By Roger Harrabin
Environment analyst, BBC News
China has changed its attitude to CO2 emissions, Mr Blair says
The G8 leaders are set this week to deliver their strongest statement so far on global warming.
They are likely to agree that the world ought to cut greenhouse gas emissions by 50% by 2050 - with rich nations reducing them by 80%.
The group will probably also say that any human-induced temperature rise should be held to 2C - a level considered to be a danger threshold.
The US has previously objected to such a clause.
But it looks as though the G8 will fall short of agreeing the short-term targets scientists say are essential to ensure that the 2C threshold is not breached.
Environmental campaigners accuse the G8 of willing the ends on climate change but not willing the means.
American officials have privately told BBC News they cannot cut emissions as fast as the science requires, because the issue is still too politically contentious in the US Congress.
I understand that the US is also delaying the G8 climate communique in the hope of obtaining more commitment from emerging nations on the issue.
On Thursday, US President Barack Obama chairs a meeting of the G8 members with the leaders of the emerging economies, including India and China, under a process known as the Major Economies Forum (MEF).
That meeting will produce a declaration separate from the G8. Opinions among the emerging economies vary widely. India opposes commitments on cutting emissions. It has millions living in poverty and considers that the problem should be solved by rich nations. India is suspicious of signing up to the 2C warming threshold because it implicitly puts a cap on Indian growth.
China is committed to achieving a low-carbon economy, but slowly so as to cause minimum social and economic upheaval.
"We have to persuade China that it is in China's interests to move quickly to a low-carbon economy - that will be be key," a western diplomatic source said.
Brazil is the most significant of the emerging nations to sign up to the 2C threshold. "This is extremely significant," said the source. "It is an acknowledgement from political leaders to their peoples that there are scientific limits to how far we can push the planet."
The head of the Intergovernmental Panel on Climate Change (IPCC), Professor RK Pachauri, told BBC News: "I don't think we can hold out great hopes for the MEF - it is G8 that had to make the key decisions here on emissions cuts and on funding to help poorer countries to adapt to climate change and obtain clean energy supplies."
Recently, the UK Prime Minister Gordon Brown proposed that rich nations should put $100bn into a fund to help poor countries deal with climate change; but I understand the figures on the table so far at the G8 are very much lower than this.
The former UK premier Tony Blair has urged the group's leaders to seize the moment, to tackle climate change with major emissions cuts by 2020.
He has been working on a private initiative with a business-oriented NGO called The Climate Group.
It has produced a new report which champions green technologies, arguing that they offer the chance of "substantial job creation and growth".
The report also says the technologies needed to meet emissions reduction goals set for 2020 are "already proven, available now and the policies needed to implement them known".
This means ramping up existing policies on energy efficiency, new appliance standards and renewable energy.
Mr Blair told BBC News that significant emissions cuts could be achieved by halting deforestation and the degradation of forests; something that could be done if rich nations paid poor nations to protect their forests (though this seemingly simple policy is fraught with practical difficulties).
"I think it's very understandable at a time of major economic crisis that people are very daunted by the additional challenge of climate change," Mr Blair said.
"I think the single most important thing we found is that almost three-quarters of what you want to do can come from existing and known technologies and actions. It has to be done. There is no option."
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Mr Blair said there had been a huge change in the attitude of world leaders to climate change: "In 2005, there was a lot of resistance when I put it on the G8 agenda. We were able to come out with some broad, general principles - it was a big step forwards.
"But in the intervening period, this has moved a long way. I think leaders are now focused on practical policy implications.
"This is now at the stage where it's been taken out of the hands of campaigners (although they are still important) and into the hands of the people who are going to have to get the job done.
"We have an American administration committed to tackling climate change. We have a Chinese administration that's no longer saying, 'you guys have created the problem - you solve it', but has immersed itself in this challenge.
"And you have a general acceptance on the part of most sensible people that we have to deal with it. I think you will see a significant move forward before Copenhagen." (The UN conference to seal a new global climate deal in December)
Campaigners will welcome Mr Blair's intervention but may be sceptical about his confidence in the outcomes.
In his term of office the UK Treasury adopted a laissez-faire energy policy which has left the UK with one of the lowest shares of renewable energy in Europe, despite having one of the best potentials for renewable power.
I asked Mr Blair if he would have pursued a different energy policy with hindsight. He declined to comment.
A group of 22 leading climate scientists has written to G8 and MEF leaders calling for policies that would see global emissions peak by 2020, and shrink by at least 50% by 2050.
"Unless the burden of poverty in developing nations is alleviated by significant financial support for mitigation, adaptation, and the reduction of deforestation, the ability of developing countries to pursue sustainable development is likely to diminish, to the economic and environmental detriment of all," the scientists said.