“I hope world leaders will hear them and come to Glasgow ready to answer them with decisive action.

“We need to recognise the scale of the consequences if we fail.

“The difference between restricting the growth in temperatures to 1.5C and allowing them to increase by 4C is potentially absolutely catastrophic for our children, our grandchildren and succeeding generations. We cannot discount their welfare.

“Together, we can mark the ­beginning of the end of climate change — and end the uncertainty once and for all.”

Mr Johnson and wife Carrie ­visited the Colosseum, where he gloomily predicted the best he can hope for by any action agreed is to slow the increase in global temperatures.

His biggest stumbling block is the fact that China — responsible for more than a quarter of all carbon emissions — decided not to attend the summit in the first place.

The communist giant plans to build 43 new coal power plants and 18 blast furnaces, adding 1.5 per cent to its emissions, despite a pledge to be carbon neutral by 2060.

A massive 58 per cent of its energy is generated from coal as the world is being urged to switch to more expensive green power.

Mr Johnson accepts the cost of going green won’t come cheap, but said there were the added benefits of hundreds of thousands of high-wage, high-skilled jobs.

He added: “When I say that going green is easier, look at what we have done.

“The UK has cut CO2 by 44 per cent on 1990 levels and yet our economy has grown by 78 per cent.

“The point I made to our Chinese friends yesterday is you can cut your reliance on coal and you can still grow your economy and create many, many hundreds of thousands of high-pay, high-skilled jobs.

“They’ve made progress on overseas financing of coal.

“What China needs to do is find ways of making a more ambitious, nationally determined contribution.

“What the whole world needs to understand is you can reduce dependence on coal very fast.

“I reminded President Xi that the first time I went to Beijing, as the Mayor of London, the UK was 40 per cent reliant on coal to generate power.

“Today, it’s only one per cent.

“You can make progress very fast.”

He admits the chance of a breakthrough is no better than the six out of ten he gave it last month, but says Glasgow can be a “stopping point” for eventual success.

He said the fight was going to be very difficult, but added, “the whole of humanity is in the ring” squaring up to the “foes of humanity”.

Mr Johnson declared: “I think that everybody needs to focus.

“What the UK has been trying to do is take the abstract concepts of Net Zero we talked about in Paris six years ago and turn them into hard, sharp deliverables — reducing coal and internal combustion engine use, planting millions of trees and getting the cash the world needs to finance green technology.”

Mr Macron and Mr Johnson are set for a showdown over a furious fishing row

Boris Johnson warned the world faces ­’absolute catastrophe’ if leaders fail to strike a deal to tackle climate change

• COP26 opens today at the Scottish Event Campus, with 30,000 delegates, 10,000 police — and as many as 200,000 protesters for the 13-day conference.

Carrie Johnson visits the Colosseum alongside partners of world leaders during the G20 spousal programme in Rome

The PM’s wife at Rome’s iconic Colosseum