The teen-age activist Greta Thunberg is pure spirit, committed to the foremost emergency of our time and to the science behind it. Photograph by Luis Filipe Catarino / Redux

On December 3rd, Greta Thunberg, the sixteen-year-old climate activist from Sweden, completed her second transatlantic voyage, by almost entirely emissions-free sailboats, in the span of four months. Her small figure, dressed in black, stood, waving, on the bow of a catamaran, as it approached the port of Lisbon. Hundreds of people, standing onshore, cheered, welcoming her back to Europe. “I’m not travelling like this because I want everyone to do so,” she told reporters after walking off the boat onto dry land. “I’m doing this to send a message that it is impossible to live sustainably today, and that needs to change.” The scene felt both ancient and precisely of this moment, like Thunberg herself, who writes regularly in a paper journal but has mastered social-media virality, who can seem ageless and androgynous (the fierce stare) while also strikingly young and girlish (the braids), who acts with an otherworldly grace while delivering an outraged message grounded in the latest, best climate science. Her lightning-strike emergence as the planet’s hero, her capacity to inspire students around the world—all in the span of little more than a year—can seem like a prophesied story, an epic poem, a fable. Margaret Atwood (and others, including me) have compared her to Joan of Arc—if the teen-age medieval warrior, who was burned at the stake in part for impersonating a man, had been inspired by scientific reports instead of divine voices and visions of angels. Centuries from now, we hope, people will live in a thriving, equitable civilization and tell Thunberg’s tale, too.

But it is, as Thunberg says repeatedly, precisely what we do during this century that will determine the fate of those future centuries, and what we do during the next decade that will determine the climate for the nearly two billion children alive today. They are the ones Thunberg represents, whom she is fighting for, and whom she has mobilized, since August, 2018, when she first sat outside the Swedish Parliament with a simple handwritten sign that read, in black letters, “SKOLSTREJK FOR KLIMATET.” Hundreds of thousands of students (and, gradually, their parents), in cities around the world, have followed her lead, striking from school and marching in the streets to protest for climate action. “You say you love your children above all else,” she said in her first big address, at last December’s United Nations climate talks. “And yet you are stealing their future in front of their very eyes.”

From Lisbon, Thunberg took a train to Madrid, where leaders from around the world were gathering for another round—the twenty-fifth since 1995—of U.N. climate negotiations (known as the Conference of the Parties, or COP25). The point of this year’s talks was for countries to lay the groundwork for ambitious new targets in the reduction of their greenhouse-gas emissions. By the end of 2020, according to the terms of the Paris Agreement, countries are to commit to new nationally determined contributions (N.D.C.s, in U.N.-speak) that reflect the scale of global decarbonization necessary to limit global heating to two degrees Celsius. (The current pool of N.D.C.s, which many countries are not even meeting, would lead to more than three degrees warming by century’s end.) A related issue at the talks has involved carbon markets—detailed in Article 6 of the Paris Agreement—in which one country can pay another country for its emissions reductions (the equivalent of buying a carbon credit) and then count those reductions toward its own N.D.C. Australia, Brazil, Saudi Arabia, and India have, reportedly, all been blocking text that would provide strong regulations of these kinds of markets and accounting mechanisms. Though the final text of this year’s agreement is due Friday, the deliberations will likely continue at least until Saturday.

Thunberg, meanwhile, has increasingly referred, in mathematical detail, to carbon budgets, or the amount of carbon dioxide that we have left to emit into the atmosphere if we want to limit global temperature rise to 1.5 degrees Celsius. In her speech to world leaders in Madrid, on Tuesday, she referred her audiences to page 108, chapter 2, of the 2018 Intergovernmental Panel on Climate Change report, and she said that, if we are to have a sixty-seven per cent chance of achieving that goal, we had, as of the first of January, 2018, four-hundred-and-twenty gigatons of carbon dioxide left in our carbon budget. That number is now much lower, considering that we emit approximately forty-two gigatons of carbon dioxide every year. This means that we have roughly eight years left to burn fossil fuels at current levels before our budget is empty. For all the efforts under way to reduce greenhouse-gas emissions, they are nowhere near enough. Global emissions again hit a record high in 2019. As Thunberg also said, in the same speech, “The biggest danger is not inaction. The real danger is when politicians and C.E.O.s are making it look like real action is happening, when in fact almost nothing is being done, apart from clever accounting and creative P.R.”

On Wednesday, Time named Thunberg the magazine’s Person of the Year. Donald Trump, who is famously obsessed with being on the cover of Time, could not stand it. He has campaigned on fossil-fuel expansion, has betrayed on numerous occasions that he does not understand what climate change is, and, on November 4th, he officially began proceedings to remove the U.S. from the Paris Agreement. (Every other country in the world remains a signatory to the pact.) On Thursday, in response to Thunberg’s news, he tweeted, “So ridiculous. Greta must work on her Anger Management problem, then go to a good old fashioned movie with a friend! Chill Greta, Chill!” Thunberg, as always, took the President’s mockery in stride, changing her Twitter bio, minutes later, to “A teenager working on her anger management problem. Currently chilling and watching a good old fashioned movie with a friend.”

This is not the first time that Thunberg has one-upped Trump’s mocking tweets. In September, she gave a historic speech with the kind of rhetorical vigor that exemplifies her gifts as an orator. “This is all wrong,” she said. “I shouldn’t be up here. I should be back in school, on the other side of the ocean. Yet you all come to us young people for hope. How dare you!” Later, Trump retweeted a video clip of her remarks, adding, “She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!” The same day, Thunberg put the exact words in her Twitter bio: “A very happy young girl looking forward to a bright and wonderful future.”

Thunberg is Trump’s perfect foil. She is pure spirit, committed to the foremost emergency of our time, to the science behind it, and to the people who are working every day to rapidly change our energy systems and consumption patterns so that we avert climate change’s deadliest impacts and destabilizing tipping points. Thunberg is devoted to learning, writing, and understanding the world around her. She constantly lifts up other young climate leaders—especially those from indigenous and frontline communities—and begs reporters to focus on them, not her. (On Monday, she and Germany’s most prominent youth activist, Luisa Neubauer, hosted a press conference with young leaders from the Marshall Islands, the Philippines, Russia, and Uganda.) She is a gifted public speaker, not because she stirs up chaos and hate through incoherent rants but because she speaks elegantly and intelligently, in logical, pithy, unmuddied sentences. Her rhetorical gifts are, perhaps, all the more remarkable considering that, when she was younger, she fell into a major depression concerning climate change and stopped speaking altogether for months. As she said at the start of her speech on Tuesday, “A year and a half ago, I didn’t speak to anyone unless I really had to. But then I found a reason to speak.”