As they made their pledges on Monday at the Climate Action Summit, though, they and others conceded it was not enough.

And even before they spoke, Sweden's Thunberg, 16, shamed them over and over for their inaction, accusing them: "How dare you?"

Sixty-six countries have promised to have more ambitious climate goals, and 30 swore to be carbon neutral by mid-century, said Chilean President Sebastian Pinera Echenique, who is hosting the next climate negotiations later this year.

Businesses and charities also got in on the act, at times even going bigger than major nations.

Microsoft founder Bill Gates announced that his foundation, along with The World Bank and some European governments, would provide $US790 million in financial help to 300 million of the world's small farmers adapt to climate change.

The Gates foundation pledged $US310 million of that.

"The world can still prevent the absolute worst effects of climate change by reducing greenhouse gas emissions and developing new technologies and sources of energy," Gates said.

"But the effects of rising temperatures are already under way."

As the day went on and the promises kept coming, the United States seemed out in the cold.

Before world leaders made their promises in three-minute speeches, Thunberg gave an emotional appeal in which she scolded the leaders with her repeated phrase, "How dare you."

"This is all wrong. I shouldn't be up here," said Thunberg, who began a lone protest outside the Swedish parliament more than a year ago that culminated in Friday's global climate strikes.

"I should be back in school on the other side of the ocean. Yet you have come to us young people for hope. How dare you. You have stolen my dreams and my childhood with your empty words.

"We are in the beginning of a mass extinction and yet all you can talk about is money," Thunberg said. "You are failing us."

Later, she and 15 other youth activists filed a formal complaint with an arm of the UN that protects children, saying that governments' lack of action on warming is violating their basic rights.

Outside experts say they heard a lot of talk on Monday, but not the promised action needed to keep warming to a few tenths of a degree. They say it won't produce the dramatic changes the world requires.

"Sometimes I feel that Greta is still out in front of the Swedish parliament out on her own," said Stanford University's Rob Jackson, who chairs the Global Carbon Project, which targets carbon emissions across the world.

Bill Hare, who follows national emissions and promises for Climate Action Tracker, called what was said "deeply disappointing" and not adding up to much.

"The ball they are moving forward is a ball of promises," said economist John Reilly, co-director of MIT's Joint Centre for Global Change.

"Where the 'ball' of actual accomplishments is, is another question."

Of all the countries that came up short, World Resources Institute Vice President Helen Mountford said one stood out: the United States for "not coming to the table and engaging".

"What we've seen so far is not the kind of climate leadership we need from the major economies," Mountford said, but added that businesses, as well as small- and medium-sized countries had "exciting initiatives".

Nations such as Finland and Germany promised to ban coal within a decade.

Several also mentioned goals of climate neutrality - when a country is not adding more heat-trapping carbon to the air than is being removed by plants and perhaps technology - by 2050.

US President Donald Trump dropped by the summit and left without saying anything.

The United States did not ask to speak at the summit, UN officials said. And Secretary-General Antonio Guterres had told countries they couldn't be on the agenda without making bold new proposals.

He opened the summit Monday by saying: "Earth is issuing a chilling cry: Stop."

"Time is running out," he said. "But it is not too late."

Australian Associated Press