This story is part of the optimistic argument for the future of the planet in our special issue on Earth Day. Read the pessimistic argument and the rest of our stories here.

Editor's Note: Greta Thunberg, the Swedish climate activist, announced March 24 that she had experienced mild flu-like symptoms and may have had COVID-19. In an Instagram post, Thunberg said she was not tested, but isolated herself for two weeks and has recovered. She urged young people to take precautions not to pass the virus to others. “Our actions can be the difference between life and death for many others," she wrote.

Before Greta, there was Severn.

Their photos often appear side by side, like bookends framing the long campaign by young people to persuade adults to take significant steps to fight climate change. Greta Thunberg, the Swedish teen activist, is the latest child to sound the alarm. Severn Cullis-Suzuki, the daughter of an environmental scientist in Vancouver, Canada, came first.

In 1992, when Severn was 12, she traveled with three other young activists to the United Nations climate conference in Rio de Janeiro. The science of global warming had just begun to resonate. The UN had created the Intergovernmental Panel on Climate Change, now the leading authority on climate science, just four years earlier, and world leaders weren’t accustomed to listening to children lecture them.

Severn became known as “the girl who silenced the world for six minutes,” setting a precedent for young activists to express their sense of impending doom in the clear-eyed way that only children can. “You must change your ways,” Severn told the delegates. “Losing my future is not like losing an election or a few points on the stock market.”

When Greta delivered her scold at the UN’s climate summit in New York City last September, the similarities were striking. One could be forgiven for concluding that nothing at all had occurred in the intervening 27 years to stave off the existential threat to humanity.

Yet much has changed that might finally prompt action. The accelerating number and intensity of catastrophes not visible three decades ago has focused global attention on what’s at stake. Tellingly, the population that will live with the consequences took to the streets last year to stage some of the largest environmental protests in history.

Young people are well positioned, by the strength of their numbers and the organizing power of social media, to provoke action. Worldwide, there are more than 3 billion people under 25, two-fifths of the total population. In the United States during the cultural unrest of the late 1960s and early 1970s, Americans between the ages of 18 and 29 numbered 41 million. Today the same age group is 52 million strong. Youth protests also have broadened into a movement that includes a mash-up of so many social causes, including racial justice and gun control, that it invites comparison with the social activism of the late 1960s that roiled countries around the world.

View Images Greta Thunberg After capturing the world’s attention at the United Nations in New York City last September, Thunberg, now 17, spoke in December at the UN’s climate change conference in Madrid. Her main theme: science. “I’ve given many speeches and learned that when you talk in public, you should start with something personal or emotional to get everyone’s attention,” she said. “But today I will not do that because then those phrases are all that people focus on. They don’t remember the facts, the very reason why I say those things in the first place.” Photograph by TOM JAMIESON

Millions of children have come of age watching ice sheets melt and temperatures rise, and they are fed up with waiting for government leaders to act. “The Vietnam War served as a trigger to radicalize a generation,” says Stephen Zunes, a University of San Francisco political science professor. “Climate is going to do the same thing.”

Delaney Reynolds, 20, who lives in Florida, one of the places most vulnerable to climate change, is increasingly frustrated with the lack of action. “A lot of adults in power today are way too focused on money and profits,” she says. “As soon as we can replace them, we will replace them.”

Now a student at the University of Miami, Reynolds grew up when Florida’s leadership hadn’t faced up to the flooding that will inevitably remake the coastline of their sandspit of a state; then Governor Rick Scott promoted an unofficial policy to avoid even mentioning the words “climate change.” Reynolds founded the Sink or Swim Project and began educating Floridians about the risks of sea-level rise, giving hundreds of talks to everyone who would listen. “It is incredible that kindergartners can grasp this as a problem and politicians can’t,” she says.

View Images Kehkashan Basu Born in Abu Dhabi of Indian heritage, Basu, 19, now lives in Toronto, Canada. A National Geographic Young Explorer, she started the Green Hope Foundation to give voice to young people. She helped children replant mangroves in the deforested Sundarbans on the Bay of Bengal and planted trees in a refugee camp in Bangladesh. Her optimism in the future is reflected in her foundation’s name. Photograph by REBECCA HALE, NGM STAFF

Felix Finkbeiner, a 22-year-old German activist, is another old-timer in the youth climate change movement. He found his way to advocacy as a nine-year-old who had a toy polar bear and was moved by photos of starving polar bears struggling to hunt for food as the Arctic ice disappears.

Finkbeiner wanted to help: He planted a tree at his school. Now he’s pursuing a doctorate in climate ecology while heading the nonprofit he founded in 2007. Plant-for-the-Planet has planted eight million trees in 73 countries and is part of a global effort to plant one trillion.

“There’s no reason this movement had to wait this long or be a youth thing,” he says. “What’s happening is phenomenal. This could be the tipping point we were hoping for.”

Last fall he met and shared tips with Lesein Mutunkei, a 15-year-old soccer player in Nairobi who planted a tree after every goal he scored to do his bit to help Kenya recover its forests. Mutunkei expanded his project to involve other youths who celebrate their own achievements by planting trees. “If you are good at music and reached a certain point, you can plant a tree for that. If you get an A in a subject, you can plant a tree,” he says.

