Former President Obama on Monday marked Earth Day by praising a group of young activists who are “stepping up to save the one planet we’ve got.”

“Young people all over the world are leading the way in the fight to protect our planet because they know their future depends on it,” Obama said on Twitter. “This Earth Day, I want to celebrate the courageous, committed young leaders who are stepping up to save the one planet we’ve got.”

Obama went on to mention figures such as Greta Thunberg, the Swedish teen who helped spark a youth movement to combat climate change.

“They’re people like 16-year-old @GretaThunberg, whose protests at Swedish parliament sparked a movement,” Obama added in separate tweet. “Inspired by Greta’s action, Fridays for Future brought together more than a million strikers on every continent last month to demand action on climate.”

They’re people like 16-year-old @GretaThunberg, whose protests at Swedish parliament sparked a movement. Inspired by Greta’s action, Fridays for Future brought together more than a million strikers on every continent last month to demand action on climate. https://t.co/un7nBhEF8i
— Barack Obama (@BarackObama) April 22, 2019

Obama also recognized other youth climate change activists, including Luisa Neubauer of Germany, Alexandria Villasenor of New York, Haven Coleman of Colorado and Hisra Hirsi of Minnesota.

“Luisa, the organizer behind the student movement in Germany, says, ‘It’s our future and we’re not going to stop fighting for it,’” Obama said, before saying in a separate tweet that he hopes more individuals will set the example set by these activists.

I met 23-year-old @luisamneubauer in Berlin earlier this month. Luisa, the organizer behind the student movement in Germany, says, “It’s our future and we’re not going to stop fighting for it.” https://t.co/hS03PCWYAq
— Barack Obama (@BarackObama) April 22, 2019

“This generation of climate activists is tired of inaction, and they’ve caught the attention of leaders all over the world,” Obama concluded. “So while this challenge is only getting more urgent, they show us the kind of action it’ll take to meet this moment.

This generation of climate activists is tired of inaction, and they’ve caught the attention of leaders all over the world. So while this challenge is only getting more urgent, they show us the kind of action it’ll take to meet this moment.
— Barack Obama (@BarackObama) April 22, 2019

Thunberg, a 16-year-old from Sweden last year began refusing to go to school as a way to draw attention to climate change. She instead would hand out leaflets outside Parliament with the message: “I am doing this because you adults are shitting on my future,” according to reports.

Her activism helped spark student-led demonstrations in other countries, including the United Kingdom and Belgium. Students took part in protests in more than 1,500 cities around the world in a global demonstration on March 15.

Obama has been an outspoken supporter of policies meant to combat climate change.

The U.S. signed on to the historic Paris climate accord during his presidency, committing to curb global greenhouse gas emissions through an individual, nonbinding national plan. But Trump announced that the U.S. would withdraw from the plan in 2017.