"Well, I am telling you there is hope. I have seen it," she told the audience. "But it does not come from the governments or corporations. It comes from the people."
Thunberg has drawn the support of millions of people, including demonstrators who coordinated an international protest day in September when she was visiting Washington and other U.S. cities.
The Swedish teenager has spoken with the pope, the U.N. secretary general and other influential figures. But Thunberg has also been mocked or criticized by climate skeptics — and by powerful leaders such as President Trump, Brazil's President Jair Bolsonaro and Russia's President Vladimir Putin. She has shrugged off those attacks, saying she won't be silenced.
"We need a balance of optimism and outrage" in the environmental movement, Thunberg said in Madrid. She added, "We need optimism to keep going and to not give up ... and we need outrage to be able to step outside our comfort zones."
Time says the award is based on the idea that powerful individuals shape the world. It chose Thunberg, the magazine says, for raising the alarm on climate change and "showing us all what it might look like when a new generation leads."
In the past, Thunberg has spoken about how her generation is being burdened both with climate change's effects and the task of educating people about it. And if the rise in average temperatures goes unchecked, she says, the effects will be cataclysmic.
"For about a year, I have been constantly talking about our rapidly declining carbon budgets — over and over again," Thunberg said on Wednesday. She added, "But since that is still being ignored, I will just keep repeating it."
Because of the clarity and energy she has brought to the discussion over climate change, Thunberg has been named as a contender for a number of international honors, including the Nobel Peace Prize. But she recently rejected an environmental prize and cash award from the Nordic Council, saying, "It is a huge honor. But the climate movement does not need any more awards."
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