One of the most consequential efforts is playing out in the courts of the world, including in Norway and Pakistan, where young people are pursuing litigation to win climate protections. In a case that’s ongoing, 21 young Americans have sued the federal government for its role in creating a “dangerous climate system.”

The most recent wave of climate protests began to build several years ago in Europe. Young activists in Germany organized school strikes that attracted few numbers and little attention but helped build the foundation for the movement sparked by Greta Thunberg’s solitary school strike in August 2018, which swept the world. Unknown when she sat outside the Swedish parliament in Stockholm, the 17-year-old has become the face of a global movement that has seen school strikes in most countries and over 7,000 towns and cities. By the time she arrived in New York, after sailing across the Atlantic on a no-emissions yacht, she had achieved the kind of one-name celebrity usually afforded to rock stars.

View Images Rosie Mills Mills, 19, led a petition drive that persuaded the local council in Lancaster, England, to declare a “climate emergency” after catastrophic flooding. Last year she ran for a seat in the European Parliament as a Green Party candidate. She lost, but didn’t finish last. “One of the weirdest things is when a teacher comes up to you and says, ‘I’m going to vote for you.’ Then she assigned me an essay the next day.”

Photograph by TOM JAMIESON

Thunberg is plainspoken and blunt, perhaps in part because she has Asperger’s syndrome. She doesn’t engage in the contorted language so common in political discourse. When she testified before the U.S. Congress, she submitted a UN climate panel report instead of prepared remarks. “I don’t want you to listen to me, I want you to listen to the scientists,” she said.

Elizabeth Wilson, a human rights lawyer and visiting scholar at Rutgers Law School in New Jersey, has watched young activists find their footing. “I think it is extraordinary where we have persuaded ourselves we’re living in a post-truth world, and these kids are saying, ‘We believe in facts. We believe in science. What you are telling us is not an alternative reality; it’s a lie,’” she says. “It’s breathtaking.”

It’s easy to forget that, for all their media savvy and tactical organizing skills, many of the climate activists are still just children. Many struggle with anxiety and depression. Their attention is riveted on alarming reports—a 2018 UN analysis that concluded carbon emissions must be cut almost in half by 2030 to hold global warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit), and research by the World Meteorological Organization and the journal Nature published late last year warning that temperatures rising beyond that threshold will lead to worsening hurricanes, floods, droughts, and wildfires, as well as agricultural disasters that could shrink the world’s food supply.

View Images Jerome Foster II He’ll be old enough this November to vote for the first time in a U.S. presidential election, and he sees registering young people to vote as the best way to spur action on climate change. To do that, the Washington, D.C., resident founded OneMillionOfUs. His organization combines voter registration with activism on climate change, gun control, immigration reform, and gender and racial equality. Photograph by REBECCA HALE, NGM STAFF

“It’s not hard to find kids who say they don’t want to have children because of the chaos they believe the world will be in,” says Lise Van Susteren, a psychiatrist who has studied how youth are coping with climate change. “This is a shaky time for children. They have seen it for themselves. They have seen the fires. They have seen the storms. They’re not stupid, and they are angry.”

Alexandria Villaseñor, 14, who has skipped school on Fridays since December 2018 to strike at UN headquarters in New York, and Jamie Margolin, 18, founder of the group Zero Hour, candidly described their fears for the future at a symposium last fall at Twitter’s Washington, D.C., office. Villaseñor said she’s worried that, by the time she’s able to vote and help elect leaders who will act on climate change, it will already be too late. Margolin, who lives in Seattle, described bouts of despair that have sent her to bed. “Climate anxiety is real for me,” she said.

Will the movement finally succeed? History argues against it. Social movements waged against identifiable villains, such as despots, often succeed. But it’s more difficult to force societies to make structural changes, which can consume decades. Remaking the world’s energy system presents an almost Sisyphean task.

“The hallmarks of a movement that is going to be successful are sustaining it and turning it into public policy,” says Kathleen Rogers, president of the Earth Day Network and a longtime environmental activist. “If you don’t turn it into political power, it will just die.”

In Europe, activists have changed the political landscape more easily than they have in the United States. “In Germany, there has been a fundamental shift in policy and scale,” Finkbeiner says. “Every German politician has understood that elections can no longer be won without green policies.”

Severn Cullis-Suzuki, now 40, doesn’t fear the climate movement will fizzle. “What strikes me now is how much right now feels like where we were back in 1992. Rio was a success. We got all the leaders to sign on,” she says. “We’re back at that same moment. Awareness has been raised. We now have to translate that into nothing short of a revolution.”

Cullis-Suzuki, who earned a degree in ecology, now lives with her husband and two children on Haida Gwaii, an island cluster off the coast of British Columbia, Canada. She’s working on a doctorate in linguistic anthropology, studying the language and culture of the Haida, an indigenous people whose stewardship of their environment has enabled them to endure for more than 10,000 years. She pauses. Does she need to say more?

View Images Severn Cullis-Suzuki Through speaking, writing, and filmmaking, she promotes a return to values that will sustain the Earth. Her 1992 speech to the UN climate conference in Rio de Janeiro, delivered when Cullis-Suzuki was 12, stills draws viewers on YouTube. In 2017 she celebrated its 25th anniversary by encouraging young people to give her speech, or parts of it, and to upload the video to her “I’m Only a Child, but ...” YouTube page. Photograph by KARI MEDIG

This story appears in the April 2020 issue of National Geographic magazine.