BBC environment correspondent
Tony Blair said climate change could soon become "irreversible"
The debate the government promised on the climate bill has begun - online.
A conversation between Tony Blair and the Friends of the Earth director Tony Juniper about the government's climate change bill has been posted on the environmental group's website.
Cynics have argued the bill is a statutory manifestation of the panic brought on within the Labour Party by the way David Cameron and the Tories have seized the environmental agenda.
But officials within Downing Street argue this bill has been on the cards for some time, and that New Labour were promising to cut greenhouse gas emissions way before the Conservatives had climate change at the heart of their political agenda.
Central to this parliamentary point scoring has been the environmental group, Friends of the Earth, which has moved from being considered dangerously wacky a few years ago, to now being wooed by all party leaders.
Mr Cameron asked to share a platform with the group earlier this year to campaign for the climate change bill, and now Tony Blair has launched a debate on their website.
But do they like what Mr Blair says?
Well, the prime minister is obviously keen for the greens to be his friends as well as the planet's.
"I suspect I share many more views on climate change with Friends of the Earth supporters than you might imagine," he begins.
"I am fully aware of just how serious a challenge it is to our planet and way of life. I agree as well there is a huge amount more to do both at home and internationally to cut emissions.
"Above all, I share your view that there is very little time to waste. There is, I promise you, no complacency in government.
"We recognise that we have little time before climate change becomes irreversible and possibly catastrophic."
So far, so environmentally sound, the green groups would say.
The prime minister then goes on to talk about the UK's record on reducing carbon emissions - not brilliant, but we are "on course to double our promised cuts in greenhouse gases. No other country will get anywhere near this reduction."
"We also intend," he continues, "to put in place a raft of new measures, including new targets for renewable energy and energy efficiency, which Friends of the Earth support, and replacing nuclear plants which it does not - to continue progress towards our long-term goal of a 60% reduction by 2050."
Ah, the nuclear question - that's likely to elicit a very unfriendly response from the Friends.
One of the key demands of environmental groups - and, it has to be said, the other main political parties - when campaigning for a climate change bill was that there should be annual targets for reductions in greenhouse gases - and again, the prime minister knows he is going to upset them.
Campaigners have called for carbon cuts for a long time
"I know Friends of the Earth are disappointed that annual targets for reductions will not be part of the bill," he said.
"A year is too short a period with too many factors such as a cold winter or a sudden hike in energy prices outside government control to make this a meaningful trigger for action."
"The cold winter of 2001, for example, saw emissions increase by 3% over the previous year only to drop back again to the same level within 12 months.
"So annual targets would force action to be taken which could hit both families and business to tackle a problem which may already be disappearing by the time the measures are put in place."
So, what does the politician's green group of choice think of all this? The director of Friends of the Earth, Tony Juniper, replies in fairly trenchant terms.
"I welcome that fact that you, prime minister, recognise the threat posed by climate change, as thankfully all our major political parties do now," he says.
"Unfortunately the problem has not been seeing the threat but making changes to address it.
"You make much of the government's record to date on cutting carbon emissions but the figures reveal a more patchy record.
"For example, you say you are committed to cutting emissions of the main greenhouse gas, carbon dioxide, by 20% by 2010.
"Unfortunately I don't see how this will happen. Between 1990 and 2005 we managed only a 5% reduction in carbon dioxide emissions.
"To meet this target, we must now cut carbon emissions almost 10 times as fast. Is that really possible?
"And thank goodness we are on track to meet our Kyoto target, but doubling it looks doubtful.
"It has taken six years to cut our Kyoto emissions by 1% - to double the target we need a 10% cut in the next five years.
"The hard fact is our current approach simply isn't working. We need a new one."
In other words, your approach isn't working, prime minister, and you need a new one...
And he doesn't think much of the prime minister's arguments against annual targets.
Mr Juniper said they "are needed to ensure that we remain on track to deliver the small cuts that in the end will add up to big ones".
He added: "Right now we have some good targets, but they are all in the future. When the future arrives and we find that we didn't meet them, then it is too late. The damage to the climate will be done."
On to the nuclear issue: "Yes, we disagree on nuclear. But not for the reasons you probably think.
"Our major argument against nuclear is that we could cut carbon emissions more cheaply and quickly if investments were instead encouraged in energy efficiency and renewable power," he said.
"Of course there are serious worries about waste, accidents and terrorist threats. Weighing those risks against climate change is never going to be easy but as alternatives to nuclear would be more effective, we actually don't need to."
But after taking Mr Blair to task, Mr Juniper does ends with warm words - but he's not going to let that point on annual targets go: "You are one of very few world leaders who has seen the scale of threat and who has advocated action.
"You have been successful in establishing a political debate that has helped to create perhaps our last chance to avoid the worst effects of rapid global warming.
"Seizing the opportunity provided by the new bill, enacting annual targets, and in so doing showing the world that you and the British people are serious about protecting our way of life, would be an act of leadership that would rally the full support of Friends of the Earth, and most of the wider environmental community.
"If you act boldly here, if you lead in Britain, then the rest of the world will very likely follow you."
The idea is that these are the opening positions in a web-based debate to which everyone can respond - go to www.foe.co.uk if you want to take part.