Warming oceans due to climate change could soon jeopardize restoration of coral reefs

Anyone who tends 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 “replanting” or transplanting them to restore damaged reefs.

The goal of planting is to help the natural recovery process of coral reefs by growing new corals and moving them to damaged areas. It’s the same idea as replanting forests that have been heavily exploited or depleted agricultural fields that were once grasslands.

I have studied how global stressors such as ocean warming and acidification affect marine invertebrates for over 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 increased sea surface temperatures to such an extent that it will be very difficult for the implanted 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 their young. Today, the warming of the oceans induced by climate change is straining reefs around the world.

Rising ocean temperatures cause bleaching phenomena – episodes in which the corals expel the algae that live inside 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 can eventually die.

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

Planting coral is expensive: according to a recent study, the median cost is around US $ 160,000 per acre, or $ 400,000 per hectare. It is also time consuming, with divers placing each planted coral by hand. It is therefore important to maximize the survival of the corals by choosing the best locations.

We used data from the National Oceanic and Atmosphere Administration’s Coral Reef Watch program, which collects daily satellite measurements of the sea surface temperature. We associated this information with the survival rates of hundreds of coral plantation projects. in the whole world.

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

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

When reefs experience temperatures just a few degrees above long-term averages for a few weeks, stress can cause corals to bleach and die. Increases of a few degrees above normal have caused three episodes of massive bleaching since 2016 that have devastated Australia’s Great Barrier Reef.

Warmer oceans

Climatologists predict that the oceans will warm to 3 ° C by the year 2100. Scientists are working to create coral explants that are able to better survive increases in temperature, which could help increase the success of the catering in the future.

When coral restoration experts choose where to plant, they usually take into account what’s on the seabed, algae that could choke the coral, predators that eat the coral, and the presence of fish. Our study shows that using temperature data and other information collected remotely from airplanes and satellites could help optimize this process. Remote sensing, which scientists have used to study coral reefs for nearly 40 years, can provide information at much larger scales than aquatic 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 temperature research will help increase the survival and restoration success of coral explants.

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