Pablo Blazquez Dominguez/Getty Images

Time on Wednesday revealed teen climate activist Greta Thunberg as its Person of the Year. Thunberg, a 16-year-old Swedish student, became the face of a movement of young people protesting inaction on climate change. Time described her as the "most compelling voice on the most important issue facing the planet."

Over the past two years, Thunberg's unflinching demand for world leaders to take action on climate change has inspired millions to participate in climate strikes. During world climate strike events in September, several tech giants promised new devotion to sustainability and blacked out portions of their websites in so-called "green out" digital strikes. The next month, Google committed $150 million to renewable energy projects. In March, Thunberg was nominated for a Nobel Peace Prize. She's even getting her own mural in San Francisco.

"Her rise in influence has been really extraordinary," Time editor-in-chief Edward Felsenthal told Today. "She was a solo protester with a handpainted sign 14 months ago. She's now led millions of people around the world, 150 countries, to act on behalf of the planet, and she's really been a key driver this year taking this issue from backstage to center."

When Thunberg sat in front of the Swedish parliament building with that sign reading "Skolstrejk för klimatet," it drew global attention.

The magazine first selected a Person of the Year in 1927. Last year, it named The Guardians and the War on Truth, a collection of journalists and a news organization that did their jobs despite great personal cost. In 2017, Time chose The Silence Breakers, which included people who spoke up against sexual assault and harassment.

Former Secretary of State Hillary Clinton congratulated Thunberg on Wednesday. "I am grateful to Greta for all she's done to raise awareness of the urgency of the climate crisis and for her willingness to tell hard, motivating truths," Clinton posted on Instagram.

Los Angeles Mayor Eric Garcetti similarly said Thunberg is "inspiring young people to fearlessly demand immediate action to see the planet."

Thunberg was chosen from a shortlist that included House Speaker Nancy Pelosi; President Donald Trump; the Hong Kong protesters; and the whistleblower, the anonymous CIA officer who kickstarted the Trump impeachment inquiry.

Donald Trump Jr, the US president's son, tweeted Wednesday that Time should have awarded the Hong Kong protestors with the honor, calling Thunberg "a teen being used as a marketing gimmick."

Back in September, the president had called Thunberg a "very happy young girl looking forward to a bright and wonderful future" following her impassioned speech to the UN on the need for change before mass extinction occurs.

CNET's Corinne Reichert also contributed to this report.

Now playing: Watch this: Jeff Bezos announces climate pledge

Originally published Dec. 11, 5:23 a.m. PT.

Updates, 5:49 a.m., 6:43 a.m., 9:18 a.m.: Adds more background on Thunberg; 11:36 a.m: Adds social reactions.