ocean heat that has led to mass coral bleaching on the Great Barrier Reef.

UNESCO's World Heritage Committee recently handed down its final decision on the state of the Great Barrier Reef, declining to list the Reef as in danger. However, the scientists have pushed back and say, based on their new evidence, the Great Barrier Reef is absolutely in danger.

Dr. Henley and his team combined sea surface temperature reconstructions using geochemical data from coral cores previously collected from the region. They also analyzed climate model simulations of sea surface temperatures, run with and without climate change, finding that human-caused climate change is to blame for the rising temperatures in the region.

The recent mass bleaching events coincide with five of the six hottest years in the new 400-year-long record. In the years 2024, 2017 and 2020, the Coral Sea reached 400-year highs, with 2024 being the warmest on record by a large margin.

The recent heat events in 2016, 2004, and 2022, were the next warmest three years on record. The impact on the Great Barrier Reef's unparalleled ecology and biodiversity over repeated sequences of mass coral bleaching is devastating.

"When I plotted the 2024 data point, I had to triple check my calculations—it was off the charts—far above the previous record high in 2017. I could almost not believe it. Tragically, mass coral bleaching has occurred yet again this year," Dr. Henley said.

"In the absence of rapid, coordinated and ambitious global action to combat climate change, we will likely witness the demise of one of Earth's most spectacular natural wonders.

"When you compile all of the evidence we have, it's the inevitability of the impacts on the reef in the coming years that really gets to me."

Professor Helen McGregor, from UOW's Environmental Futures, who is the second author of the study, said urgent action is needed to prevent devastation of one of the world's most important ecosystems.

"There is no 'if, but or maybe'—the ocean temperatures during these bleaching events are unprecedented in the past four centuries."

"The Great Barrier Reef is facing catastrophe if anthropogenic climate change is not immediately addressed. The very corals that have lived for hundreds of years and that gave us the data for our study are themselves under serious threat," Professor McGregor said.

"Our climate model analysis confirms that human influence on the climate system is responsible for the rapid warming in recent decades," said Dr. Henley, who undertook most of the study as a post-doctoral researcher at UOW, where he is now an Honorary Fellow.

"Without urgent intervention, our iconic Great Barrier Reef is at risk of near-annual bleaching from high ocean temperatures. The Reef's fundamental ecological integrity and outstanding universal value are at stake.

"We have many of the key solutions to stop climate change ; what we need is a step change in the level of coordinated national and international action to transition to net zero.

"We can never lose hope. Every fraction of a degree of warming we avoid will lead to a better future for the human and natural systems of our planet.

"We hope that our study equips policymakers with more evidence to pursue deeper cuts in greenhouse gas emissions internationally."

Coral bleaching occurs when stress causes the corals to expel the algae that live in their tissue. The algae give corals their vibrant colors and without them the coral's white skeleton is exposed.

Stress from environmental disturbances and declining water quality can lead to bleaching, but recent warming in sea temperatures has led to bleaching on a mass scale. Corals can recover from bleaching events if the stress trigger, such as extreme ocean warming, is reduced for a significant period. The reef has experienced five major mass coral bleaching events since 2016.

While the Great Barrier Reef is the world's largest coral reef system, the groundbreaking research has implications for coral reef systems throughout the world, highlighting the link between the long-term trajectory of extreme ocean temperatures and the ecological health and biodiversity of these complex and crucial ecosystems.

Researchers from Securing Antarctic's Environmental Future, UOW, University of Melbourne, The University of Queensland, the Australian Institute of Marine Science, Tulane University (U.S.), and Columbia University (U.S.) contributed to the paper.

Journal information: Nature

Provided by University of Wollongong

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Interventions to help coral reefs under global change—A complex decision challenge

Kenneth r. n. anthony.

1 Australian Institute of Marine Science, QLD, Australia

2 School of Biological Sciences, The University of Queensland, St. Lucia, QLD, Australia

Kate J. Helmstedt

3 ARC Centre of Excellence in Mathematical and Statistical Frontiers, School of Mathematical Sciences, Queensland University of Technology, QLD, Australia

Line K. Bay

Pedro fidelman.

4 Centre for Policy Futures, The University of Queensland, QLD, Australia

Karen E. Hussey

Petra lundgren.

