Coral reefs are built by tiny animals called polyps. Each polyp secretes a calcium carbonate skeleton. Over thousands of years, these skeletons pile up into structures that can be seen from space. The Great Barrier Reef stretches over 2,300 kilometers. Reefs cover less than 1 percent of the ocean floor but support about a quarter of all marine species.

Corals live in symbiosis with algae called zooxanthellae. The algae provide food through photosynthesis and give corals their color. When water gets too warm, corals expel the algae and turn white. This is bleaching. A bleached coral is not dead, but it is starving. If temperatures stay high, it dies.

Reefs are in trouble. Mass bleaching events have hit the Great Barrier Reef repeatedly since 1998. Pollution, overfishing, and ocean acidification add to the stress. Scientists estimate that half of the world's coral has been lost in the past 30 years. Restoration projects plant coral fragments and breed heat-tolerant strains, but they cannot keep up without cutting emissions.

Key threats

• Ocean warming
• Ocean acidification
• Pollution and runoff
• Overfishing
• Coastal development