Science

Scientists estimate that over the past two decades at least 20% of Australia's Great Barrier Reef – the world's largest – has been destroyed, and up to 90% of coral has been lost in the Indian Ocean from East Africa to island states of Maldives and Seychelles.

Human-induced global warming has a double whammy effect on corals: higher ocean temperatures force the marine organisms to expel the colourful zooxanthallae algae that live inside its skeleton and that provide most of its nutrients; and oceans become more acidic as a result of absorbing higher levels of atmospheric carbon dioxide, slowing down the rate at which corals can build their calcium carbonate skeletons. During a coral bleaching event, reefs lose so much zooxanthallae that they become white and experience massive die-offs.

As if that weren't enough, pollution, over-fishing and boat-anchor damage also destroys reefs. Reefs are far more likely to recover from a global warming event if they are located in a protected area than if they are exposed to further human-related damage.  Reef survival in coming years will depend largely on the next-generation corals – on how well coral larvae settle on rock, metamorphosise into feeding polyps, and establish new reefs after extensive damage.

Scientists at the Smithsonian Tropical Research Institute in Panama have been growing coral polyps in different water temperatures and acid levels to see how it affects polyp growth.

"The biggest surprise was that neither temperature alone, nor acidification alone had a big effect on the growth or survival rate of the coral, even though the warming prompted zooxanthallae expulsion as expected," says researcher Aaron O'Dea. "Once we combined this moderate warming and acidification, though, we saw significant impacts: growth rate plummeted to almost half of the rate seen under the other conditions, and they were twice as likely to die."

The findings provide hope for one proposed conservation strategy: reducing ocean acidity by adding bicarbonates or lime to the water. Trials show that this could succeed, but we'd need to add around 10 cubic kilometres of lime (around 9,000 million tonnes) to the oceans every year to offset the effects of our carbon emissions. At the moment, we produce around 300 million tonnes of lime annually, mainly for the concrete industry, so the problem quickly becomes apparent.  Other scientists have suggested ways of reducing temperatures in the shallow waters that reefs grow, including erecting giant canopies to shade them.

Replanting Efforts

A more practical method might be to garden the reefs in a similar way to how we tend our agricultural plots on land. To that end, scientists are breeding corals in special facilities that simulate different growing conditions, looking for varieties that are tolerant to higher temperatures, ultraviolet light (a problem since we created a hole in the ozone layer), and acidity, for example. One way of doing this is by exchanging the coral's algae species for more robust types.  The next stage is to plant the corals in the sea. Scientists around the world have created all manner of artificial reef for transplantation, from concrete balls to steel cages. One interesting design incorporates an electric current, which helps stimulate coral growth. I saw one of these on Vabbinfaru island in the Maldives, where researchers had submerged a huge steel cage called the Lotus on the sea floor. The 12-metre structure, weighing around 2 tonnes, is connected to long cable which supplies a low-level electric current. The electricity triggers a chemical reaction, which draws calcium carbonate out of solution in the water and it gets deposited on the cage structure .  Corals seem to find this irresistible because they use the same material to grow their protective skeletons, and the Lotus has been so thoroughly colonised by coral that it is difficult now to make out the steel shape beneath all the elaborate shapes and colour.  The El Nino Pacific-warming phenomenon of 1998 killed 98% of the reef around Vabbinfaru, so the scientists there have been able to compare the growth rates for corals grafted on to concrete structures on "desert" patches of seafloor, and those stuck on to the Lotus. Abdul Azeez, who led the Vabbinfaru project, said coral growth on the structure is up to five times as fast as that elsewhere.  The electric reef may also make the corals fitter and better able to withstand warming events, perhaps because the creatures waste less energy on making their skeletons. A smaller prototype device was in place during the 1998 warming event and more than 80% of its corals survived, compared to just 2% elsewhere on the reef.  The original designer of the Lotus, Wolf Hilbertz, who died in 2007, believed that his structures could be submerged across the world to repopulate reefs and protect shorelines. In reality, the cost and effort involved make it impossible to do except on a small scale. However, it is a useful technique for tourist resorts and to help small areas of reefs recover that have been hit by temporary damage, such as an oil spill or boat impact. 

The Anthropocentric – the Age of Man – will not only be far less colorful place, it will also be far poorer in terms of fish diversity, islands and coastlines will be more prone to erosion, and millions of livelihoods will be threatened. Coral reefs support a quarter of marine life on Earth – they contribute to half of the GDP in many Caribbean countries, from creating sandy beaches to harboring the fish people eat. And as the sea levels rise, they provide vital protection against storm surges and inundation.

We can turn things around if we focus on three goals: ending overfishing, controlling by-catch and protecting our ocean nurseries. Help us ensure that the oceans remain bountiful and beautiful for generations to come.