Bigger Toolbox Needed to Save Coral Reefs

The U.S. National Archives

Tropical corals reefs around the globe are in decline. They need new solutions to survive climate change – and they need them desperately for three reasons.

First, coral reefs are among the most climate-sensitive ecosystems on Earth1 as corals – the ecosystem engineers – already live close to their upper thermal limits2. And climate change has impacted coral reefs already. In recent years, corals have been lost on reefs around the globe at an unprecedented scale after the warmest and most prolonged El Niño event in recorded history3. The heat wave led to large-scale bleaching: a stress response where corals lose their internal symbiotic microalgae that provide energy for reef growth4. The largest and most intensely managed reef system – Australia’s Great Barrier Reef – experienced widespread bleaching and mortality following back-to-back warming events in 2016 and 20175. Corals can recover from bleaching, but the extent, magnitude, and frequency of recent events left large areas damaged.

Eric Matson, AIMS
Eric Matson, AIMS
Fig. 1. The colours of climate stress for corals on the Great Barrier Reef during the 2015/16 warming event. Heat waves induce a stress response whereby the symbiotic endocellular algae within the coral host become toxic and get expelled. This bleaching response can be fatal as the coral is left in a state of starvation and vulnerable to disease. The loss of coral means loss of habitat for fish and other biodiversity. New interventions that can enhance the climate resilience of reef corals could improve the prospects for reef ecosystems that feed hundreds of millions of people and support a trillion dollar economy.

Second, coral reefs are the richest ecosystem in the world’s oceans, biologically and economically. They are home to around a million species6,7 and are valued at around U$10 trillion annually based on the ecosystem services they provide to society from tourism, fishing and coastal protection8.

Third, model projections see coral reefs and their values decline further under climate change (e.g. Ortiz et al. 2014, Wolff et al. 2018). This expected decline will be rapid without carbon mitigation; around 90% of the world’s coral reefs are likely to be at high risk by 203011. Strong carbon mitigation will only slow the decline if the highest aspiration of the Paris Agreement is achieved12 – i.e. global warming kept within 1.5°C above pre-industrial levels13. Under this best-case scenario, tropical surface ocean waters where coral reefs live would still see another 0.3°C warming in coming decades2. That’s likely to be too much for the most sensitive coral species14, including those that form critical habitats for fish and other biodiversity15. While the less ambitious target of 2°C warming may not mean complete loss of coral reefs, it will mean less scope to sustain coral reefs as we know them.

This reality check is confronting but informative. To minimise climate impacts on coral reefs now and in the future, any solution must do three things: mitigate carbon emissions AND build climate resilience AND act now16. The key to success is tackling this issue from all three fronts. Both logic and science supports such a proactive, multi-tiered strategy. Yet, it’s a hard sell. There is a tendency to replace “AND” with “OR” in the search for a silver bullet, and to delay intervention due to uncertainties and risks associated with new solutions. In a time of rapid climate change, delayed intervention could represent high risk.

Proposals for new interventions surface daily, and ideas vary in degrees of effectiveness, spatial scale, time horizon and cost. There is a sense of desperation mixed with opportunism as the new climate reality sinks in. Confusion and scepticism grow as it becomes harder to sort the wheat from the chaff. What will work and what won’t? What are the risks and the rewards? What is the price tag?

Here we attempt to bring some clarity around the science behind options available to build climate resilience on coral reefs. We ask questions about the risks and rewards associated with new technologies, which ones show promise, and why they are needed in addition to conventional ones. We suggest a path for research and development to guide safe management and policy decisions on the issue of new interventions. We argue that the sooner an expanded toolset can be developed, risks and rewards can be fully understood and results socialised with the broader community, the more opportunities we will have to sustain coral reefs and their values for economies and people.


Building resilience

Coral reefs are naturally resilient. They have historically bounced back from major natural disturbances such as cyclones17 and outbreaks of coral-eating Crown-of-Thorns Starfish18. But too much stress – in frequency and intensity – can overwhelm that natural resilience. With mass coral bleaching increasing in both intensity and severity, there is diminishing time and capacity for recovery19. If combined with chronic pressures such as pollution, coral reefs may reach new lows from where any recovery is difficult2.

