“Humanity as a whole, at half time is about 5-1 down,” he said.
“We have got a long way to go but we can do it.
“We have the ability to equalise, to save the position, to come back but it will take a huge amount of effort.”
In a historical explanation of how global warming could damage civilisations, he highlighted the rise and fall of the Roman Empire.
He stressed: “We could consign our children, our grandchildren, our great grandchildren to a life in which there are not only huge movements of populations and huge migrations, but also shortages of food, shortages of water, of conflict caused by climate change and there is absolutely no question that this is a reality that we must face.”
Mr Johnson spoke to China’s President Xi Jinping today as he sought to persuade world leaders to ensure their countries’ carbon emissions peak earlier and then seek to get to net zero by 2050.
“I made a couple of other points. First of all the moment for peaking, and whether they’re going to get to that. He said 2030. I pushed a bit on that 2025 would be better than 2030. I wouldn’t say he committed on that,” explained the Prime Minister.
“The other point where I was evangelical was the potential to move away from coal. He said China depends on it for our domestic economy. I said so many times from the UK moving from 80 percent dependency on coal for electricity to one percent today.
“I said when I came to see you in Beijing in 2008 we were 40 percent dependent on coal and it shows how fast you can make the transition.”
Mr Johnson visited Beijing as Mayor of London 13 years ago.
China, the world’s biggest polluter by volume rather than head of population, has committed to net zero by 2060.
Mr Xi stressed to the PM how China’s domestic economy is dependent on coal.
Mr Johnson is seeking to rally world leaders to unite to “keep alive” the goal of limiting global temperature rises to 1.5 degrees C above pre-industrial levels.
He added: “America has got a massive role to play as well.
“Joe Biden totally buys into this agenda and I thought that what they did at UNGA was very promising.
“We won’t get anywhere unless the developing world feels that the West is making the commitments in cash that they need to see.
“That American commitment of £11 billion dollars was very important.”