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.
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.