Former President Barack Obama expressed support for the climate activists who took part in Friday’s global climate strike and proclaimed that climate change is the “one challenge” that will “define the future for today’s young generation more dramatically than any other.”

Activists in over 150 countries gathered to protest climate change on Friday, in part inspired by 16-year-old Swedish climate activist Greta Thunberg, who made waves after skipping school to protest climate change.

Obama, who met with Thunberg this week – calling her “one of our planet’s greatest advocates” – weighed in on the worldwide protests and proclaimed climate change as the “one challenge” that will “define the future for today’s young generation more dramatically than any other.”

“One challenge will define the future for today’s young generation more dramatically than any other: Climate change,” Obama wrote.

“The millions of young people worldwide who’ve organized and joined today’s #ClimateStrike demand action to protect our planet, and they deserve it,” he added:

One challenge will define the future for today’s young generation more dramatically than any other: Climate change. The millions of young people worldwide who’ve organized and joined today’s #ClimateStrike demand action to protect our planet, and they deserve it. — Barack Obama (@BarackObama) September 20, 2019

Social media buzzed with clips Friday, showing rallies in major cities like Dublin, Melbourne, Cologne, New York City, and Washington D.C.:

The estimate is at least 70,000 on #ClimateStrike in beautiful Cologne, Germany. The numbers around the world are astonishing https://t.co/BfGN3bCaU9 — Bill McKibben (@billmckibben) September 20, 2019

This is what I saw right before my speech at NYC #ClimateStrike. Y’all are so beautiful!!! 💚 Early estimates are 300,000+ striking in New York City! pic.twitter.com/CnYJ1owQmU — Dr. Ayana E. Johnson 🌎 #ClimateStrike Sept. 20th! (@ayanaeliza) September 20, 2019

Were on the ground in NYC, DC, and hundreds of other cities all around the world marching and striking because #ThisIsZeroHour to act on climate change! Millions upon millions of youth are marching and striking worldwide, stay tuned for more updates and coverage!#ClimateStrike pic.twitter.com/mfktZv9ypO — Zero Hour (@ThisIsZeroHour) September 20, 2019

Activists in D.C. could be heard chanting, “Don’t eat cows! Eat the rich!” with accompanying signs reading “Stop denying earth is dying” and “Burning our future.”

Several speakers took the stage on the West Front of the U.S. Capitol, with one activist deeming the ecosystem in D.C. “destroyed” – linking climate change to colonization and slavery – and another activist going on a frenzied rant, questioning the point in sitting in school and learning “about a future that might not exist.”

One activist at the D.C. rally told Breitbart News that the fight against climate change is directly linked to the push for socialism, calling the two “inseparable.”

“We’re fighting for the broadest possible mobilization of youth, students, and workers to fight against climate change and to fight for socialism, which in our view is inseparable,” the man said, adding that it is “fundamentally impossible” to be a capitalist and a climate change activist.

“You might hold those views, but they’re contradictory,” he argued. “There is no way to reconcile the fight for the environment with a capitalist drive for profit.”