A Coral reefs cover only a tiny fraction of the sea floor, yet they shelter a disproportionate share of marine life and protect coasts from storm waves. Reefs are built by corals, tiny animals that live in colonies and form hard skeletons. Most reef building corals depend on microscopic algae that live inside their tissues. The algae use sunlight to make food, and the coral supplies them with shelter, which gives the partnership its bright colours.
B Rising sea temperatures put this partnership at risk. When water stays unusually warm for weeks, corals expel their algae, turn white and, if conditions do not improve, starve. This process is called bleaching. Bleaching is not always fatal, and corals can recover if temperatures fall in time, but a reef that suffers repeated events has little opportunity to regrow. Several major bleaching events in recent decades have affected reefs across entire oceans.
C Warming is not the only threat. Run off from farms carries nutrients into coastal water, which encourages seaweed that smothers young corals. Fishing methods that damage the reef, and the removal of fish that graze on seaweed, make the problem worse. Because these pressures reinforce each other, researchers argue that local protection, such as limiting pollution and fishing, can give reefs a better chance of surviving warming, even though it cannot prevent it.
D Some scientists are experimenting with ways to help reefs adapt. Teams grow corals that tolerate heat in nurseries and transplant them to damaged sites, and others study whether tolerant algae can be passed to vulnerable corals. These techniques are promising but expensive and slow, and they work only at small scales. Most experts therefore see them as a supplement to, not a replacement for, cutting the emissions that cause warming.
