The magazine announced the 16-year-old as its choice Wednesday exclusively on the "TODAY" show.
"She became the biggest voice on the biggest issue facing the planet this year, coming from essentially nowhere to lead a worldwide movement," Time editor-in-chief Edward Felsenthal told the show, adding that Thunberg is the magazine's youngest choice ever to be named Person of the Year.
Thunberg quickly bloomed into one of the world's most notable climate change activists, sparking a collective movement to fight the issue after protesting alone outside the Swedish Parliament during school hours on Fridays when she was 15. The teen held up a now universally recognized hand-painted sign that read “skolstrejk för klimatet,” which translates to “School strike for the climate.”
Her initiative to strike galvanized students to protest against climate change throughout Europe and that momentum quickly fanned across the globe, becoming the “Fridays For Future" movement.
Her solo protest, Fensenthal noted, eventually prompted millions of people in 150 countries "to act on behalf of the planet."
He also said that Thunberg "represents a broader generational shift in the culture" — with youth standing up for what they believe in — from Hong Kong to Chile.
"Young people are demanding change, and urgently," Felsenthal said. "She embodies youth activism."
Thunberg's signature no-nonsense blunt style of speaking made her a force that could not be ignored by world leaders and she was asked to speak in front of several high-profile entities, including the United Nations and the U.S. Congress.
Thunberg, who has Asperger’s syndrome, first learned about climate change at 8 years old and said she became instantly concerned to the point that she plunged into depression over it.
“I remember thinking that it was very strange that humans that are an animal species, among others, could be capable of changing the Earth’s climate,” she said during a 2018 Ted Talk.
She gave up eating meat and traveling via airplane, among other things, to reduce her carbon footprint.
In October, Thunberg was the recipient of another honor — an environmental award at a Stockholm ceremony held by the Nordic Council. But she declined it, explaining in an Instagram post, "The climate movement does not need any more awards."
"What we need is for our politicians and the people in power start to listen to the current, best available science," she wrote.
When asked what he thought Thunberg's response would be after learning that she is Time's Person of the Year, Felsenthal said, "I don't know, but I think what she has done, her rise in influence, has been really extraordinary."
The young activist sailed for just over two weeks on a zero-emission boat with her father in August to New York City.
When she appeared before Congress in September, Thunberg refused to read prepared remarks and instead submitted a 2018 United Nations global warming report to lawmakers, telling them, “I don't want you to listen to me. I want you to listen to the scientists, and I want you to unite behind the science."
One of her most notable appearances occurred at the U.N. Climate Action Summit in September when she excoriated global leaders, including U.N. Secretary-General Antonio Guterres by telling them they had “stolen her dreams and childhood” with their “empty words.”
"People are suffering. People are dying. Entire ecosystems are collapsing," she said. "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!"
The “how dare you!” sentiment reverberated universally, emboldening climate change activists while making politicians uncomfortable.
Her fiery words drew ire and sarcastic responses from several detractors, including President Donald Trump.
“She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!” Trump commented after retweeting a video of Thunberg's speech.
Despite being a frequent target of criticism, Thunberg has trudged forward on her quest for environmental justice.
Earlier this year, Thunberg, along with 15 other young climate activists, filed a legal complaint with the United Nations against five countries under the U.N. Convention on the Rights of the Child alleging the nations are not doing enough to combat climate change and that their inaction is affecting their right to thrive.
Her efforts also earned her a nomination for the 2019 Nobel Peace Prize. While the teen lost the award to Ethiopian Prime Minister Abiy Ahmed Ali, she remains one of the youngest nominees for the illustrious prize.
The other 2019 finalists for Time's Person of the Year were Democratic House Speaker Nancy Pelosi, President Donald Trump, the Hong Kong protesters, and the anonymous whistleblower whose memo on Trump's dealings with Ukraine triggered the impeachment inquiry.
Instead of doing runners-up this year, the magazine gave awards in different categories, including the U.S. women's soccer team as athlete of the year, Lizzo as entertainer of the year, and Disney CEO Bob Iger as businessperson of the year.
The magazine has selected a Person of the Year annually since the 1920s. Last year, Time chose "The Guardians and the War on Truth,” which included four journalists and one news organization who paid a hefty price, either with their lives or freedoms, to be journalists at time when the profession has been under attack on several fronts.