Reducing pollution and managing no-take areas that control fishing and maintain populations of algal-eating (herbivorous) fish is the traditional recipe for keeping coral reefs resilient 20,21. These will continue to be essential. But with climate change placing extra pressure on the ecosystem, this traditional recipe is no longer enough. The system now needs extra resilience. Corals must become more tolerant and recover faster to keep up with the added climate pressures – perhaps more than ever before.

Boosting coral reef resilience in the face of climate change means coral reef managers will need to consider new interventions beyond the traditional toolbox. Although natural processes of selection, acclimation, and genetic adaptation are all in play 22,23, they may not be fast enough in most coral species to keep up with the rate of global warming, even under strong carbon mitigation.


New interventions

The question is then: what new interventions should be on the table?

This is where the discussion becomes heated, controversial and often stuck. New initiatives are seen by some people as opportunities and by others as threats. This includes attempts to climate-harden species by moving warm-adapted specimens to cooler places24, selective breeding programs and promoting the natural processes of acclimatisation and adaptation25 to new gene technologies26. It also includes geoengineering approaches that attempt to lower local seawater temperatures and light (a co-factor in the coral bleaching response) and increase seawater local pH to counter ocean acidification (another consequence of carbon emissions)27.

Risks associated with new interventions, whether perceived or real, and their potential side effects, costs, and uncertainties lead to quick judgements. It is no secret that some biological interventions in the past have been poorly planned. The managers who introduced cane toads to Australia to manage the cane beetle did not consider the catastrophic invasive potential of the toads. However, science has come far in recent decades in fighting diseases and pests safely. For example, innovative biological management of an invasive weed (Mimosa pigra) in northern Australia is now protecting the Kakadu National Park from that invasive28. The risk of negative outcomes of any ecological management must be considered fully and transparently. But the discussion of risks often trumps the discussion of the opportunities we now have to help conserve species critical for ecosystems and people. Humans are notoriously poor judges of risks – even to ourselves and the ones we care about – until fully informed29,30. Clarity in the risks and ecological benefits of any interventions will be a critical first step towards producing solutions.

In the wake of a recent paper we co-authored on the topic16 and the launch of a new project to test and develop new interventions for coral reef conservation (, a series of pointed questions have arisen about risks and rewards associated with new interventions on coral reefs. Those questions, which we address below, offer an opportunity for discussion, and help to illustrate ways forward.


Are coral reefs too big to make resilient with any new technology?

Reefs are connected by ocean currents, so resilient corals with dispersive larvae can spread to larger areas with some careful planning. Climate resilience may only need to be boosted on a subset of well-selected reefs from where offspring can be dispersed to other reefs by currents31. For example, reefs in the far northern Great Barrier Reef (GBR) have higher temperature tolerance than reefs further south32. The natural flow of larvae from the north to the south is limited by the South Equatorial Current breaking into the south-flowing East Australian Current and the north-flowing Gulf of Papua Current33. Moving enough warm-adapted corals across this bottleneck for larval exchange could help boost the climate resilience of the southern half of the GBR, an area larger than Nepal. However, reef managers may eventually have to prioritise what reefs to protect. Criteria for such prioritisation could be based on what biodiversity a reef supports, its importance for fisheries, tourism value and/or world heritage values. This is now the approach adopted broadly in nature conversation – forced in part by limited resources34.


Can we climate-harden and protect all species?

With close to 600 species of corals on the GBR we will have to prioritise which ones to target for active intervention. How to prioritise target species will come down to analyses of how they contribute to the reef values society would like to see protected, what level of climate-hardening is achievable technologically and biologically, and what resources are available for research and development, and for implementation. But there are ways to be effective. By targeting “keystone” species that support other species – for example as habitat and food – many dependent species can be given some climate protection. These huge opportunities are of course dependent on the predicted climate future. If global warming is kept within 1.5°C (in alignment with the Paris Agreement), then there is potential scope to climate-harden many coral species. Above 1.5°C global warming, sensitive coral species may need to give way to naturally hardier, or more responsive ones.


