The world is 33 months into its biggest recorded coral bleaching event with little sign of it ending, raising the prospect that coral mortality on the Great Barrier Reef will increase "significantly" from the quarter already lost in the past year, scientists say.

"Is this the global bleaching event that doesn't go away?", said Mark Eakin, coordinator of the Coral Reef Watch run by the US National Atmospheric and Oceanic Administration. "The patterns that we are seeing in the [forecast] models are looking very similar to what we saw in the last two years."

Dr Eakin's comments come as the first major study of the current bleaching event is published on Thursday in the journal Nature. The paper, of which Dr Eakin is one of 46 authors, examined the 2015-16 bleaching event compared with the two big previous ones in 1998 and 2002.

The current event alone killed two-thirds of the relatively pristine northern third of the Great Barrier Reef in 2016 and has returned now, particularly in the central region between Cairns and Townsville.