5 Great Barrier Reef Foundation, QLD, Australia

Ian M. McLeod

6 TropWATER, James Cook University, QLD, Australia

Peter J. Mumby

Maxine newlands, britta schaffelke, kerrie a. wilson.

7 ARC Centre of Excellence for Environmental Decisions, The University of Queensland, QLD, Australia

Paul E. Hardisty

Associated data.

All data referred to in this paper are available via a web portal associated with the Reef Restoration and Adaptation Program: https://www.gbrrestoration.org/reports#technical-reports .

Climate change is impacting coral reefs now. Recent pan-tropical bleaching events driven by unprecedented global heat waves have shifted the playing field for coral reef management and policy. While best-practice conventional management remains essential, it may no longer be enough to sustain coral reefs under continued climate change. Nor will climate change mitigation be sufficient on its own. Committed warming and projected reef decline means solutions must involve a portfolio of mitigation, best-practice conventional management and coordinated restoration and adaptation measures involving new and perhaps radical interventions, including local and regional cooling and shading, assisted coral evolution, assisted gene flow, and measures to support and enhance coral recruitment. We propose that proactive research and development to expand the reef management toolbox fast but safely, combined with expedient trialling of promising interventions is now urgently needed, whatever emissions trajectory the world follows. We discuss the challenges and opportunities of embracing new interventions in a race against time, including their risks and uncertainties. Ultimately, solutions to the climate challenge for coral reefs will require consideration of what society wants, what can be achieved technically and economically, and what opportunities we have for action in a rapidly closing window. Finding solutions that work for coral reefs and people will require exceptional levels of coordination of science, management and policy, and open engagement with society. It will also require compromise, because reefs will change under climate change despite our best interventions. We argue that being clear about society’s priorities, and understanding both the opportunities and risks that come with an expanded toolset, can help us make the most of a challenging situation. We offer a conceptual model to help reef managers frame decision problems and objectives, and to guide effective strategy choices in the face of complexity and uncertainty.

Introduction

Climate change is impacting tropical coral reefs globally. Solutions are needed urgently to help reefs cope—and for three reasons. First, coral reefs are biologically the richest ecosystem in the world’s oceans [ 1 , 2 ]. Second, they provide ecosystem services that support livelihoods, recreation and economic activities worth hundreds of billions of dollars annually [ 3 – 6 ]. Third, coral reefs are among the most climate-sensitive ecosystems on Earth [ 7 , 8 ].

The recent marine heat wave exacerbated by the 2015/16 El Niño event led to extensive episodes of coral bleaching [ 9 , 10 ]. On Australia’s Great Barrier Reef, back-to-back bleaching in 2016 and 2017 led to unprecedented loss of coral cover [ 11 , 12 ]. While corals, the reef ecosystem engineers, can recover from severe disturbances [ 13 ], the projected shortening of interludes between increasingly severe bleaching events under even optimistic climate futures [ 14 , 15 ] will diminish the scope for net reef recovery. Growing pressure from ocean acidification, a chemical consequence of carbon emissions, will further diminish this scope [ 16 ].

Reducing greenhouse gas emissions will be necessary to sustain coral reefs in the long term. However, global emissions increased in 2017, 2018 and 2019 [ 17 , 18 ]. Current unconditional climate-mitigation pledges would see the world warm by 2.9 to 3.4°C above pre-industrial levels this century [ 19 ]. Even if global warming could be kept below 1.5°C–currently with less than 1% chance given pledges [ 20 ]–the surface waters of tropical oceans would warm another 0.3°C in coming decades [ 16 ]. Even such minimal continued warming would damage the sensitive coral species [ 21 ] that drive reef recovery [ 22 ] and form critical habitats [ 23 ]. Thus, as it currently stands, the Paris Accord will not protect coral reefs.

