Coral reefs are much more than just a pretty place to visit. They are among the world's richest ecosystems, hosting about a third of all marine species .
Authors
- Corey J. A. Bradshaw
Matthew Flinders Professor of Global Ecology and Node Leader in the ARC Centre of Excellence for Indigenous and Environmental Histories and Futures, Flinders University
- Clelia Mulà
PhD student in Marine Ecology, The University of Western Australia
- Giovanni Strona
Doctoral program supervisor, University of Helsinki
These reefs also directly benefit more than a billion people , providing livelihoods and food security, as well as protection from storms and coastal erosion.
Without coral reefs, the world would be a much poorer place. So when corals die or become damaged, many people try to restore them. But the enormity of the task is growing as the climate keeps warming.
In our new research , we examined the full extent of existing coral restoration projects worldwide. We looked at what drives their success or failure, and how much it would actually cost to restore what's already been lost. Restoring the reefs we've already lost around the world could cost up to A$26 trillion.
Global losses
Sadly, coral reefs are suffering all over the world. Global warming and marine heatwaves are the main culprits. But overfishing and pollution make matters worse.
When sea temperatures climb above the seasonal average for sustained periods, corals can become bleached . They lose colour as they expel their symbiotic algae when stressed, revealing the white skeleton underneath. Severe bleaching can kill coral.
Coral bleaching and mass coral deaths are now commonplace. Last month, a massive warm-water plume bleached large areas of Ningaloo Reef on Australia's northwest coast just as large sections of the northern Great Barrier Reef were bleaching on the northeast coast.
Since early 2023, mass coral bleaching has occurred in throughout the tropics and parts of the Indian Ocean .
Over the past 40 years, the extent of coral reefs has halved . As climate change continues, bleaching events and coral deaths will become more common . More than 90% of coral reefs are at risk of long-term degradation by the end of the century.
Direct intervention
Coral reef restoration can take many forms , including removing coral-eating species such as parrot fish , transferring coral spawn , or even manipulating the local community of microbes to improve coral survival.
But by far the most common type of restoration is " coral gardening ", where coral fragments grown in nurseries are transplanted back to the reef.
The problem is scale. Coral restoration can only be done successfully at a small scale. Most projects only operate over several hundred or a few thousand square metres. Compare that with nearly 12,000 square km of loss and degradation between 2009 and 2018. Restoration projects come nowhere near the scale needed to offset losses from climate change and other threats.
Sky-high costs
Coral restoration is expensive , ranging from around $10,000 to $226 million per hectare. The wide range reflects the variable costs of different techniques used, ease of access, and cost of labour. For example, coral gardening (coral fragments grown in nurseries transplanted back to the reef) is relatively cheap (median cost $558,000 per hectare) compared with seeding coral larvae (median $830,000 per hectare). Building artificial reefs can cost up to $226 million per hectare.
We estimated it would cost more than $1.6 billion to restore just 10% of degraded coral areas globally. This is using the lowest cost per hectare and assuming all restoration projects are successful.
Even our conservative estimate is four times more than the total investment in coral restoration over the past decade ($410 million).
But it's reasonable to use the highest cost per hectare, given high failure rates, the need to use several techniques at the same site, and the great expense of working on remote reefs. Restoring 10% of degraded coral areas globally, at $226 million a hectare, would cost more than $26 trillion - almost ten times Australia's annual GDP .
It is therefore financially impossible to tackle the ongoing loss of coral reefs with restoration, even if local projects can still provide some benefits.
Location, location, location
Our research also looked at what drives the choice of restoration sites. We found it depends mostly on how close a reef is to human settlements.
By itself, this isn't necessarily a bad thing. But we also found restoration actions were more likely to occur in reefs already degraded by human activity and with fewer coral species.
This means we're not necessarily targeting sites where restoration is most likely to succeed, or of greatest ecological importance.
Another limitation is coral gardening normally involves only a few coral species - the easiest to rear and transplant. While this can still increase coral cover, it does not restore coral diversity to the extent necessary for healthy, resilient ecosystems.
Measuring 'success'
Another sad reality is that more than a third of all coral restoration efforts fail . The reasons why can include poor planning, unproven technologies, insufficient monitoring, and subsequent heatwaves.
Unfortunately, there's no standard way to collect data or report on restoration projects. This makes it difficult - or impossible - to identify conditions leading to success, and reduces the pace of improvement.
Succeed now, fail later
Most coral transplants are monitored for less than 18 months . Even if they survive that period, there's no guarantee they will last longer. The long-term success rate is unknown.
When we examined the likelihood of extreme heat events immediately following restoration and in coming decades, we found most restored sites had already experienced severe bleaching shortly after restoration. It will be difficult to find locations that will be spared from future global warming.
No substitute for climate action
Coral restoration has the potential to be a valuable tool in certain circumstances: when it promotes community engagement and addresses local needs. But it is not yet - and might never be - feasible to scale up sufficiently to have meaningful long-term positive effects on coral reef ecosystems.
This reality check should stimulate constructive debate about when and where restoration is worthwhile. Without stemming the pace and magnitude of climate change , we have little power to save coral reefs from massive losses over the coming century and beyond.
Other conservation approaches such as establishing, maintaining and enforcing marine protected areas , and improving water quality , could improve the chance a coral restoration project will work. These efforts could also support local human communities with incentives for conservation.
Reinforcing complementary strategies could therefore bolster ecosystem resilience, extending the reach and success of coral restoration projects.
Corey J. A. Bradshaw receives funding from the Australian Research Council.
Clelia Mulà receives funding from the Australian Institute of Marine Science.
Giovanni Strona does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