Will it be too expensive to deploy emerging technologies?

Not if we are smart about it. If all the damaged corals after a bleaching event are to be replaced manually by divers, then the bill could run toward a trillion dollars. The exercise would be futile and prohibitively expensive at the scale of the GBR. Other more cost-effective technologies are available if we explore options with their long-term cost, scalability, and likelihood of success in mind. For example, instead of manual coral gardening, helping the spread and establishment of warm-adapted coral larvae and juveniles to reefs where good management measures are in place to protect them could be both a feasible and cost-effective solution at a large scale. Future feasibility is not set by past practices, and only our imagination limits our ability to produce solutions here. The technology to help up-scale and automate processes previously dependent on manual intervention are evolving fast. For example, forest recovery in remove or inaccessible areas can now be helped by high-speed seed injection from drones ( With focused research and development, modern manufacturing and automation systems could be leveraged to provide the required deployment scales at a cost far less than current methods.


ill it be too hard to find resilience solutions for reefs despite substantial investment?

It will be hard, but possible. The Reef Restoration and Adaptation Program proposes to use a process whereby a large number of intervention approaches (as individual interventions or bundles representing strategies) are initially screened, filtered and subjected to research and testing (Fig. 2). By allowing non-viable concepts to fail early, research and development efforts and resources can be focused in on a diminishing, but increasingly promising, pool of interventions. Given the immeasurable value of the GBR and the risk it now faces in a changing climate, even if only one intervention strategy out of a hundred truly works in building resilience, then that one strategy will more than pay for all other R&D efforts by helping to secure the values of the system.

Fig. 2. Process by which the Reef Restoration and Adaptation Program starts by examining a broad range of potential concepts and then narrow in on increasingly viable and effective options.


Don’t new interventions take attention away from carbon mitigation and other management efforts?

Without carbon mitigation, no intervention will be truly successful in the long run. And no single intervention can solve the problem on all reefs at the same time. A portfolio of interventions that enhance each other’s benefits safely is what’s needed. This means efforts to reduce pollution, manage marine parks and deploy new interventions that can enhance climate tolerance must be combined. We have to keep those three balls in play at all times. Dropping one could mean game over. Communication around new interventions need to reflect this. A focus on solutions that involve local communities, industry and governance organisations gives people hope and increase engagement in broader challenges, including climate change mitigation.


hould we only consider new technologies as a last resort?

This is a tough question. Some of the most important habitat-building coral species are also the most sensitive to climate change. While we delay new interventions because of uncertainty around side effects, the risk of losing important species and functions increases with time.

The “right” time to intervene is when the risk of inaction is greater than the risk of action35. The sooner we start research and development programs around a broad set of interventions that show promise, the more informed policy decisions can be about whether to deploy, delay or dismiss an option for intervention.

Importantly, the motivation and social license to start conservation programs typically come when ecosystems or species are in advanced decline36. Such delayed action represents a lost opportunity as (1) intervention strategies take time to develop, and (2) delayed restoration is less cost-effective than damage-prevention 16.


Won’t the use of emerging technologies compromise the pristine nature of coral reefs?

Pristine reefs are no more37. The challenge now is to best preserve what we can, as best we can, and as soon as we can. And using science and technology to preserve reef species and ecosystems would make remaining coral reefs even more valuable. Without an expanded toolset we are likely to just be observing coral reefs degrade further, and along with them the values that support economies and livelihoods.



An expanded toolbox of interventions will give us more opportunities to build coral reef resilience against continued climate change. Given the benefits that coral reefs provide for people in both developing and developed countries, such climate resilience could translate into sustained economies at the scale of trillions and human livelihoods at the scale of hundreds of millions. The challenge is to develop technologies that with focused research and community consultation can be made both safe and effective – and within years rather than decades. And while we grow that toolbox, AND continue conventional management AND curb carbon emissions, we can start to roll out the safest new interventions to help stem further decline.



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