An ambitious agenda; complicated and nuanced subject matter; a U.S. president given to going off script; and 40 world leaders, including some with beefs against the United States, at an open mic. All on the equivalent of a giant Zoom call. What could go wrong?

As it happened, not much, despite the oddity of President Biden’s task: reestablishing the country as a reliable global leader on climate change at a marquee global summit conducted via pandemic-mandated video remote.

A few participants took the opportunity to criticize Washington as an inconsistent advocate for action. Others puffed up their nation’s past contributions or pleaded that their smaller, poorer nations should be held to a different standard.

But the overriding sentiment from the global leaders was one of relief and receptivity to having the United States back in the fold after four years of President Donald Trump. Biden collected plaudits for recommitting the United States to focus on the crisis of a warming planet and reversing the policies of his predecessor. Trump had called climate change a hoax and dismayed allies by withdrawing the United States from its pledges to work with its partners, chiefly the Paris climate accord.

President Biden on April 22, emphasized the economic benefits of fighting climate change during the first day of his administration's global climate summit. (The Washington Post)

“The signs are unmistakable. The science is undeniable. But the cost of inaction is — keeps mounting,” Biden said as he opened the two-day session. “The United States isn’t waiting.”

Biden committed the United States to cutting its greenhouse gas emissions as much as 52 percent by the end of this decade. Global carbon emissions are expected to surge this year as parts of the world begin to rebound from the coronavirus pandemic.

The event is a milestone for Biden as he makes climate change one of his main international priorities. The administration opted to hold the session early in Biden’s term, despite the restrictions forced by the pandemic, to showcase him as a leader on the issue.

That made for a weird debut.

There were no whispered ­tete-a-tetes off to one side, no chance encounters in the hallway, no awkward “family photo” of the assembled leaders. Biden, a glad-handing pol who relished representing the United States on the global stage during his time as vice president, was left fidgeting at his seat, placed at a socially distant remove from climate envoy John F. Kerry and Secretary of State Antony Blinken.

The slick production value of videos produced for the session, showing renewable energy operations and gorgeous natural vistas, was not always matched by world leaders Zooming in from sometimes badly lighted offices around the world.

Joining live from far-flung time zones, they sometimes missed their cues or forgot about the mute button. There was echoey audio, buzzy audio, no audio. Russian President Vladimir Putin sat in awkward silence for nearly two minutes, unable to hear as Blinken tried several times to introduce him.

When the technology was smooth, sometimes leaders’ rhetoric was not.

British Prime Minister Boris Johnson got tangled up as he made the point that combating climate change can offer economic opportunities.

“It’s vital for all of us to show that this is not all about some expensive, politically correct agreement of bunny-hugging,” he said.

Mexican President Andrés Manuel López Obrador floated a novel trade — U.S. work visas for those who help with a reforestation program he hopes to expand to Central America. Biden had no immediate response to the trees-for-visas idea.

Biden’s most-watched guests, sometime U.S. adversaries Putin and Chinese President Xi Jinping, kept it classy with a call for global cooperation in pursuit of carbon reductions, although only Putin came with new commitments.

“I’m confident that despite Russia’s size, its geography, climate and economic structure, this task is achievable,” Putin said of a pledge, announced earlier this week in Moscow, that Russia would significantly reduce its greenhouse gas emissions by 2050.

Putin did not mention recent bad blood with Biden over U.S. sanctions and accusations that Russia had menaced neighbor Ukraine and is mistreating jailed dissident Alexei Navalny.

But he did get in what appeared to be a dig at the United States for its past role as the world’s top greenhouse gas polluter.

“It is no secret that the conditions that facilitated global warming and associated problems go way back,” Putin said.

Canada, Japan and other nations made specific new commitments to carbon reduction, as Biden had hoped.

Brazilian President Jair Bolsonaro, a populist and opponent of coronavirus restrictions who had styled himself as the “Trump of the Tropics,” thanked Biden for his efforts. Bolsonaro also made new commitments to counter Amazon deforestation that were met with skepticism by climate advocates in Brazil and elsewhere but welcomed by Kerry.

German Chancellor Angela Merkel, who had a testy relationship with Trump, didn’t disguise her pleasure at the turn in U.S. politics.

“I’m delighted to see that the United States is back to work together with us in climate politics, because there can be no doubt about the world needing your contribution,” she said.

Italian Prime Minister Mario Draghi echoed Merkel with a warm welcome for Biden.

“It’s a complete change. Now, we are confident that together we will win this challenge,” Draghi said.

European Commission President Ursula von der Leyen grinned as she addressed Blinken and Kerry.

“It is so good to have the U.S. back on our side in the fight against climate change,” she said.

The first day was not all rose petals for Biden, however.

An impassioned climate activist lectured everyone, her U.S. hosts included, for ignoring the consequences of official actions to protect fossil-fuel industries and build pipelines and “extractive infrastructure.”

Greenhouse gas emissions should be eliminated by 2030, not just slashed as Biden is proposing, Xiye Bastida told the session.

“You are the naive ones if you think we can survive this crisis with the current way of living,” said Bastida, 18.

She added that decision-makers should “stop thanking” young activists and do something instead.

Blinken, looking slightly startled, thanked her nonetheless.