The world still has a chance of staving off the worst impacts of climate breakdown but only if governments step up in the next few months with stronger commitments on cutting greenhouse gas emissions, the US envoy for climate change has said.

John Kerry, appointed by Joe Biden to spearhead the US’s international efforts to tackle the crisis, urged all large economies to come forward with new plans to cut emissions before the Cop26 UN climate talks in Glasgow this November.

“The climate crisis is the test of our own times and, while it may be unfolding in slow motion to some, this test is as acute and as existential as any previous one. Time is running out,” he said.

He called Cop26 “a pivotal moment” and 2021 “a decisive year”, as the world must get to grips with the climate crisis and rapidly slash emissions in the 2020s to have a chance of a safe future.

Speaking as floods have devastated parts of Europe and heatwaves and wildfires swept North America, Kerry drew a parallel between the ruins of Europe after the second world war and the ravages of the climate crisis.

“The world order that exists today didn’t just emerge on a whim. It was built by leaders and nations determined to makes sure that never – never – again would we come so close to the edge of the abyss,” he said.

Kerry said his earliest memory, aged four, was of the ruined skeleton of a burned-out building in Europe, where he had been taken by his mother, who fled the Nazis. “That journey has always given me the bedrock confidence that we can solve humanity’s biggest threats together.”

Staying within 1.5C of global heating, the aspirational goal of the 2015 Paris climate agreement, was still possible, he insisted.

“There is still time to put a safer 1.5C future back within reach. But only if every major economy commits to meaningful absolute reductions in emissions by 2030. That is the only way to put the world on a credible track to global net zero by mid-century,” he said.

The Paris agreement targets an upper limit of holding global temperature rises to well below 2C above pre-industrial levels, with an aspirational lower limit of 1.5C.

Kerry made it clear that the Cop26 summit must aim for the lower threshold, and warned that current government pledges on emission cuts would lead to 2.5C or 3C rises.

“We’re already seeing dramatic consequences with 1.2C of warming. To contemplate doubling that is to invite catastrophe,” he said.

Kerry used his landmark speech at the Royal Botanic Gardens, Kew, in London, with just over 100 days to go before Cop26, to make an impassioned plea for a unified global effort. “We can’t afford a world so divided in its response to climate change when the evidence for compelling action is so strong.”

He singled out China, the world’s biggest emitter and second largest economy, which has yet to submit to the UN a national plan for emissions cuts before 2030. “It’s imperative that we and China, and the rest of the world, are pulling in the same direction on this critical effort,” he said.

Kerry told the Guardian in an interview after his speech that he was hopeful China would realise the need to act quickly. “When China has set targets before, it has outperformed them, so that is very hopeful,” he said.

But he made it clear he also had other countries in his sights. He said the US was working with “allies, partners, competitors and even adversaries all too aware that some things happening today threaten to erase the very progress so many are struggling to advance”.

UN climate summits proceed by consensus so recalcitrant counties can thwart agreement. For Cop26 to be a success, countries such as Russia, Brazil and Saudi Arabia will need to acquiesce at least – Kerry’s remarks will be seen as warning them not to disrupt the process.

Climate experts and campaigners told the Guardian the US was still lagging behind in providing finance for poor countries, to help them cut emissions and cope with the impacts of extreme weather. Rich countries promised a decade ago to provide at least $100bn a year in climate finance by 2020, a pledge that has not been met.

“The US is not pulling its weight – it’s the only country holding up the $100bn pledge,” said John Sauven, executive director of Greenpeace UK. “If the US does not put its hands in its pockets and make up the shortfall, Glasgow will be in jeopardy.”

Kerry told the Guardian in response that the Biden administration was “working hard” on finding more financial assistance for poor countries. “It’s very important that the US should provide finance. Our internal process on this is not complete yet.”

He added: “We are very conscious of the sensitivities around this. The US obviously plays a key role, and our absence in the last four years [from climate action] heightens that sense of responsibility and the imperative to find a way.”

Ed Miliband, the shadow business secretary, who was at the speech – which no government ministers attended – said Kerry had shown the US was determined to lead the way on climate action. “He made it clear he is focused on 1.5C – and he’s absolutely right, that’s ambitious but essential,” he said.

Kerry also called on governments to invest in clean energy, holding out the prospect of a clean energy boom worth $4tn a year by 2030, and said new technologies such as hydrogen and carbon capture and storage would also be needed.