Another avenue is to build ecosystem resilience by further improving conventional management interventions and their governance [ 6 ]. Reducing nutrient pollution [ 24 , 25 ], limiting herbivore overfishing [ 26 ] and removing coral predators [ 27 ] can support resilience by enhancing coral growth and survival. This is so because (i) sediments have direct negative effects on coral recruitment and growth [ 28 , 29 ], and (ii) nutrient run-off in combination with herbivore overfishing reduce coral resilience by favouring the growth and survival of algae which prevent coral recruitment [ 30 , 31 ]. Reducing nutrient run-off may also reduce bleaching risks [ 32 – 34 ] and dampen outbreak risks of coral-eating crown-of-thorns starfish [ 35 ]. A problem, however, is that climate change—in addition to causing increased mortality via bleaching events [ 11 ] and storms [ 36 ]—erodes two key biological processes that underpin coral resilience: growth rate [ 37 , 38 ] and recruitment rate [ 39 , 40 ]. Thus, increasing conventional management action cannot compensate for the climate-driven decline in coral survival, growth and recruitment of many coral species in many places [ 16 , 22 , 41 , 42 ]. The situation is analogous to that of a cancer patient: good care helps, but it is only a solution when combined with a cure.

Both climate mitigation and intensified conventional management are indispensible to sustaining healthy coral reefs into the future. But more is needed. While natural processes of physiological acclimation may improve coral heat tolerance [ 43 , 44 ] genetic adaptation generally acts on longer timescales [ 45 ]. Warm-adapted traits may not spread fast enough in most coral species to keep up with the rate of global warming, even under strong carbon mitigation [ 14 , 46 – 48 ].

To build the biological resilience required to tolerate and recover from the projected escalation of marine heat waves [ 49 ] and increasing pressure from ocean acidification [ 50 ], high rates of coral adaptation will be needed. Active interventions to assist adaptation include ways to enhance coral performance including thermal tolerance [ 51 – 53 ] and/or lowering the exposure of corals to bleaching conditions–i.e. dampening heat waves locally and shading against strong solar radiation. A recent review by the National Academy of Sciences, Engineering and Medicine identified 23 candidate interventions with varying scope to become effective, feasible and safe [ 54 ]. While such measures are often referred to as restoration, they go beyond classical restoration techniques by altering biological and ecological resilience or stress exposure, or both. A similar review completed for the Australian Government’s Reef Restoration and Adaptation program (RRAP) examined 160 such interventions across a range of scales (from a few square metres to hundreds of reefs), concluding that 43 warranted more research and development (Box 1 ) and that the possibilities for positive impact overall were promising enough to warrant further investment [ 55 ].

Box 1. Categories of intervention based on functional objective as used in the Reef Restoration and Adaptation Program (RRAP) on the Great Barrier Reef [ 55 ]

The questions are then: what new interventions should be developed and added to the management toolbox for coral reefs? And once developed, when and where should they be deployed? How should performance expectations, risks and uncertainties be managed? We argue that an expanded intervention toolbox, as an adaptation strategy, presents at least three core challenges for reef managers, policy-makers and regulators: (1) framing the problem and setting the right objectives, (2) managing risks and uncertainties given the urgency, and (3) assessing and making necessary trade-offs ( Fig 1 ). Here we address each of these challenges. We close with a discussion of how fast and effective research and development (R&D) strategies provide options in a time of crisis and how the governance of on-reef intervention will face unprecendented challenges of coordination and integration. We conclude that the sooner we step up to this challenge, the closer we will be to producing solutions.

The framework is centred on an adaptive management cycle of intervention research and development (R&D), stakeholder and regulatory consultation, and governance. Two-way arrows indicate that steps in the structured decision-making framework form adaptive links with R&D, consultation and the governance of how resources are allocated and actions implemented given updated information. Adapted from: [ 56 – 58 ].

Challenge 1: Setting the right objectives to solve the right problem

Pristine coral reefs are no more [ 59 , 60 ]. Even under best-case emissions trajectories, coral reefs will likely be transformed by climate change [ 11 ], so striving to retain or recreate historical levels of biodiversity and richness in a warming world may be futile. The most a conservation program may hope to deliver are sustained, yet altered, ecosystem services and priority values. And the results of any program will ultimately depend on how successful emission reductions become. These considerations affect our problem framing and the objectives we can achieve ( Fig 1A ). For example, is the objective to stem the decline in reef biodiversity, is it to sustain ecosystem services, or perhaps to create new ones? Is the objective to stem the decline of key (prized) species, or to sustain the key ecological functions they underpin? Perhaps provocatively, is it really coral reefs we seek to sustain, or is it the benefits they provide for society? We can’t have one without the other, but asking the question helps clarify objectives, and ultimately what we are willing to trade off. Different answers to these questions would lead to very different reef conservation programs.

