Joe Biden has warned that the climate crisis poses “the existential threat to human existence as we know it” and urged other world leaders to embark upon a transformational shift to clean energy, as questions linger over the US president’s ability to deliver this vision at home.

Biden, addressing a sparse chamber at crucial UN climate talks that have begun in a frigid and drizzly Glasgow, said that the conference must act as a “kickoff of a decade of ambition and innovation to preserve our shared future”.

The president added: “We meet with the eyes of history upon us. Will we do what is necessary? Or will we condemn future generations to suffer?”

Biden’s administration is attempting to reassert America’s credibility at the gathering of nearly 200 countries in Scotland, known as Cop26, after Donald Trump’s decision to pull the US from the Paris climate agreement and his dismissal of climate science. Scientists have warned the world is badly off track to avoid disastrous climate change, with leaders of poorer, vulnerable countries using the talks to warn their populations face looming cataclysm.

“We will demonstrate to the world the United States is not only back at the table but hopefully leading by the power of our example,” Biden said in his speech, in a tacit acknowledgement of Trump. “I know it hasn’t been the case, which is why my administration is working overtime to show our climate commitment is action not words.”

“Right now, we are falling short, there’s no time to hang back, sit on the fence or argue amongst ourselves,” the president continued. “This is the challenge of our collective lifetimes, an existential threat to human existence as we know it and every day we delay the cost of inaction increases.”

Biden said that wealthy, major polluters such as the US have an “overwhelming responsibility” to aid smaller countries that are struggling to cope with growing floods, fires and heatwaves spurred by global heating.

Before arriving in Glasgow, Biden also took aim at some other leading emitters for not doing enough to prevent global heating surpassing 1.5C. He said these countries are “not only Russia but China (which) basically didn’t show up in terms of commitments to deal with climate change. I found it disappointing myself”.

At a side event, Biden also effectively apologized for his predecessor. “I guess I shouldn’t apologize, but I do apologize for the fact that the United States – the last administration – pulled out of the Paris accords and put us sort of behind the eight ball,” Biden said.

But climate activists, many of whom gathered outside the Glasgow venue that hosted more than 120 world leaders on Monday, argue that Biden is failing to live up to his own words. The president touted vast proposed climate legislation that would be the “most significant investment to deal with the climate crisis that any advanced nation has made, ever,” but the bill remains stalled in Congress, after being winnowed away by a senator who has extensive ties to fossil fuels.

“Biden is at Glasgow empty handed, with nothing but words on paper,” said Varshini Prakash, executive director of Sunrise Movement. “It is humiliating and fails to meet the moment that we’re in.”

Biden has also been attacked over his administration’s reluctance to drastically scale back oil and gas drilling in the US. The president’s narrative of “climate leadership” contradicts the daily suffering by communities on the frontline of gas and oil production in the US, activists say. In the first six months of the Biden administration, about 2,500 new oil and gas permits were authorized – a figure Trump’s administration took a year to reach.

Speakers outside Cop26 on Monday – only 23 civil society observers were allowed in to hear the leaders’ speeches – included Black and Indigenous leaders whose communities are on the frontline of fossil fuel extraction impacts, including air pollution and contaminated drinking water and land across the US.

Tom Goldtooth, Native American leader from the Indigenous Environmental Network, said: “We’re here as the original people of the US to denounce the polluters conference – it’s not a climate conference – it’s been taken over by corporate interests. If we Indigenous people don’t come we’ll be on the menu. We’re here to defend our people, we want to live.”

Biden’s speech came shortly after an official opening of Cop26 that acknowledged the growing anguish over the escalating, and largely unchecked, climate crisis. Boris Johnson, the British prime minister, said “the people who will judge us are children not yet born”, adding “if we fail they will not forgive us”. Antonio Guterres, the secretary general of the UN, warned that “we are digging our own graves” due to the failure to dramatically cut planet-heating emissions.