Boris Johnson has likened the globe’s battle against the climate emergency to a football team losing 5-1 at half-time, as he flew to Rome for a world leaders’ gathering seen as crucial for setting the tone for next week’s Cop26 climate summit.Speaking to reporters on the flight to Italy for the G20 meeting, Johnson conceded that he had not always been convinced about climate change, and that his mind had been changed in part by a briefing given by government scientific advisers soon after he became prime minister.Johnson also summoned up the image of the Roman empire to argue that if sufficient progress was not made over the next fortnight at Cop in Glasgow, then the modern world could “also go backwards” in a dramatic way.“There is absolutely no question that this is a reality we must face,” Johnson said of climate change, saying he had pressed China’s president, Xi Jinping, about his country’s use of coal during a phone call earlier on Friday.“I would say that humanity as a whole is about 5-1 down at half-time,” he said. “We’ve got a long way to go, but we can do it. We have the ability to come back but it’s going to take a huge amount of effort.” He added: “Team World is up against a very formidable opponent in climate change.”Rome’s ancient monuments, Johnson argued, could be seen as “a memento mori to us”, demonstrating how quickly civilisations can decline.“Humanity, civilisation, society, can go backwards as well as forwards, and when things start to go wrong they can go wrong at extraordinary speed,” he said.“You saw that with the decline and fall of the Roman empire, and I’m afraid to say that it’s true today that unless we get this right in tackling climate change, we could see our civilisation, our world also go backwards.”An activist sings Abba’s Money, Money, Money flanked by cardboard cutouts of Boris Johnson and President Joe Biden in front of the Colosseum in Rome. Photograph: Luca Bruno/APSpeaking about his “road to Damascus” after a journalism career in which he regularly questioned climate science, Johnson said he had been briefed by government scientists early during his time inside No 10.He said: “I got them to run through it all, and if you look at the almost vertical kink upward in the temperature graph, the anthropogenic climate change, it’s very hard to dispute. That was a very important moment for me.”Asked if he was eating less meat as a way to limit his environmental footprint, Johnson said: “I’m eating a bit less of everything, which may be an environmentally friendly thing to do.”Johnson will have a series of meetings with other world leaders at the G20, which groups together leading industrialised nations, before flying on to Glasgow on Sunday.He reiterated the UK’s goal of keeping on track the goal, set in the Paris climate accord, of limiting warming to 1.5C above pre-industrial levels, saying that this Cop in Glasgow was part of a process.He said: “We’re not going to stop global warming either at this meeting in Rome or at Cop. The most we can hope to do is slow the increase, begin the end of the problem. What we need to do is take steps now that give us the ability in the future to come back and make further commitments.”People power would also play a role, he predicted. “The great force in all this is not governments, it’s the people. You’re seeing populations around the world starting to demand this, and that’s increasingly true everywhere.”