Defining multiple, and often conflicting, objectives for complex social-ecological systems such as coral reefs is challenging, but critical. Within objectives, which values can be sustained with the capabilities and resources available? Coral reefs produce numerous value streams to society [ 61 , 62 ]. Bona fide adaptation solutions would be those that strike a balance across such value streams–monetary and otherwise. Altered, but functionally resilient, ecosystems are increasingly being embraced in terrestrial and freshwater conservation programs [ 63 – 66 ]. The time may now be right to explore such options for coral reefs also. We revisit this challenge under Prioritisation and trade-offs . With a clear understanding of objectives and values, the decision-making process around developing and applying new and potentially contentious intervention options, in combination with mitigation and conventional management ( Fig 1 , step B), can become informed and transparent [ 57 ].

Challenge 2: Balancing benefits and risks in the face of uncertainty

Developing new technologies for environmental management and conservation is risky: it is expensive, takes a long time, and success is not guaranteed. Risks associated with emerging technologies, whether perceived or real, and their potential side effects, costs, and uncertainties trigger precaution [ 67 ]. There is good reason for this as history is replete with examples of how interventionist management can result in destructive outcomes [ 68 ]. The managers who introduced cane toads to Australia in 1935 to manage the cane beetle did neither have experience nor foresight to consider the catastrophic invasive potential of the toads. Today, the scientific and regulatory communities are much more informed about the biological, ecological, ethical, legal and social implications of new and emerging technologies [ 69 ]. Examples of advancement in the management of risk and uncertainty across a diversity of fields include the protection of nature reserves against invasive species [ 70 ], managed readiness levels of new technologies that enter aviation and space programs [ 71 ], risk assessments of new drugs prior to approval [ 72 , 73 ] and the adoption of driverless cars [ 74 ]. Applied coral reef research and development can and should learn from these and other fields. Doing so can help identify options that, when implemented in a coordinated approach after rigorous development and consultation ( Fig 1 , step C), are effective, safe, acceptable to the public and regulators, and economically rational.

Critically, in a time of rapid climate change, being risk averse can be risky [ 75 ]. Delaying new interventions because of uncertainty around side effects could mean losing key species and functions. However, the risk associated with status quo under different climate futures must be balanced against the risk of premature intervention, especially with technologies that are not yet ready for deployment [ 76 , 77 ]. Premature deployment of untested interventions (e.g. genetic engineering, assisted migration, solar radiation management) may cause ecosystem disruptions [ 54 , 78 , 79 ]. The sooner research and development programs evaluate the potential risks and benefits of interventions, the more informed policy decisions can be about whether to deploy, delay, or dismiss an intervention. This approach is the basis for NASA’s assessment of readiness levels of new technologies entering space programs [ 80 ], for expanding the number of options for medical treatments [ 81 ], and most recently for Australia’s Reef Restoration and Adaptation Program for the Great Barrier Reef [ 55 ]. Unfortunately, the motivation and social license to start conservation programs typically come when ecosystems or species are already in advanced decline [ 75 ]. Such delayed action represents a lost opportunity as interventions take time to develop, and because damage-prevention and restoration are now both needed to sustain ecosystems [ 82 , 83 ]. For example, coral populations in the northern Great Barrier Reef (GBR) are adapted to 1–2°C higher temperatures than populations in the central section [ 84 ], but the North-to- South larval spread is limited by diverging currents [ 47 , 85 ]. Under expectations of escalated GBR-wide warming [ 86 ], building resilience in the central and south using warm-adapted coral stock from the north will be a race against time as both donor reefs and receiving reefs are at risk. While classical reef-restoration approaches using local coral stock or larvae may enhance reef recovery following disturbances [ 87 , 88 ], enhanced climate tolerance is needed to support coral resilience under climate change [ 54 ].

