A visibly frustrated prime minister admitted the Rome gathering had fallen short of what was required to put the Cop26 summit on course for success in Glasgow.
Promises made to tackle the climate crisis are “starting to sound hollow”, Mr Johnson told a press conference – when the “solution is clear”.
And he agreed a pledge for all the biggest economies to achieve net zero emissions was “vague”, after the G20 failed to set a target date of 2050.
The commitments made at the G20 were “drops in a rapidly warming ocean when you consider the challenge we’ve all admitted is ahead of us”, the prime minister warned.
“We have made reasonable progress at the G20, all things considered – but it is not enough,” Mr Johnson admitted.
For the first time, he named-and-shamed the US for not contributing enough money to the $100bn climate crisis fund for poor nations, saying it was “well down”.
And on the prospects for Cop26, the prime minister said: “If Glasgow fails then the whole thing fails. The Paris Agreement will have crumpled at the first reckoning.”
It would be “holed beneath the waterline”, he warned, calling it “just a piece of paper” and adding: “We need to fill that piece of paper – to populate it with real progress.”
Mr Johnson was speaking after G20 leaders agreed merely to “enhance when necessary” plans to cut carbon emissions by 2030 – the cut-off point for averting disaster, scientists say.
Far from agreeing to “consign coal to history” – the UK’s aim for the Glasgow summit – it sets no date for phasing out the fossil fuel, which will happen only “as soon as possible”.
There is also no deadline for ending fossil fuel subsidies, which would have soared since the easing of the Covid pandemic, with an aim to achieve that only “over the medium term”.
In the run-up to Cop26, the UK had urged all countries to follow it in committing to net zero emissions by 2050 – but the communique also fails to do that.
The omission reflects the reality that China, the world’s biggest carbon emitter, has set a target date of 2060, while India and Russia are also not committed to the 2050 date.
Greenpeace attacked the lack of progress, warning that “if the G20 was a dress rehearsal for Cop26, then world leaders fluffed their lines”.
“Their communique was weak, lacking both ambition and vision, and simply failed to meet the moment,” said Jennifer Morgan, the organisation’s executive director.
Mohamed Adow, director of the think tank Power Shift Africa, said: “This weak statement from the G20 is what happens when developing countries who are bearing the full force of the climate crisis are shut out of the room.
“The world’s biggest economies comprehensively failed to put climate change on the top of the agenda ahead of Cop26 in Glasgow.”
A downbeat Antonio Guterres, the UN secretary-general, tweeted: “I leave Rome with my hopes unfulfilled – but at least they are not buried.”
Mr Johnson said: “Just 12 G20 members have committed to reach net zero by 2050 or earlier.
“Barely half of us have submitted improved plans for how we will cut carbon emissions since the Paris Summit in 2015.
“And we have also failed to meet our commitment to provide $100bn a year to support developing countries to grow in a clean and sustainable way.
“The UN says emissions will rise by 15 per cent by 2030 – and they need to halve by then.”
Accusing the world’s first industrialised nations of “not doing their fair share of the work”, Mr Johnson added: “If we are going to prevent Cop26 from being a failure then that must change.”