Environmental activist Greta Thunberg addressed world leaders at the UN Climate Action Summit on Monday to highlight the impact of climate change and stress the urgency of action.

"People are suffering; people are dying; entire ecosystems are collapsing," Thunberg said. "We are in the beginning of a mass extinction and all you can talk about is money and fairytales of eternal economic growth."

Later that day, President Donald Trump tweeted that Thunberg seems like a "very happy girl looking forward to a bright and wonderful future" after she delivered her tearful speech.

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President Donald Trump said on Twitter that environmental activist Greta Thunberg seems like a "very happy girl looking forward to a bright and wonderful future" the same day that she delivered an impassioned speech about the impact of climate change.

"You have stolen my dreams and my childhood with your empty words," Thunberg said on Monday at the United Nations Climate Action Summit. "Yet, I'm one of the lucky ones."

Thunberg addressed world leaders at the summit to highlight the urgency of acting now.

"The popular idea of cutting our emissions in half in 10 years only gives us a 50% chance of staying below 1.5 degrees and the risk of setting off irreversible chain reactions beyond human control," she said.

"So a 50% risk is simply not acceptable to us, we who have to live with the consequences," she continued.

Read more: Greta Thunberg addressed world leaders through tears: 'How dare you! You have stolen my dreams and my childhood with your empty words.'

"People are suffering; people are dying; entire ecosystems are collapsing," Thunberg said. "We are in the beginning of a mass extinction and all you can talk about is money and fairytales of eternal economic growth."

Trump, who also made a brief and unanticipated appearance at the climate summit, retweeted a video of Thunberg's speech, saying she seemed like a "very happy girl" and that it was "so nice to see."

Last Friday, Thunberg inspired a global movement as millions of — especially young — people around the world participated in climate strikes and marches, calling for action against climate change. Thunberg began skipping school and protesting outside of the Swedish Parliament in August 2018, inspiring Fridays for Future protests.