Precaution is central to policy and regulation, but social science research indicates the need to interpret and understand risks more broadly [ 68 ]. Risk assessments of new interventions need to consider views that go beyond those of scientists and regulatory experts. Thus, decision makers and management agencies need to consult reef stakeholders (e.g. tourism operators, commercial and recreational fisheries, conservation groups), Traditional Owners and the wider community. Risk assessments in this context need to be tackled at three levels (1) the risk regime of future climatic conditions, (2) whether interventions will really produce the intended benefits, and (3) risks and costs versus benefits of early vs delayed implementation ( Fig 1 , step D). Such assessments are complicated by the fact that different future conditions will require different solutions, timing and risk tolerance [ 53 ]. What would constitute premature intervention deployment under the expectation of 1.5°C warming this century could be too-little-too-late under the expectation of 3°C warming. Further, picking intervention solutions that are robust to climate change could be a blunt strategy because both timing and intervention type could be misaligned with the conditions that eventually unfold. The most effective solution from a risk-management perspective could be a combination of intervention hedging and improved forecasting, not unlike an investment portfolio strategy [ 89 ].

Challenge 3: Prioritisation and tradeoffs–we can’t save everything

The gap between resources available and resources needed for conservation is widening [ 90 , 91 ]. Consequently, investment prioritisation is necessary [ 92 , 93 ]. How this is done needs to be anchored in the problem framing and by clearly defined ecological, economic and social objectives. Further, prioritisation needs to have line of sight to outcomes that can be achieved given climate uncertainty and funding contraints ( Fig 1 , step E). As an example, consider two extreme yet realistic prioritisation alternatives for a large reef system such as the Great Barrier Reef. Should we aim to sustain a minimum of 5% coral cover over a 1000 km 2 area of reef, or a minimum of 25% coral cover over a 200 km 2 area? Logistics will differ, but the net result is the same in terms of coral area sustained: 50 km 2 . However, depending on the spatial configuration of the saved corals, these alternatives would produce very different ecological outcomes and values for society. Spreading efforts across a large area would speak to system integrity and perhaps the Outstanding Universal Value of the Great Barrier Reef World Heritage Area [ 94 ]. Downsides of spreading efforts thinly include reduced capacity to sustain critical ecological functions such as net reef accretion [ 95 ], and reduced fitness via a reduced demographic Allee effect [ 96 , 97 ]. Conversely, concentrating efforts on a selection of just a few but glorious reefs could sustain parts or all of the GBR’s tourism industry, which is spatially concentrated [ 98 ]. It would enable managers to support ecological functions and services on those focal reefs more easily, and perhaps create spill-over effects to other reefs [ 99 ]. Taken to the extreme under severe climate change, spatial prioritisation under resource constraints could reduce the Great Barrier Reef to a fragmented (and therefore vulnerable) network of coral oases in an otherwise desolate seascape.

Other options might involve targeting reefs that are gateway nodes in the spatial reef network–in other words, investing in well-connected reefs located in the least thermally stressed environments [ 100 ]. Here, efforts to support population growth of climate-hardy corals on source reefs (larval donors) may allow export of their beneficial traits to reefs downcurrent through paths of natural dispersal [ 99 , 101 ]. But risks are that disease agents and potentially invasive species arising from either translocation or assisted gene flow may also spread via similar routes [ 47 ]. Selection criteria should thus favour the dispersal of desirable species only [ 99 ]. The decision challenge associated with spatial prioritisation is therefore one of maximising the spread of genes or traits that produce benefits and minimising those that represent risks. Another option may be to assemble a portfolio of reefs that have less risk of being exposed to the most damaging climate stressors [ 48 , 102 ]. Combining these options may both enhance resilience and reduce stress on priority reefs.

