Climate activist Greta Thunberg is making headlines again. On Monday, she accused world leaders of failing her generation and did so with a charismatic speech at the United Nations Climate Action Summit.

Thunberg emphasized that the science behind climate change has been “crystal clear” for more than 30 years, and yet the most powerful people on Earth had failed to take meaningful action. “You are still not mature enough to tell it like it is,” she said. “You are failing us. But the young people are starting to understand your betrayal.”

Image credits: United Nations

Donald Trump appears to have noticed it, tweeting a video of an emotional Thunberg with a sarcastic comment: “She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!”

Thunberg, who started the global School Strike movement with her weekly Friday school strikes in Sweden, has been in New York for the climate summit and briefly crossed paths with Donald Trump at the as he arrived to a meeting on religious freedom.

When Thunberg came to New York, she said she had little hope she would be able to convince the president to take action on the climate emergency: “I say ‘listen to the science’ and he obviously does not do that. If no-one has been able to convince him about the climate crisis and the urgency, why would I be able to?” the activist said.