The world is facing a “moment of truth” in Glasgow, Boris Johnson has said, as leaders from more than 100 countries gather in the city for the COP26 climate summit to avert an “absolute catastrophe” for the planet.

In an exclusive interview with The Herald, the Prime Minister did not exude his usual bullish confidence but, rather, warned of the “massive challenge” facing world leaders, and made clear getting them all to agree to deliver on the aim of the 2015 Paris accord – to limit global warming to a rise of 1.5C – would be extremely difficult.

“I’m not going to give a kind of rosier, optimistic gloss to this… I’m saying this is extremely difficult but countries can achieve extraordinary things.”

Indeed, Mr Johnson emphasised what he regards as Britain’s global leadership on battling climate change. The country has cut its CO2 emissions by almost half since 1990 while seeing its economy grow by almost 80% over that period. This, he insists, shows a green transformation is entirely possible.

“Don’t forget how fast the transition can take place once you have the political will to do it. So, when I was a kid, 80% of power generation came from coal. When I was Mayor of London(2008-16) it was 40%. Today, it’s less than 1%. What I’m saying to you is when the transition comes, when people are willing to make that transition, it can be remarkably swift.”

READ MORE: Boris Johnson on COP26: 'Countries can achieve extraordinary things'

The father-of-six spoke passionately about doing everything possible to avoid passing on a blighted planet to future generations. “Glasgow certainly is a moment of truth and people need to recognise the scale of the consequences if we fail,” he said. “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.

“We shouldn’t be so arrogant and conceited about our prosperity and happiness and discount theirs. That’s the challenge.”

The PM noted how 116 out of 190 countries have come up with detailed plans to reduce their CO2 emissions, but accepted “that’s not enough; the challenge is massive”. He added: “What they need to do and what we are encouraging them to do is to come forward with specific plans to do four things: phase out the use of coal; phase out the use of hydrocarbon internal combustion engine vehicles; plant millions of trees if not a trillion trees altogether and find the funding from the developed nations to support green technology in the developing world because otherwise this thing won’t happen; otherwise there will simply be no agreement at COP.

“The developing world will say: ‘why should we listen to you about this if you are not going to find the cash. It’s coal, cars, cash and trees’.”

READ MORE: Nicola Sturgeon - COP26 protesters 'must be heard loud and clear'

Yet all the numbers on climate change do not look good. This week, a UN report calculated CO2 emissions would only be cut by 7.5% by 2030 when what was needed was a reduction of 55% to limit global warming to a 1.5C rise. Analysis of sectors including finance, energy and agriculture showed not a single one was doing enough to meet the Paris accord target.

Asked if COP26 is already doomed to fail, Mr Johnson replied: “I have to tell you we will not be able to stop global warming. What we can do is restrict it and what we are going to try and do at COP is set some way-markers for the end of global warming. If you look at what the UK has done it’s clearly possible. That’s the message everybody needs to get.”

In the interview, the PM also -

*Expresses understanding for Vladimir Putin’s reasons for not attending COP26 in person, because of rising Covid cases in Russia but does not comment on the reasons behind President Xi Jinping of China’s absence. He announced yesterday he would attend via videolink.

*Defends the controversial Budget decision to cut Air Passenger Duty on short-haul flights within the UK, saying 96% of all CO2 emissions came from long-haul flights and arguing the cut will aid “Union connectivity”.

*Insists Nicola Sturgeon will be “fully deployed” during COP26 but again declines to reveal what her role will be, saying only: “Wait and see.”

*Urges climate protesters like Extinction Rebellion to demonstrate within the law, appealing to them not to disrupt other people from “going about their lawful business”.

*Thanks the people of Glasgow for hosting Britain’s biggest ever summit and says it will be a “great showcase” not only for the city but also for Scotland and the UK.

As the PM prepared to head off for Rome for a pre-COP meeting of the G20 – whose members account for 80% of CO2 emissions – his spokesman made clear: “COP26 needs to mark the beginning of the end of climate change.

“The next few days are a critical moment for world leaders to demonstrate they can show the climate ambition needed.”