In a speech peppered with scientific facts about global warming, the Swedish 16-year-old told negotiators at the U.N.'s COP25 climate talks in Madrid that they have to stop looking for loopholes for their countries' actions and face up to the ambition that is needed to protect the world from a global warming disaster.
"The real danger is when politicians and CEOs are making it look like real action is happening, when in fact almost nothing is being done, apart from clever accounting and creative PR," Thunberg said.
"Finding holistic solutions is what the COP should be all about, but instead it seems to have turned into some kind of opportunity for countries to negotiate loopholes and to avoid raising their ambition," she added, to wide applause.
About 40 climate activists, including representatives of indigenous peoples from several continents, briefly joined Thunberg after her speech on the conference's main stage, holding hands and demanding "Climate Justice!" through slogans and songs.
Negotiators in Madrid had one eye on Brussels, where the European Union was set to announce an ambitious plan on cutting the greenhouse gas emissions that cause global warming.
German Environment Minister Svenja Schulze said she hoped the 'European Green Deal' would "give the discussions here a boost."
"It's a really important signal if the EU puts protecting the climate center stage in this way," she told reporters in Madrid. "If (the EU) says very clearly that we will raise our targets, that we want to do more."
The climate talks in Madrid, meanwhile, entered uncharted waters Wednesday with ministers trying to agree on rules for a global carbon market and possible ways to compensate vulnerable countries for disasters caused by global warming.
World leaders agreed in Paris four years ago to keep global warming below 2 degrees Celsius (3.6 degrees Fahrenheit), ideally no more than 1.5 C (2.7 F) by the end of the century. Scientists say countries will miss both of those goals by a wide margin unless drastic steps are taken to begin cutting greenhouse gas emissions next year.
Johan Rockstrom, director of the Postdam Institute of Climate Studies and one of the most revered scientists on the issue, said for 20 years "we have underestimated the pace of change and we have underestimated the risks we are facing."
Addressing the heads of delegations, activists and non-governmental organizations at the climate talks, Rockstrom said under the current scenario, the planet is heading to warming by 3 to 4 degrees in only 80 years. He said that could create an environment unseen in Earth for more than 4 million years and could trigger disastrous domino effects for human life.
"We stand on an unprecedented mountain of truth," he said. "If nature fails, we fail as well."
Following him, Thunberg cited the same reports, insisting that national pledges to reduce emissions weren't enough. She said to avoid disaster, carbon needs to remain underground and that the greenhouse gases responsible for rising temperatures need to be zeroed.
"This is not leading, this is misleading," she told the plenary, adding that "every fraction of a degree matters."