Ocean warming threatens coral reefs and soon could make it harder to restore them



Climate-driven ocean warming threatens healthy coral reefs, like this one in Hawaii.
Shawna Foo, CC BY-ND

Shawna Foo, Arizona State University

Graphic stating that at 86.9 degrees Fahrenheit, the chance of transplanted corals surviving falls below 50%

CC BY-ND

Anyone who’s tending a garden right now knows what extreme heat can do to plants. Heat is also a concern for an important form of underwater gardening: growing corals and “outplanting,” or transplanting them to restore damaged reefs.

The goal of outplanting is to aid coral reefs’ natural recovery process by growing new corals and moving them to the damaged areas. It’s the same idea as replanting forests that have been heavily logged, or depleted farm fields that once were prairie grasslands.

I have studied how global stressors such as ocean warming and acidification affect marine invertebrates for more than a decade. In a recently published study, I worked with Gregory Asner to analyze the impacts of temperature on coral reef restoration projects. Our results showed that climate change has raised sea surface temperatures close to a point that will make it very hard for outplanted corals to survive.

Coral gardening

Coral reefs support over 25% of marine life by providing food, shelter and a place for fish and other organisms to reproduce and raise young. Today, ocean warming driven by climate change is stressing reefs worldwide.

Rising ocean temperatures cause bleaching events – episodes in which corals expel the algae that live inside them and provide the corals with most of their food, as well as their vibrant colors. When corals lose their algae, they become less resistant to stressors such as disease and eventually may die.

Hundreds of organizations worldwide are working to restore damaged coral reefs by growing thousands of small coral fragments in nurseries, which may be onshore in laboratories or in the ocean near degraded reefs. Then scuba divers physically plant them at restoration sites.

Outplanting is the process of transplanting nursery-grown corals onto reefs.

Outplanting coral is expensive: According to one recent study, the median cost is about US$160,000 per acre, or $400,000 per hectare. It also is time-consuming, with scuba divers placing each outplanted coral by hand. So it’s important to maximize coral survival by choosing the best locations.

We used data from the National Oceanic and Atmosphere Administration’s Coral Reef Watch program, which collects daily satellite-derived measurements of sea surface temperature. We paired this information with survival rates from hundreds of coral outplanting projects worldwide.

We found that coral survival was likely to drop below 50% if the maximum temperature experienced at the restoration site exceeded 86.9 degrees Fahrenheit (30.5 degrees Celsius). This temperature threshold mirrors the tolerance of natural coral reefs.

Globally, coral reefs experience an annual maximum temperature today of 84.9˚F (29.4˚C). This means they already are living close to their upper thermal limit.

When reefs experience temperatures only a few degrees above long-term averages for a few weeks, the stress can cause coral bleaching and mortality. Increases of just a few degrees above normal caused three mass bleaching events since 2016 that have devastated Australia’s Great Barrier Reef.

Map of global sea surface temperatures, color coded to show bleaching risks.
Sea surface temperatures on Aug. 3, 2020, measured from satellites. Warning = possible bleaching; Alert Level 1 = significant bleaching likely; Alert Level 2 = severe bleaching and significant mortality likely.
NOAA Coral Reef Watch

Warmer oceans

Climate scientists project that the oceans will warm up to 3˚C by the year 2100. Scientists are working to create coral outplants that can better survive increases in temperature, which could help to increase restoration success in the future.

When coral restoration experts choose where to outplant, they typically consider what’s on the seafloor, algae that could smother coral, predators that eat coral and the presence of fish. Our study shows that using temperature data and other information collected remotely from airplanes and satellites could help to optimize this process. Remote sensing, which scientists have used to study coral reefs for almost 40 years, can provide information on much larger scales than water surveys.

Coral reefs face an uncertain future and may not recover naturally from human-caused climate change. Conserving them will require reducing greenhouse gas emissions, protecting key habitats and actively restoring reefs. I hope that our research on temperature will help increase coral outplant survival and restoration success.

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Shawna Foo, Postdoctoral Research Scholar, Arizona State University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

The science and art of reef restoration



File 20180715 27042 jlzpgh.jpg?ixlib=rb 1.1
Silent Evolution by Jason deCaires Taylor. Taylor makes sculptures and sinks them beneath the sea to create artificial reefs.
© Jason deCaires Taylor

Adam Smith, James Cook University and Ian McLeod, James Cook University

Coral reefs around the world are in crisis. Under pressure from climate change, overfishing, pollution, introduced species and apathy, coral colonies and fish communities are steadily deteriorating.

