Can people grow a coral reef back?

People can grow some coral back, but not yet a whole reef. The best-known method is coral gardening: coral fragments are grown in nurseries and then replanted on the reef. Gardening skips the early stages of a coral's life, when it is most at risk of dying. A second approach tries to give corals a better chance against heat, using naturally heat-tolerant partners or breeding.

Gardening works because corals can fragment and keep growing if the pieces anchor to a new surface. Baruch Rinkevich first tested it in 1995. A newer trick called microfragmentation, developed in 2014 at Mote Marine Laboratory in Florida, cuts corals into small pieces with a diamond band saw. After two years one species had grown to 6.5 times its original size, and the lab produced 25,000 corals in a single year.

Heat tolerance is the other hope. Some zooxanthellae, for example the group called clade D, survive better in modern bleaching events, and researchers are trying heat-tolerant symbionts and interbreeding colonies of the Great Barrier Reef to see whether offspring resist warmer water.

The honest limits are large. Nurseries cannot stop climate stressors: warm water or hurricanes can still kill nursery corals. Manipulating the algae partners has worked only in young corals, and experiments on older ones have all failed. Assisted evolution is believed to be only a temporary solution. And by one estimate, shallow tropical reefs have already declined by 50 percent since 1950. Restoration can repair patches and buy time, but it cannot remove the heat and storms that kill corals.