Buying time for the Barrier Reef – shading coral and controlling starfish show promise at large scale

By Mary O’CallaghanApril 29th, 2021

Shading corals to reduce mass bleaching and expanding the control of coral-eating crown-of-thorns starfish—if socially acceptable and done on a large scale—could buy at least 10 to 20 years for the Great Barrier Reef, according to scientists who have for the first time modelled all of the world’s biggest marine ecosystem.
diver collects a starfish

A diver collects a starfish for research. Credit: David Westcott, CSIRO

Modelling showed that these two interventions combined could delay the ecological collapse of the Reef, allowing time for meaningful global action on climate change to take effect.

The delay would also give the Reef more time to adapt naturally to the warming that is already baked into the planet’s climate system.

No intervention, no coral

The cumulative impacts of cyclones, marine heatwaves and outbreaks of crown-of-thorns starfish (CoTS) have severely depleted coral cover on the Reef, while the impacts of ocean acidification are yet to play out.

If we don’t intervene, the model shows, coral cover will plummet within 50 years—under all climate scenarios.

CSIRO’s Scott Condie is the lead developer of the model, known as CoCoNet (Coral Community Network), and was part of the Reef Restoration and Adaptation Program’s (RRAP) Concept Feasibility Study, where most of the modelling work was done. He co-authored the first ever description of the model, published today in Royal Society Open Science.

“Modelling is the only way to see how effective an intervention is likely to be before undertaking expensive field deployments,” he says.

“The Great Barrier Reef is one of the best studied marine systems in the world. However, we had no comprehensive models of the entire Reef ecosystem. And with no one point where all process and understanding came together, we didn’t have a platform to see where we might intervene. You really need a systems modelling capability to be able to project into the future and test different management actions.”

With about 600 species of coral, never mind the reef fish and invertebrates, one can’t hope to model everything, says Condie: “The art is to capture those things that control the main trends in the system. We have to include the interactions that impact coral the most—tropical cyclones, heatwaves, bleaching events, CoTS and, over the longer term, acidification.”

Shading – the only single intervention effective over large areas

Condie modelled five interventions aimed at protecting and restoring coral communities to see which ones would be most effective at a large scale, under realistic environmental conditions, over the next 50 years.

Most interventions that worked well on individual reefs were largely ineffective over areas comprising several hundred reefs (the Great Barrier Reef comprises more than 3700 reefs).

This is not to suggest that small-scale interventions on individual reefs are not worthwhile—they can have social and economic value locally, says Condie: “Tourism [operators] undertake their own CoTS control, for example, and are very effective in maintaining good coral cover on dive reefs.”

The notable exception at scale was shading, using ‘cloud brightening’ or similar technologies to reduce the solar radiation reaching the coral at times when the risk of bleaching is high. It stood out as the only single intervention to be effective at maintaining coral cover over the next 20 years—not surprising given that more frequent and more severe heatwaves are predicted to be the main cause of coral death, through bleaching, during this period.

The cloud brightening process has similarities to snow making and cloud seeding, says Dr Daniel Harrison, a senior lecturer at Southern Cross University and co-author of the paper: “Vessels spray microscopic droplets of seawater into the atmosphere over the ocean. When the droplets evaporate, the salt crystals left behind can then interact with low-lying clouds to increase their reflectivity.”

It’s important to note that, while small-scale experiments designed to test the feasibility of cloud brightening on the Reef began last year, with encouraging results, the technology is still being developed and its practicality and effectiveness are yet to be tested at larger scales.

Any strategy must include controlling CoTS

Interventions were more effective when combined, and a clear message from the modelling is that any strategy for protecting and restoring coral must include expanded control of CoTS, which might otherwise consume any extra coral.

Perversely, adding heat-tolerant corals in the model without CoTS control only served to beef up the CoTS populations which, in turn, reduced the coral cover to less than it would have been with no intervention.

