Mr Obama told the University of Queensland audience on the sidelines of the G20 meeting he wanted the reef to still exist "50 years from now" so his grandchildren could visit. Scientists say rising temperatures and acidity are two long-term threats to the Great Barrier Reef. While Ms Bishop and other Coalition leaders have criticised the US President's intervention, leading scientists have come to his support. Mr Obama was "right on the money", Ove Hoegh-Guldberg, director of the university's Global Change Institute, said. "He was stating a fact. "We have one of the jewels of the planet in our possession and we should care a lot about climate and he wasn't getting that from our leader [Prime Minister Tony Abbott]," Dr Hoegh-Guldberg said.

Peer-reviewed research published by Dr Hoegh-Guldberg in 2012 said the global agreement to limit CO₂ concentrations to 450 parts per million in a bid to keep global warming to under 2 degrees from pre-industrial times would not be enough to protect the reefs. Any increase above 1.5 degrees would be devastating, the research found. The reef has already shrunk by half in 30 years, he added, with climate change a factor in its retreat. Threats Charlie Veron, a former chief scientist at the Australian Institute of Marine Science, went further, saying the Abbott government was downplaying the dire future facing the Great Barrier Reef and coral reefs everywhere.

In the long term ... we are going to have the Great Barrier Reef slaughtered. "In the long term, that is the whole of this century, we are going to have the Great Barrier Reef slaughtered," Dr Veron, a world authority who has scientifically named about one-quarter of all known corals, said. "There's no doubt about that at all, if carbon-dioxide emissions keep on tracking as they are." CO₂, as the major greenhouse gas, traps radiation, heating up the planet. While natural variability plays a role, the background warming continues apace with 2014 on course to be the hottest year on record, the US National Atmospheric and Oceanic Administration reported on Thursday. The immediate threat is another outbreak of bleaching – some of which is now being detected – as ocean temperatures warm, disrupting coral ecosystems, Dr Veron said.

"In the short-term, the Great Barrier Reef is incredibly at risk from mass bleaching from the warming of the oceans," he said. He also dismissed the federal and Queensland governments' Reef 2050 Long-Term Sustainability Plan, released earlier this year. "It's basically a five-year plan to head off UNESCO," Dr Veron said, referring to the UN body's review of the Great Barrier Reef's World Heritage status due for completion by mid-2015. Acid effect Dr Hoegh-Guldberg said government efforts to curb run-off of farm chemicals and the recent decision to ban off-shore dumping of dredge spoil – after approving major works to expand the Abbot Point coal port – would go some way to aid the reef's ability to cope with near-term challenges.

The increased concentration of CO₂ not only heats the atmosphere, it also results in an increase in the acidity of the world's oceans as carbon gets absorbed by the seas. "It's just chemistry ... you can't deny that," Dr Hoegh-Guldberg said. "It's dropped the amount of crucial building blocks for skeletons and shells by 26 per cent," he said. "That's then had an impact on the abilities of corals and other organisms to build their skeletons and to rebuild them after storms, after damage from crown of thorns [starfish] and so on." Work at the James Cook University has shown that altered chemistry affects some fish in ways that reduce their ability to identify predators and even find their way home.

"It's just one of many, many significant changes that we are yet to discover," Dr Hoegh-Guldberg said. The health of the reef was also covered in the Intergovernmental Panel on Climate Change reports, which predicted "significant change in community composition and structure of coral reef systems in Australia". "The ability of corals to adapt naturally to rising temperatures and acidification appears limited," the chapter on Australasia said. Contradiction Jon Brodie, a chief research scientist from James Cook University, said Ms Bishop's comments contradicted the government's own report on the state of the reef.

The Great Barrier Reef Outlook Report 2014 "found the reef to be in poor condition and the outlook is for continuing deterioration," Dr Brodie said. "It's obviously in danger." "Climate change remains the most serious threat to the Great Barrier Reef and is likely to have far-reaching consequences in the decades to come," the report said. Dr Brodie said government efforts to improve the reef's water quality had made some headway "but it's small progress". He noted that the government had extended targets out to 2018 because goals set in 2009 had not been met. The government's 2050 action plan, still in a draft stage, will "absolutely not work", Dr Brodie said, adding that it so far fails to set goals to address long-term threats such as climate change. "Under that plan, the reef will continue to decline," he said.