1Coral reefs cover less than one percent of the ocean floor, yet they support a quarter of all marine species.
2These underwater cities are built slowly, over thousands of years, by tiny animals called polyps.
3Rising sea temperatures are now forcing corals to expel the colourful algae that keep them alive.
4When this happens, the coral turns a ghostly white, in a process that marine scientists call bleaching.
5A bleached reef is not dead, but it becomes weak and may starve without its algae.
6In the past three decades, the world has already lost roughly half of its coral reefs.
7The loss of reefs threatens not only fish but also the millions of people who depend on them.
8Coastal communities around the tropics rely on healthy reefs for food, tourism income, and protection from violent storms.
9Scientists are now breeding heat-resistant corals and planting them back onto damaged reefs by hand.
10Whether these restoration efforts will ultimately succeed depends largely on how quickly humanity reduces its carbon emissions.