Australia's Great Barrier Reef has only a 50 per cent chance of survival if global CO2 emissions are not reduced at least 25 per cent by 2020, accordiung to a coalition of Australia's top reef and climate scientists.

The 13 scientists said even deeper cuts of up to 90 per cent by 2050 would be necessary if the reef was to survive future coral bleaching and coral death caused by rising ocean temperatures.

"We've seen the evidence with our own eyes. Climate change is already impacting the Great Barrier Reef," Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, said in a briefing to Australian MPs.

Australia, one of the world's biggest CO2 emitters per capita, has only pledged to cut its emissions by five per cent from 2000 levels by 2020.

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 next month, but this is looking increasingly unlikely with legally binding targets now off the agenda.

"This is our Great Barrier Reef. If Australia doesn't show leadership by reducing emissions to save the reef, who will?" asked scientist Ken Baldwin, in calling for Australia to lead the way in cutting emissions.

But the Australian government is struggling to have a hostile Senate pass its planned emission trading scheme. A final vote is expected next week.

The World Heritage-protected Great Barrier Reef sprawls for more than 345,000 square kolimetres off Australia's east coast and can be seen from space.

The Australian scientists said more than 100 nations had endorsed a goal of limiting average global warming to no more than 2°C above pre-industrial temperatures, but even that rise would endanger coral reefs.

The scientists said global warming was already threatening the economic value of the Great Barrier Reef which contributes A$5.4 billion to the Australian economy each year from fishing, recreational usage and tourism.