Coral cover in the Great Barrier reef has declined by an alarming 50% since the 1980s. Some leading scientists believe that the Great Barrier Reef is at a terminal stage.




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One way to address this is through reef restoration. At its simplest, this involves the addition of coral or habitat to a reef. It’s generally undertaken on existing coral reefs, but can also be done on rocky reefs or bare sand.

We have looked back through the decades to celebrate the history of reef restoration, not just in science but also in art, business and politics.

Gardener, by Jason deCaires Taylor.
© Jason deCaires Taylor

Band-aid or reef revolution?

Just as there is no magic solution in human healthcare, there is likewise no magic solution in caring for corals. You do what you can with the resources you have.




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Some scientists have argued that reef restoration is a Band-Aid for the enormous problems that reefs face. We can agree with this point of view, but there are times when a band aid is very useful – and may prevent much more serious injuries.

Reef restoration makes an important local difference, as seen here at Koh Tao, Thailand.
Author provided

Earlier this year the federal government allotted an unprecedented A$500 million dollars to the Great Barrier Reef. This included A$100 million focused on restoration to improve the health of the reef.

Reef restoration science and projects complement community efforts. There is an increasing focus on addressing local issues such as water quality, overfishing, and outbreaks of crown-of-thorns starfish.




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When scientists, industry and government work with local communities we can accelerate the recovery of local reefs.

To do this, we need people who want to make a difference. Once we recognise a degraded ecosystem, we work to reduce stress (like pollution in the water) and add new habitat or helpful species.

Artist Jason deCaires Taylor builds breathtaking underwater sculptures that double as artificial coral reefs.

The history of reef restoration

People have been restoring ecosystems and degraded land for thousands of years. Reef restoration, on the other hand, is relatively new and rarely documented.

Our research indicates that in the modern era there have been three major waves of reef restoration. The first wave started in the 1970s and ‘80s, as scientists were able to easily SCUBA dive and new protective legislation was introduced around the world. This largely involved the addition of new habitats. These could be coral transplants, or artificial constructs likes shipwrecks, concrete pipes, tyres and a purpose built structure called a reef ball.

The second wave from 2000-2010 was associated with scientists and conservationists responding to local concerns from cyclone damage, overfishing, introduced species and over-crowding at tourism sites, particularly in the Caribbean. Restoration methods at this point expanded to removing items as well as adding them, including algae, crown-of-thorns and lionfish.

Reef restoration has evolved over decades.
Author provided

The third wave, from 2016, has focused on new scientific technology such as micro-fragmentation: breaking coral into small pieces so it grows faster. It also emphasises partnerships between government-business-community to reduce threats and restore reefs.

This era also sees a huge increase in communication. Increasingly, we are influenced by social sciences and marketing rather than science and biology in our search for coral reef solutions. Organisations such as Rare, Citizens of the GBR and Reef Check are using citizen scientists, campaigns and pledges to reduce human impact and improve reefs’ health. As an example, the rapid phase out of plastic bags has been led by social media – not science.

Celebrating the Reef restoration Leaders

Documenting the history of reef restoration is important because it allows us to understand our past and be more informed and inspired to take action in the future.

Sculpture at the Underwater Museum at Lanzarote Rubicon.
© Jason deCaires Taylor

The great men and women in our history were innovators who responded to crisis and went against convention by restoring reefs.

We reviewed academic literature and conducted a global survey to find the pioneers who led reef restoration science, management, business and communication. These include Drs Austin Bowden-Kerby, David Vaughan, Todd Barber, Barach Rinkievich and Kristen Marhaver.




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An idea without action is just a dream. Similarly, an idea that has not been communicated widely and is not known and adopted by the general community cannot result in changed behaviour. Increasingly we recognise that good science and management is not enough without community support and action.


The authors would like to acknowledge the valuable contribution of Nathan Cook, Senior Marine Scientist at Reef Ecologic, to this article.

A presentation on the history of Reef Restoration will occur at the Great Barrier Reef Restoration Symposium, July 16-19, Cairns.

Thanks to Jason deCaires Taylor for the use of images. See more at underwatersculpture.com.

The ConversationThis article was updated on July 25 to clarify the location of the reef pictured demonstrating the impact of restoration.

Adam Smith, Adjunct Associate Professor, James Cook University and Ian McLeod, Senior Research Scientist – Coastal Restoration, James Cook University

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