Swedish teenage climate activist Greta Thunberg told world leaders at the opening of a United Nations conference in New York City that they had stolen her childhood with “empty words.”

“You have stolen my dreams and my childhood with your empty words,” Thunberg said at a U.N. climate change summit, admonishing them for not doing enough to protect the environment.

Thunberg, who traveled from Europe to New York for the summit on a zero-emissions sailboat, said she should be in school in her native Sweden rather than at the UN telling world leaders what to do to address climate change.

Listen to the science, she told them, which has been “crystal clear” for 30 years. She admonished them for leaving her generation with the task of sucking billions of tons of carbon dioxide out of the air “with technologies that barely exist.”

“This is all wrong, I shouldn’t be up here,” said Thunberg, who spoke along with UN Secretary-General Antônio Guterres, India Prime Minister Narendra Modi and others. “The eyes of all future generations are on you,” she said.

Leaders from around the world, including French President Emmanuel Macron and German Chancellor Angela Merkel, have gathered in New York this week to make new pledges to curb global-warming emissions. US President Donald Trump, who wasn’t originally scheduled to attend, made a surprise appearance at the summit on Monday.

President Trump took to Twitter, citing the first half of that statement then saying Thunberg “seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!”

Guterres is calling on countries to step up commitments to reduce their dependence on fossil fuels. “We face at least three degrees Celsius of global heating by the end of the century,” he said. “I will not be there, but my granddaughters will. And your grandchildren, too. I refuse to be an accomplice in the destruction of their home and only home.”

French President Emmanuel Macron acknowledged in his speech after Thunberg’s at the summit the frustration she and other activists are expressing. “I was very struck by the emotion in the room,” Macron said. “I think they’ve identified an absolute urgency that we have to respond to here.”

Thunberg recently celebrated the one-year anniversary of the start of her climate change movement. Last August, she began striking by herself outside the Swedish parliament, and soon, students around the world began walking out of school, demanding action from their governments. She’s been called “the voice of the planet,” and has even been nominated for the Nobel Peace Prize.