New York (CNN) The United Nations gathered world leaders to present concrete plans to tackle the climate crisis Monday, making made clear it didn't want to hear platitudes, as scientists, activists and religious leaders warned that urgent steps are needed to slow rising temperatures that intensify devastating natural disasters.

President Donald Trump paid a flying visit but said nothing.

"If we don't urgently change our ways of life, we jeopardize life itself," Secretary-General António Guterres said, while youth leader Greta Thunberg excoriated leaders.

"People are suffering. People are dying. Entire ecosystems are collapsing," she said, days after cities worldwide were galvanized by climate protestors. "We are in the beginning of a mass extinction and all you can talk is the money and fairy tales of eternal economic growth."

Absent from the conversation: the United States, one of the world's largest emitters of greenhouse gases.

'Very important'

Trump appeared at the Climate Action Summit in New York, in an impromptu stop on his way to his administration's priority event on religious freedom. But the US did not speak at the event and the President, who has repeatedly said he thinks climate change is a hoax, left after 15 minutes.

Trump pulled the US out of the 2015 Paris Climate Agreement, which has been ratified by at least 186 nations and calls for action to stop global temperatures from rising more than 2 degrees. His administration has systematically moved to roll back US regulations meant to protect water and air quality, and limit greenhouse gas emissions.

Asked why he'd put in an appearance at the summit, the US President told reporters that he is "a big believer in clean air and clean water, and all countries should get together and do that, and they should do it for themselves. Very, very important."

Guterres has made climate change a signature issue and told leaders attending the 74th United Nations General Assembly to "come with concrete plans" and not just "beautiful speeches."

The UN leader drew a concrete line, telling leaders they would not get time at the podium unless they came ready to present concrete, realistic ways their countries could help lower emissions by 2020 as part of the goal of reducing greenhouse gas emissions by 45% over the next decade and getting to net zero emissions by 2050.

'Stop'

In his opening remarks, Guterres told the audience to look no further than the last few months for the impact the changing climate was having on the world.

June through August, he noted, marked "the hottest summer in the Northern Hemisphere ever; and the second hottest winter in the Southern Hemisphere ever. The years 2015 to 2019 — the five hottest years on the books ever. Our warming earth is issuing a chilling cry: Stop."

The UN released a World Meteorological Organization report to mark the Monday summit showing that a slew of worrying indicators have been accelerating, including sea-level rise, warming global temperatures, shrinkage of ice sheets and carbon pollution.

The report said average global temperatures from 2015 through to the end of 2019 are on pace to be the "warmest of any equivalent period on record" and said they are at 1.1 degrees above pre-industrial levels.

Countries and cities came ready with plans. German Chancellor Angela Merkel announced her country would double its climate protection funding. "Germany sees its responsibility on the international stage and on the national stage," she said.

Chile's President Sebastián Echeñique and Montreal Mayor Valerie Plante announced their respective plans to achieve carbon neutrality by 2050 or sooner. Costa Rica, Indonesia and China spoke about the economic and business case for shifting to renewable energy and the health case for phasing out coal.

A question of political will

Maine Gov. Janet Mills and Kenyan President Uhuru Kenyatta spoke about their plans for transitioning to a decarbonized environment and sustainable transit systems. Others spoke about climate finance, small island developing states and other issues.

Pope Francis called international efforts on climate change "weak" to date and asked whether leaders had the necessary political will to bring about change. "It is necessary to ask whether there is a real political will to allocate greater human, financial and technological resources to mitigate the negative effects of climate change and to help the poorest and most vulnerable populations, who suffer most," he said.

Thunberg was less polite.

"How dare you," she said in her remarks at the summit. "You are failing us, but the young people are starting to understand your betrayal. ... We will not let you get away with this. Right here, right now, is where we draw the line."