Prioritisation of species adds to the decision challenge for reef restoration and adaptation. Without significant climate mitigation, sensitive coral species will give way to naturally hardier ones [ 11 ], or to species that can adapt faster [ 45 , 103 ]. Picking who should be winners, and ultimately who will be losers, under continued but uncertain climate change is perhaps the biggest challenge facing R&D programs tasked with developing reef rescue interventions. Unfortunately, sensitive coral species tend to be the ones underpinning high-value ecosystem services, including habitat provision for a rich biodiversity [ 23 ] that in part underpin tourism [ 5 ]. Should we invest in making sensitive species hardier but risk failing by not making them hardy enough, thereby wasting resources? Or should we pursue a potentially less risky pathway and support the more climate-hardy species and help them adapt to the consequently altered ecosystems and the different goods and services they provide? Importantly, our best efforts to build coral resilience under severe climate change will not prevent reefs from transitioning to altered ecosystems [ 6 , 60 ]. Strategies to help humans adapt to a changed ecosystem need to combine with strategies that help reefs [ 104 ]. Lastly, can robust keystone species be found that can give climate protection to many other dependent species [ 105 ], thereby sustaining ecosystem services? The latter may ultimately be the most effective choice if species compositions allow the ecosystem to remain functionally resilient [ 64 ]. How these priorities are set ultimately depends on what society wants (objectives and values), what options can be achieved technically, institutionally and socially, and what compromises and risks we are willing to accept. The preferred strategy would be the one that delivers the most positive outcomes to priority objectives (and the values they encompass) with low or manageable risks and within resource constraints [ 57 , 58 , 106 ].

R&D provides options, but choose carefully

The likelihood that the world will warm more than 2°C (air) since preindustrial levels this century was recently 95 percent [ 17 , 20 ]. With this outlook, new intervention options for coral reefs will be in growing demand. Importantly, however, no new intervention can be added to the operational management toolset without significant R&D; it is the prerequisite for intervention effectiveness, safety and cost efficiency [ 67 ]. How interventions are chosen for R&D and progressed through to deployment is both complex and critical because it will determine what options will ultimately be available for managers and when ( Fig 1 , steps B-E). Three questions are at the centre of reef intervention R&D. First, which interventions should be prioritised for development? Second, how should they be queued in time? Third, should intervention strategies be robust or targeted?

Limited resources for R&D means not all interventions can be assessed nor progressed. Complicating this problem is that the more the world warms, and the more ecosystems become affected, the greater the overall demand for intervention resources will be. Misguided investment choices can lock up vital resources in inferior solutions, hampering or preventing the development of superior ones [ 107 ]. Prioritising no-regrets options because they are inexpensive or less challenging technologically [ 77 ] could lead to regrets downstream by preventing or delaying the development of more effective solutions. Prioritisation of interventions for R&D should ideally be a fast adaptive process (indicated by multiple adaptive cycles in Fig 1 ) whereby combinations of interventions are continuously assessed for their combined benefits and risks against environmental, social and economic objectives [ 53 ]. In general terms, the right time to implement an intervention strategy following R&D would be when the cumulative (time-integrated) benefit-to-risk ratio of deployment exceeds the cumulative benefit-to-risk ratio risk of not deploying. Here, benefits are defined as positive outcomes (as likelihood and consequence) for ecosystem services and values for society, and risks as negative outcomes. The benefit-to-risk ratio of these contrasting strategies, however, will depend on the climate future ( Fig 1 , step D).

Robust strategies work across a range of climate change scenarios. While investing in a robust R&D strategy will give some return regardless of climate future, the strategy may eventually underperform because it trades off effectiveness for reduced risk. In contrast, targeted strategies are tuned to different climate scenarios. This involves betting on, and planning for, a specific climate trajectory. This represents high risk, but potentially also high reward. For example, a strategy that buys 1°C thermal tolerance for sensitive and valued coral species (on top of today’s 1°C global warming) may give high ecological, social and economic returns if global warming is kept below 2°C relative to preindustrial. If the world warms much more than 2°C, however, the strategy will be ineffective unless these species continue to adapt. Conversely, a strategy that bets on severe climate change and focuses on helping the hardiest species only (or develops artificial reefs) will miss the opportunity to protect biodiversity if a milder climate scemario unfolds in reality. Developing a portfolio of interventions that allows hedging and a staged roll-out of interventions as climate change unfolds may be ideal, but may again be constrained by resource availability for R&D, and the demands of urgency—real or perceived.

