A diver swims above a garden of stony corals on the Great Barrier Reef, which is more than 1,250 miles long. Climate change poses a multitude of threats to this international treasure.

An anemonefish peers from its home in the Great Barrier Reef. As fossil fuel emissions get taken up by the ocean, they can change its chemistry, which can alter how tropical fish see, hear, smell, and avoid predators.

Cardinalfish zip by a loggerhead turtle as it rests among feathery invertebrates called hydroids. As marine waters warm, they can lead to more bleaching events on reefs, reducing hiding spots and sources of food.

Currents in Challenger Bay push and pull a school of diagonal-banded sweetlips. Rising seas and warming oceans can change the reef’s ability to block storm surges.

A bumphead parrotfish and a grouper hover near the sandy bottom.

The clownish grin of a bridled parrotfish reveals its power tools: grinding teeth used to scrape away algae, a mostly beneficial move. As more CO2 enters the sea, it can cause drive the growth of more algae, which threatens to smother more of the reef.

Moray eels battle for a hiding spot in Challenger Bay. As oceans sour from absorbing more CO2, some corals struggle to grow, which can reduce hiding spots for a variety of marine creatures.

A two-foot-long sea cucumber, kin to sea stars, shoots thousands of ova into the current to spawn en masse.

A scuba diver hovers above a black coral tree bursting with cardinal fish. Divers travel from around the world to dive on the Great Barrier Reef, an economic boon to Australia that is threatened by climate change.

The humphead wrasse is among the reef's many thousands of species.

The Great Barrier Reef is among the most diverse marine environments in the world, hosting 5,000 types of molluscs, 1,800 species of fish, and 125 kinds of sharks. Much of this diversity is threatened by warming seas and ocean acidification.