The bleaching - or loss of algae4
- affects a 1,500km (932 miles) area of the reef, according to scientists.
The latest damage is concentrated in the middle section, whereas last year's bleaching hit mainly the north.
Experts fear the proximity5
of the two events will give damaged coral little chance to recover.
Prof Terry Hughes, from James Cook University, said governments must urgently address climate change.
"Since 1998, we have seen four of these events and the gap between them has varied6
substantially, but this is the shortest gap we have seen," Prof Hughes told the BBC.
"The sooner we take action on global greenhouse gas emissions7
and transition away from fossil fuels to renewables, the better."