As more than a million youth poured into the streets worldwide on Friday for the weekly #SchoolStrike4Climate demonstrations, well-known adult climate activists answered a call to action from school strikers with a pledge to join global protests in September.

"On 20 September, at the request of the young people who have been staging school strikes around the world, we're walking out of our workplaces and homes to spend the day demanding action on the climate crisis, the greatest existential threat that all of us face," 350.org co-founder Bill McKibben, author Naomi Klein, actor Mark Ruffalo, and more than 50 other activists declared in an op-ed published Friday by The Guardian.

"This is going to be the beginning of a week of action all over the world. And we hope to make it a turning point in history," they explained. "We hope others will join us... break their daily routines and join together in sending the one message our leaders must hear: day by day, a business as usual approach is destroying the chance for a healthy, safe future on our planet."

Honored to be a co-signatory! | By @NaomiKlein​ & @BillMcKibben​ in The @Guardian​ "We’re stepping up – join us for a day to halt this climate crisis" https://t.co/QJfgvzEGfG — Michael E. Mann (@MichaelEMann) May 24, 2019

The activists' pledge came in response to an op-ed published by The Guardian on Thursday in which school strikers—led by Swedish teenager Greta Thunberg, whose solitary protests last year sparked the global #FridaysForFuture movement—asked adults "to step up alongside us."

Once again our voices are being heard on the streets, but it is not just up to us. We feel a lot of adults haven’t quite understood that we young people won't hold off the climate crisis ourselves. Sorry if this is inconvenient for you. But this is not a single-generation job. It's humanity's job. We young people can contribute to a larger fight and that can make a huge difference. ...join us on climate strike this September. People have risen up before to demand action and make change; if we do so in numbers we have a chance. If we care, we must do more than say we do. We must act. This won't be the last day we need to take to the streets, but it will be a new beginning. We're counting on you.

The group 350.org shared the school strikers' call to action in an email to supporters on Friday.

"We will stand with and follow the lead of the incredible young people who in every state and city in the U.S., and in countries across the world have demonstrated the kind of leadership that our elected officials themselves must show in the midst of a climate crisis that is already impacting the most vulnerable in our communities," Tamara Toles O'Laughlin, 350.org's North America director, said in a statement.

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Isra Hirsi, a Somali-American youth striker from Minneapolis and co-founder of U.S. Youth Climate Strike, said Friday, "Young people like me deserve a sustainable future with air we can breathe, water we can drink, and a climate that is safe and healthy."

"I've been striking for my communities who are left out of the conversation on the climate crisis," said Hirsi, the daughter of Rep. Ilhan Omar (D-Minn.). "Our generation needs more than empty commitments, we need action."

On September 20th, we are striking for not just our futures but for future generation’s futures. Are you joining us?@usclimatestrike @GretaThunberghttps://t.co/jAS6en6EV4 — isra hirsi (@israhirsi) May 23, 2019

The adult activists, recognizing the need for broader action that Hirsi described, wrote in The Guardian Friday: "We're not comfortable letting schoolchildren carry all the weight here—they need our backing. And disrupting our normal lives seems key—it's normal life that is doing us in, the fact that we rise each morning and do pretty much the same things we did the day before, even amid an unfolding crisis."

McKibben, in a personal op-ed published Friday by The New Yorker, added that "there's something fundamentally undignified about leaving our troubles to school kids to resolve."

As he put it, "It's time for the elders to act like elders."