Teenage climate activist Greta Thunberg laid into world leaders on Monday, and environmental groups slammed a "missed opportunity," as concrete new plans to combat the worsening global climate crisis failed to materialize at the UN Climate Action Summit in New York.

"We are at the beginning of a mass extinction, and all you can talk about is the money and fairy tales of eternal economic growth. How dare you," a visibly emotional Thunberg said, her voice breaking.

"This is all wrong. I shouldn't be up here. I should be back in school on the other side of the ocean," added the 16-year-old, who has become an icon for climate action since beginning a solo protest outside Sweden's parliament in August 2018.

"You have stolen my dreams and my childhood with your empty words... You are failing us. But young people are starting to understand your betrayal," she told leaders from around 60 countries in attendance. 

Read more: What to expect from the UN Climate Action Summit?

UN chief Antonio Guterres had called the summit to press for renewed action as a new report on Monday warned that countries would need to triple their current efforts to prevent temperatures from rising more than 2.0 degrees Celsius from pre-industrial levels, and multiply them by five if they want to cap global warming at 1.5 degrees.

Despite commitments to cut them, global carbon emissions reached an all-time high last year. And experts say most countries are exceeding the pledges they made when they signed the 2016 Paris Agreement.

Ahead of the summit, Guterres said he expected "the announcement and unveiling of a number of meaningful plans on reducing emissions in the next decade and achieving carbon neutrality by 2050."

Who said what?

Taking to the podium on Monday, Indian Prime Minister Narendra Modi said his country was working on more than doubling its renewable energy capacity.

Chinese State Councilor and Foreign Minister Wang Yi called for "sustained actions" and collaborative efforts "to jointly tackle this challenge and to protect the planet we all call home," while making a veiled dig at the U.S. for its decision to withdraw from the Paris Agreement.

"The withdrawal of certain parties will not shake the collective will of the international community," Wang said.

French President Emmanuel Macron reiterated a vow to close coal-fired plants by 2022. British Prime Minister Boris Johnson announced the UK would double its climate change funding. And German Chancellor Angela Merkel repeated a previous pledge to achieve net zero emissions by 2050.

But there were few signs of the "concrete, realistic plans" Guterres had called for earlier.

Major countries like the U.S., Brazil and Australia were also largely absent from the summit.

In his closing remarks, Guterres noted that 77 countries had committed themselves to net-zero carbon emissions by 2050, and 70 vowed to boost their Paris pledges by 2020.

He also hailed businesses, development banks, and international investors that have pledged to phase out coal and move to carbon-neutral investment portfolios by 2050.

Still, "we have a long way to go, we are not yet there," he warned. "Much more is still needed… We need more concrete plans, more ambition, from more countries, and more businesses."

A disappointing result

Environmental groups were unimpressed.

"Some important transformational announcements were made.... But it is disappointing that the heavy polluting countries haven't done more," the WWF said in a statement.

"Clearly… we are yet to see the scale of change and commitment needed to address (climate action)," Manuel Pulgar-Vidal, head of WWF's global climate and energy practice, said.

The humanitarian agency Care International had even harsher words.

"Today's United Nations Climate Action Summit was a missed opportunity for world leaders to mount a response commensurate with what scientists, cities, organizations and more than four million people worldwide are declaring a climate emergency," it deplored.

While it welcomed "strong commitments" from a handful of less developed countries, it noted that "countries absent and repackaging old commitments as new did not go unnoticed."

Young activists file complaint

Young people have spearheaded global climate protests – last Friday, millions took to the streets in about 150 countries to make their voices heard – and on Monday, Thunberg and a dozen other young activists filed a complaint under the 1989 Convention of the Rights of the Child, claiming their rights were violated by world leaders' lack of action in combating the climate emergency.

The group, aged eight to 17, from 12 different countries and backed by a legal firm and an environmental group, presented their complaint to the United Nations Children's Fund (UNICEF) headquarters and shared stories about how climate change has already impacted their lives.

"We will not permit them to take our future away," said one of the plaintiffs, 12-year-old Catarina Lorenzo, from Brazil. "They had the right to have their future; why don't we have the right to have our own?"

(With input from AFP)