The chances of a global agreement to fight climate change at UN talks in Copenhagen in December are only 50/50, said Australia's leading environmentalist, who warned of "full climactic destabilisation" without a pact.
Tim Flannery, chairman of the Copenhagen Climate Council which fosters collaboration between international business and science to fight climate change, said without action in Copenhagen the world faced "runaway" climate change.
"The changes in the climate system are occurring at a very fast rate indeed and our attempts to catch up in terms of our social and economic policy are just not fast enough," said Mr Flannery.
"I'm deeply concerned that in the near future we're facing a runaway situation," added Mr Flannery, as part of Reuters Climate Change and Alternative Energy Summit.
The UN climate talks in Copenhagen aim to reach agreement on a post-Kyoto pact to reduce greenhouse gas emissions, which are blamed for global warming.
Cooperation between the US and China, the world's two biggest polluters, is considered essential for the world to reduce heat-trapping gas emissions. China's rapid growth has pushed it into first place as the world's leading source of carbon dioxide emissions. But the United States, the number two emitter, still has spewed the most heat-trapping gases into the air over time.
Their greenhouse gas emissions are driven by a huge appetite for fossil fuels.
But both Washington and Beijing have in recent months vowed to reduce emissions, prompting environmentalists like Mr Flannery to become more hopeful of a global climate pact being reached.
"I'm more optimistic than I have been for a number of years, for a number of reasons," Mr Flannery explained.
"The first is that President Obama has taken an interest in this issue, and also the Chinese have shown real leadership, so that gives us hope," he said.
The House of Representatives has approved legislation that would cut US greenhouse gas emissions 17 per cent from 2005 levels by 2020 and 83 per cent by 2050. It is unclear whether the US Senate will pass similar legislation before Copenhagen.
In China's current five-year plan, the government aims to cut energy consumption per unit of GDP by about 20 per cent by 2010 compared to 2005 levels, saving more than 1.5 billion tons of CO2 from being emitted.
The effort has fallen behind schedule, although cuts are speeding up.
Beijing is also demanding that developed countries cut greenhouse gas emissions by 40 per cent from 1990 levels by 2020 and for rich nations to commit between 0.5 and 1.0 per cent of their GDP to help developing countries address climate change.
"The obstacles in our path are still absolutely immense and I must admit our chances are still somewhere around 50/50 of us reaching an agreement," said Mr Flannery.