Can victory be snatched from the jaws of failure at G20 and COP26?

ITV News Political Editor Robert Peston speaks to Prime Minister Boris Johnson in Rome ahead of COP26

"There is no chance of stopping climate change next week," the prime minister told me in an interview for ITV News.

"There is no chance of getting an agreement to limit climate change to 1.5 degrees".

Standing in Rome's magnificent ancient Colosseum, he warned that the cost of this failure, if not somehow rectified, would be far worse than the recent pandemic: "The Romans thought they were going to go on forever...

"Then wham, the middle of the fifth century, they hit a complete crisis, uncontrolled immigration, you have the Dark Ages.

"The lesson is things can go backwards... for a long time. Unless we fix climate change, unless we halt that massive growth in temperatures, that's the risk we run".

What does the PM think he can achieve on climate change?

But if COP26 has already failed, as the PM seems to be saying - because the world's biggest emitters are such a long way from promising measures to cut greenhouse gas emissions by the necessary 55% before 2030 - why on earth are more than 100 world leaders descending on Glasgow for the COP26 negotiations?

Also what's the point of this weekend's preceding discussions in Rome of the G20 leaders of the world's most powerful nations?

For Boris Johnson, COP26 is what he calls a "weigh station", a checkpoint on a route towards future agreements that would stand a chance of reducing global warming to a safe increment.

In full: Robert Peston's interview with Boris Johnson

For him there remain possible constructive outcomes to be achieved in Glasgow, such as securing pledges to end the use of "unabated" coal (that's all burning of coal where the CO2 isn't "captured" and kept out of the atmosphere) by 2030 for developed nations and 2040 for developing nations.

But truthfully, though he refused to say this, the best possible outcome of COP26 for Johnson is that in the final conclusions there is backing for what officials call a "ratchet", which would be a mechanism such that in two or three years all countries would come back together to make new pledges to reduce greenhouse gas emissions that would exceed their current nationally determined contributions (or NDCs).

The point is that in the absence of such a ratchet, it will be another five years before new commitments to end the use of fossil fuels would be made, and most scientists warn five years would be far too late.

However, when I pressed him on the importance of the ratchet mechanism, he said he didn't want to herald it now, because it would provide dangerous reassurance to big carbon dioxide and methane emitters like China, India, South Africa and the US that they don't have to do more immediately to limit their emissions.

"We don't want at this stage to take any of the pressure off at Glasgow and what we want to achieve in Glasgow," he said.

A related question is what can be secured here in Rome at the G20 warm up event.

Finding consensus even here is also challenging, because of divergent economic interests between the mature and developing economies, and divergent cultural approaches to big politics between West and East.

"What is really striking," said an official, "is that in places like India and throughout Asia they don't see the point in making a commitment to a target like 1.5 degrees when we don't have a clear and detailed plan to achieve it".

He said many Asian governments think it's better to start with practical measures we all know we can do and then accept the degree of warming that flows from that. In the West however "we have a history of identifying the best outcome, 1.5 degrees in this case, and then working out a route to get there".

Alongside that almost ideological difference is the crude economic truth that economies like South Africa, India and China remain at an earlier stage in their development than the UK, European Union and US, and therefore retain coal power as an important engine of growth.

Which is why the PM was reassured when he spoke to China’s president Xi yesterday and urged him to move faster to reduce China’s dependence on coal that Xi signalled a willingness to at least keep talking.

But here in Rome, there are uncertainties about whether even the limited ambitions for the G20 communiqué will be achieved.

One aim is to coral all 20 leading nations to commit to a target date of 2050 to 2060 for "net zero", when there would be no further increases in the saturation of greenhouse gases in the atmosphere.

There are "two and a half hold outs", said an official, namely India (a massive emitter), Mexico and South Africa (the half, because there seems uncertainty about whether coal-dependent SA has or hasn't properly committed to net zero).

The other potentially significant clause in tomorrow's G20 communique will be around whether it commits all 20 members to the target of limiting global warming to that all-important 1.5 degrees above pre-industrial levels, rather than the 2 degree target of the PARIS COP in 2015.

Back in 2015, 1.5 degrees was what businesses would call a "stretch" target, a hope rather than an official goal.

For COP26 to start with anything that feels like a following wind, 1.5 degrees has to be commonly accepted as the baseline for all planning.

But even if it is, and as Johnson concedes, COP26 won't even get us half way towards warding off devastating damage to planet and civilisation.