What's happening

Tavishi adjusts her oxygen mask and dives into the sea. Soon, she tells herself, she will be gazing at the wonderful colours of a coral reef.

But when she reaches it she stares in horror: the coral is a ghostly white. That means it is very sick.

Coral reefs look like plants, but are actually made up of creatures related to jellyfish called polyps.

The polyps are usually found in groups known as colonies. When a large number of these join together a reef is formed.

Many reefs depend on tiny algaeA simple plant that grows in or on water. which live in the colony and produce 90% of its food.

But a rise in the ocean's temperature can mean that the algae leave a reef, and when this happens the coral turns white and could easily die. This is bad news for the whole ocean, since a quarter of all marine species live in coral.

US marine scientists have reported a "mass coral bleaching event", which means that every ocean containing coral is now affected.

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Bleaching can happen if the sea temperature rises by just 1C for a month.

Reefs can recover, although it takes years, and scientists are trying various things to help. One is to create giant tanks in which the polyps are encouraged to release eggs. These are then planted on endangered reefs.

And some corals are more resistant to heat than others. By breeding from these, scientists can increase a reef's chances of survival.

Is there still hope for nature?

Yes! Some of the ideas people are using to help it are really amazing, and they are bringing real passion to the challenge. And reefs in certain parts of the world are doing well.

No! Humans have already done too much damage. The oceans are growing warmer all the time and the too many rich countries are dragging their feet when it comes to reducing carbon emissions.