What is coral bleaching and why is it a problem?

Rising sea temperatures have caused coral bleaching across the Great Barrier Reef on a scale never seen before.

Two-thirds of Australia's world-famous reef is now damaged by bleaching.

Bleaching is becoming a real problem for the coral reefs across the world.

Prof Terry Hughes, from James Cook University, said governments must urgently address climate change to prevent further bleaching.

"Since 1998, we have seen four of these events and the gap between them has varied substantially, but this is the shortest gap we have seen," Prof Hughes told the BBC.

Find out why coral bleaching is such a problem by watching the video above.