After traveling to the U.S. via an emissions-free yacht and leading Friday's global climate strike from New York City, 16-year-old Swedish climate activist Greta Thunberg addressed the United Nations General Assembly Monday morning.

Thunberg told leaders, "We'll be watching you," during her emotional plea for action.

"I shouldn't be up here, I should be back in school on the other side of the ocean. Yet you all come to us young people for hope. How dare you," she said. "You have stolen my dreams and my childhood with your empty words, yet I'm one of the lucky ones. People are suffering, people are dying. Entire ecosystems are collapsing."

Thunberg noted the science has been clear for 30 years, yet the politics and solutions needed are nowhere in sight. The numbers are uncomfortable, she says, and leaders are "still not mature enough to tell it like it is."

"You are failing us," she said. "But the young people are starting to understand your betrayal. The eyes of all future generations are upon you, and if you choose to fail us, I say we will never forgive you.

She told leaders they say they understand the urgency, but she doesn't want to believe them. "If you really understood the situation and still kept on failing to act, then you would be evil," she said, "and that I refuse to believe."

Watch part of her speech below, via CBS News. Taylor Watson