NEW YORK – Grim faced, youth activist Greta Thunberg on Monday delivered harsh words to the world's leaders at the start of the U.N. Climate Action Summit.

"This is all wrong. I shouldn't be up here, I should be back in school on the other side of the ocean," she chided.

She spoke moments after United Nations Secretary General Antonio Guterres delivered an impassioned plea to the world to rein in emissions at an urgently convened Climate Action Summit. But the world's largest polluters either stayed on the sidelines or gave general promises to stay on track to meet the goals of the Paris treaty agreement.

Thunberg, 16, her voice cracking with emotion, told the world leaders they were failing. "People are suffering, people are dying, entire ecosystems are collapsing ... and all you talk about is money and eternal fairy tales of economic growth. How dare you?

"For more than 30 years the science has been crystal clear. How dare you continue to look away?"

Leaders from more than 100 nations attended the summit, meant to ramp up plans for the next stage of deep cuts to greenhouse gas emissions promised by countries as part of the 2015 Paris Agreement on Climate Change.

The summit featured speeches from Pope Francis (by video), French President Emmanuel Macron, German Chancellor Angela Merkel as well as leaders of Fiji, the Marshall Islands and other nations beset by climate change. But it was Thunberg's speech that galvanized the crowd, which responded with thunderous applause.

She noted the world's industries are rapidly burning through the budget for emissions that scientists say must be followed to avoid dire consequences, leaving only a 50% chance under the accord to stop devastating climate chain reactions.

And calculations to avoid catastrophe "rely on my generation sucking hundreds of billions of tons of your CO2 out of the air with technologies that barely exist," she said. "So a 50% risk is simply not acceptable to us – we who have to live with the consequences."

Adults lack the maturity to tell young people how it is, Thunberg added.

"You are failing us, but young people are starting to understand your betrayal," she said. "The eyes of all future generations are on you, and I say if you do not act, we will never forgive you. Right here, right now is where we draw the line, and change is coming, whether you like it or not."

A possible apocalyptic future

Guterres opened the summit by saying, "Nature is striking back with fury."

Describing his visits to the Bahamas, Mozambique and Pacific island nations devastated by fierce hurricanes or back-to-back cyclones, he said, "The destruction was not simply appalling, it was apocalyptic. We are seeing the future if we do not act now."

He said he was hopeful nonetheless. More than 100 cities, scores of financial investors and more than 60 nations pledged to ramp up or begin concrete actions to transition from fossil fuels and achieve carbon neutrality by 2050.

Perhaps anticipating Thunberg's remarks, he acknowledged, "young people are here above all demanding accountability. They are right. My generation has failed in its responsibility to protect our planet. That must change. Climate change is caused by us and the solutions must come from us."

He said "tools and technologies are on our side," and there are "readily available substitutions for more than 70% of all emissions. We have the Paris agreement ... and we have imperative, irrefutable science."

Before Thunberg's talk, Pope Francis spoke by video to the summit about the "honesty, responsibility and courage" needed to combat climate change, and the "very weak indeed" commitments to reduce greenhouse gas emissions. He questioned whether there is enough political will to save the Earth.

President Donald Trump, who wants to pull the United States out of the ratified 2015 Paris Agreement to cut the greenhouse gas emissions that cause global warming, did not speak at the meeting though he dropped in briefly. He has said he doesn't believe human activity is the cause of climate change.

Disappointment with large nations

In a process at times similar to an old-fashioned U.S. political convention, leaders of dozens of nations including Fiji, Nigeria and Nepal rose one by one to say they would commit to carbon neutrality by 2050 or sooner, and to slash emissions further before then. Many pledged to plant millions of trees.

But China – the world's largest emitter of greenhouse gasses – merely said it would stay the course laid out in the Paris agreement.

India's prime minister did promise to more than double the nation's clean energy output but gave no deadline or schedule.

More than 60 small and middle-size countries, many of them minuscule polluters who are facing the greatest threats from climate change, vowed to step up their efforts.

Several large banks, pension and insurance fund leaders were on hand, promising to shift trillions of dollars in financing for electricity and other infrastructure from polluting fossil fuels to "clean" and climate-resilient investments.

The lack of substantial commitments from larger nations was a disappointment, said climate analysts.

"Across the board, the large countries did not make clear what their intent is for next year," said David Waskow, international climate director with the World Resources Institute. "We've seen some hopeful signs, but next year is really the test of what these large countries are going to do."

He said he was heartened that at least 65 countries and more than 100 cities had pledged to increase their efforts in tangible ways, and said youth mobilization was playing a critical role.

He was glad China agreed to stay the course, but had hoped for more.

"They certainly didn't rule out reducing greenhouse gas emissions more, but they also didn’t say they would."

He'd worried China would falter in its overall commitment after Trump's past declarations that he would petition to withdraw the United States from the Paris agreement as soon as he can legally do it this November. If successful, that withdrawal would take effect on Nov. 4, 2020, the day after the next presidential election.

Monday's U.N. summit comes after a commitment Sunday by 130 banks holding $47 trillion in assets to climate action and sustainability, U.N. environment officials said. However, eight of the world's 10 largest banks did not sign the agreement.

On Friday, millions of young people took part in the worldwide Youth Climate Strike, demanding faster action to address the climate crisis.

Emissions continue to rise

Science continues to show the need for rapid movement toward zero-carbon emission – a mark far from being met.

A U.N. science report released Sunday shows emissions into the Earth's atmosphere continue to surge. A study released in July found that compared with the past 2,000 years, the warming seen in the past 100 years is unprecedented.

G-20 nations, the world’s largest economies, are responsible for 80% of emissions but are failing to meet already low targets. Globally, countries must triple their 2015 pledges to keep global warming well below 2 degrees Celsius (3.6 F) by 2100.

To keep temperatures from rising above the 1.5 C, a threshold seen as critical to the survival of small, low-lying countries, these nations would need to reduce emissions not just three times as much as their current pledge but five times.

Without significant action, fiercer hurricanes, heat waves, droughts, floods and related food scarcity and health impacts are predicted.