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The Oscar-winning actor and environmentalist heaped praise on the 16-year-old Swede and said it was an 'honour' to meet her.
In a post on Instagram, he said: "There are few times in human history where voices are amplified at such pivotal moments and in such transformational ways - but Greta Thunberg has become a leader of our time.
"History will judge us for what we do today to help guarantee that future generations can enjoy the same liveable planet that we have so clearly taken for granted. I hope that Greta's message is a wake-up call to world leaders everywhere that the time for inaction is over. It is because of Greta, and young activists everywhere that I am optimistic about what the future holds.
"It was an honour to spend time with Greta. She and I have made a commitment to support one another, in hopes of securing a brighter future for our planet."
Thunberg made international headlines in September after she tore into world leaders at a UN summit.
Speaking at the Climate Action Summit in New York City, she said: "This is all wrong. I shouldn't be up here. I should be back in school on the other side of the ocean. Yet you have come to us young people for hope. How dare you. You have stolen my dreams and my childhood with your empty words.
"Yet, I am one of the lucky ones. People are suffering, people are dying and entire ecosystems are collapsing. We are in the beginning of a mass extinction. And all you can talk about is money and fairy tales of eternal economic growth. How dare you?"
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