Professor Terry Hughes and representatives of the Federation of Australian Scientific and Technological Societies told a meeting at the Canberra parliament that the future of the reef, and a large chunk of Australia's tourist industry, was under grave threat from rising sea temperatures.
Just a small increase in average temperatures could cause massive coral bleaching on the reef, he said.
"We've seen the evidence with our own eyes. Climate change is already impacting the Great Barrier Reef," said Prof Hughes, of James Cook University in Queensland.
To avoid permanently damaging the delicate balance of life on the reef, and give the world's largest living organism a 50 per cent chance of survival, global carbon emissions must be cut by at least 25 per cent by 2020, he said.
Prof Hughes's grim prediction came as the Australian parliament debated the details of the government's planned Carbon Pollution Reduction Scheme. A final vote on the carbon trading bill is expected next week.
Australia, one of the world's biggest carbon emitters per capita, has so far only pledged to cut its emissions by five per cent from 2000 levels by 2020.
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It has said it would go further, with a 25 per cent cut, if a tough international climate agreement is reached at UN climate talks in Copenhagen in December, but this is looking increasingly unlikely.
John Quiggin, of the University of Queensland, has carried out research which shows the economic impact of a two degree rise in global temperatures.
He said a rise of more than two degrees would be "catastrophic" for the reef and tourism in North Queensland, which is already suffering as a result of the global recession.
The World Heritage-protected Great Barrier Reef sprawls for more than 133,000 sq miles off Australia's east coast and can be seen from space.
The UN Intergovernmental Panel on Climate Change has concluded that the Great Barrier Reef could be "functionally extinct" within decades, with deadly coral bleaching likely to be an annual occurrence by 2030.
Bleaching occurs when the tiny plant-like coral organisms die, often because of higher temperatures, and leave behind only a white limestone reef skeleton.