It comes as mankind is releasing more emissions into the atmosphere than at any point in history, triggering global weather hazards from heat waves to intense hurricanes to raging wildfires and rapidly acidifying oceans. Yet the gap between carbon reduction targets demanded by scientists to avert catastrophe and actions thus far taken is only widening.
"I shouldn't be up here. I should be back at school on the other side of the ocean," said Thunberg, 16, who has become the global face of a growing youth movement against climate inaction that mobilized millions in a worldwide strike on Friday.
"You come to us young people for hope. How dare you?" she thundered, her voice at times breaking with emotion.
"We are in 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!"
In a surprising turn of events, President Donald Trump made a brief unscheduled appearance Monday at the UN climate summit, which he had been expected to skip entirely.
Trump, who has repeatedly expressed doubt about the overwhelming scientific consensus on manmade causes of global warming, spent a few minutes in the hall of the General Assembly where he applauded Indian Prime Minister Narendra Modi's speech then left.
Earlier, opening the summit, Secretary-General Antonio Guterres said: "The climate emergency is a race we are losing, but it is a race we can win." French President Emmanuel Macron invited his counterparts from Chile, Colombia and Bolivia to a meeting where $500 million in extra funds were pledged by the World Bank, Inter-American Development Bank, and non-profit Conservation International to protect the world's rainforests.
Fewer than half the 136 heads of government or state in New York this week to attend the UN General Assembly will be present Monday.