Get people on board

Environmental problems are social problems [ 108 ]. Climate change, mass coral bleaching events and consequent coral reef decline are human-induced and require solutions from science and society. The dynamics of the current coral disease outbreak in the Caribbean are also consistent with ocean warming patterns [ 109 – 111 ]. While interventions that can build resistance to coral disease will differ from those that can build resistance to coral bleaching, a similar approach to solutions is needed. Solutions require innovative thinking and coordination between science, management and policy, and public engagement. There are concerns that restoration and adaptation are distractions from tackling global climate change, the main driver of coral reef decline [ 112 ]. Communication and engagement strategies must reinforce the message that restoration and adaptation are a health-care strategy that can only work in tandem with a cure: urgent global action to address climate change.

Any new interventions on coral reefs, in particular radical ones, will be up against hurdles to achieve social acceptance and to overcome regulatory constraints [ 104 , 105 ], leading to uncertainties that become barriers for solutions ( Fig 1 , step D). Existing regulations operate under a retrospective model that crowbars coral restoration and adaptation into existing policy and legislation. However, future policy development should accommodate risks of future climatic conditions (see challenge two above) whilst simultaneously adapting to the emerging opportunities and challenges of coral restoration and adaptation.

A handful of countries are currently developing or revising existing policy and regulatory processes to assist coral restoration and adaptation in the face of climate change: Australia, USA, Netherlands, France, Costa Rica, Japan, Columbia, and Thailand. The United Nations Environment Program have declared 2021–2030 as the UN Decade of Ecosystem Restoration. The aim is to “support and scale up efforts to prevent, halt and reverse the degradation of ecosystems worldwide and raise awareness of the importance of successful ecosystem restoration” [ 87 ].

To get people on board will require coordinated consulation and transparent decision making that considers all risks, benefits and value consequences of reef intervention in a structured way [ 106 ]. Open communication and engagement around objectives, options and trade-offs will be key.

Strong and coordinated governance needed

Applying a coordinated and well-conceived coral reef intervention program which sets the right objectives, identifies and balances risks, and aims to make optimal tradeoffs, in the face of uncertainty while getting community buy-in and support, will depend on robust and appropriate governance [ 113 , 114 ]. First, at the R&D stage, researchers will need to be provided with the resources to do the job, and the mandate to take risks. For something as new and potentially controversial as large-scale coral reef intervention, consultation and co-development mechanisms must involve regulators, reef stakeholders, Traditional Owners and the public. Internal processes must be agreed on at the outset, to allow ongoing, effective prioritisation while maintaining flexibility in the face of changing conditions and unexpected setbacks. Some of the tested interventions to be examined will simply not work. Strong governance will be particularly important if political pressure for quick action (just do something) mounts in the face of worsening climate conditions. Next, as R&D results yield prospective options for at-scale intervention, governance must adapt to a situation where the costs, profile, and risk associated with failure of the effort have grown substantially. Again, at this stage, costs, benefits, risks and community desires will have to be balanced and trade-offs made, for at-scale deployment to occur.

Conclusions

An expanded toolbox of interventions will provide opportunities to build reef resilience against continued climate change. Without carbon mitigation, no intervention strategy will be successful in the long run. And no single intervention can produce adaptation solutions. A portfolio of new and existing interventions must be combined with mitigation.

New interventions come with risks, but so does the status quo. If the potential of new interventions can be unlocked and their benefits exceed risks for reef, people and economies, they should be developed and deployed. The challenge for science, management and policy, in consultation with communities, is to develop and adopt technologies that will be both safe and effective—and within years rather than decades.

What climate trajectory will unfold is uncertain. But what is certain is that we need an expanded set of options to safeguard coral reefs and dependent people and industries. Research and development can help, but only if efforts are focused, coordinated and highly integrated. To do this will require a level of organisation, collaboration and integration across disciplines never seen before in natural sciences, conservation and policy.

Acknowledgments

The work was conducted as part of the Reef Restoration and Adaptation Program (RRAP, https://www.gbrrestoration.org ). RRAP data referred to in this paper can be accessed via: https://www.gbrrestoration.org/reports#technical-reports . We thank Prof Allen Chen and Prof Bob Steneck for insightful reviews and comments that improved the paper. Opinions expressed in the paper are those of the authors in their individual capacity.

Funding Statement

The work is supported by a grant from the Australian Government to the Reef Restoration and Adaptation Program ( https://www.gbrrestoration.org ). The funding body played no role in the writing of this paper. Opinions expressed in the paper is that of the authors in their individual capacity.

Data Availability