“CoTS need stepping-stones of coral to spread through the Reef,” explains Condie. “If you choose to outplant corals on a reef with little coral cover, you might be providing that stepping-stone. No matter what methods you use, you’ve got to control the predator as well as enhance the coral.”

Unknowns, assumptions and uncertainties

Running the model to project the future has many inherent uncertainties. For example, we know the probability of cyclones in a given year, and we know their frequency and strength is changing, but we don’t know when and where they will hit.

Condie is candid about the many assumptions made in the modelling, and the many uncertainties. Running the model 100 times, with randomised parameters each time, was a way of drawing out the array of possible trajectories.

Ultimately, he wants to hone in on the uncertainties by using several models together: “With the Reef at a critical point, several groups are developing models, all with different strengths and weaknesses. We want to be able to test interventions across several models. If it works for all of them, we’ll be more confident. You can’t ignore the uncertainty.”

The biggest unknown is how the Reef will adapt to climate change. “It is critical that we find out how corals evolve,” says Condie. “Additional fundamental science is needed before we can model that in detail.”

2.Aerial flood photographs taken in the Townsville region

Aerial flood photographs taken in the Townsville region, February 2019. Credit: Matt Curnock

Is cloud brightening socially acceptable?

Uncertainty is not limited to the technology and biology. Whether Australians support the interventions is a question that must be answered before any decisions to implement are made.

CSIRO’s Dr Bruce Taylor co-leads the RRAP engagement sub-program. “The modelling is absolutely central to the success of the RRAP. It’s important for the long-term management of the Reef and for knowing what makes a successful intervention. But it forms only part of a broader decision-making process that includes reef managers, traditional owners and stakeholders.

“The outcomes of a model are part of the science that is input to the decision-making process, but it is not the decision-making process.”

Surveys conducted during the RRAP Concept Feasibility Study (2018–19) showed that, generally, people were either undecided or cautiously accepting of technologies like cloud brightening, although questions remain among the community about how effective it’s going to be and what affect, if any, it might have on local weather.

“It’s important to understand that creating shade over individual reefs at times of high [heat] stress is a damage mitigation exercise that you can switch off at any time,” explains Taylor. “This is not global geoengineering—it is not resetting the atmosphere.”

The community is part of the program

The RRAP engagement team is currently designing ‘technology reference panels’ made up of citizens and stakeholders who will work closely with scientists over the life of the program to explore and review risks and opportunities. “It’s a community-checking mechanism”, says Taylor.

Opportunities for citizen science around demonstration sites are also being explored.

The Cairns – Port Douglas Regional Hub (a collaboration with the local community, RRAP and the Community Reef Protection program of the Reef Trust Partnership) is being established as a conduit for 2-way information sharing between stakeholders and the RRAP team, in particular for linking local small-scale interventions with the bigger program.

A critical step is to seek the free, prior and informed consent of traditional owners to run field trials on their sea country, and to collect coral samples, water samples and information for ecological modelling. This is already underway with the Indigenous Partnerships Team of the Australian Institute of Marine Science, a RRAP partner.

Intervention without global climate action will not keep the Reef alive

The effort and investment to keep the Great Barrier Reef alive over the next few decades is worth it for economic reasons alone, says Scott Condie.

“With intervention, the model suggests we can make a difference for 10 or 20 years. But intervention is not enough if we’re not taking global action in parallel. We need both.”

Whether world leaders agree will soon become evident.

1 comments

  1. Let’s not forget that the fate of all reefs is death by one or more processes. All pre-historic reefs are gone, due to their very nature……..they grow while the local conditions remain habitable, and they die when conditions change too much etc. Most of these have been attritioned by physical or biological agencies, and many are sequestered and can be found in the geologic record. As Earth cools, reefs will establish in higher latitudes. If we can delay that cooling, we will continue to enjoy the marvels of reefs. We are very fortunate that the eastern margin of the Australian plate has remained long enough in a location suitable for the establishment of a spectacular reef system, and that the necessary conditions of water depth, climate, local weather etc etc have remained stable enough for the reef organisms to colonise adapt, and survive.

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