During the week of Easter, Britain enjoyed—if that is the right word—a break from the intricate torment of Brexit. The country’s politicians disappeared on vacation and, in their absence, genuine public problems, the kinds of things that should be occupying their attention, rushed into view. In Northern Ireland, where political violence is worsening sharply, a twenty-nine-year-old journalist and L.G.B.T. campaigner named Lyra McKee was shot and killed while reporting on a riot in Londonderry. In London, thousands of climate-change protesters blocked Waterloo Bridge, over the River Thames, and Oxford Circus, in the West End, affixing themselves to the undersides of trucks and to a pink boat named for Berta Cáceres, an environmental activist and indigenous leader, who was murdered in Honduras. Slightly more than a thousand Extinction Rebellion activists, between the ages of nineteen and seventy-four, were arrested in eight days. On Easter Monday, a crowd performed a mass die-in at the Natural History Museum, under the skeleton of a blue whale. In a country whose politics have been entirely consumed by the maddening minutiae of leaving the European Union, it was cathartic to see citizens demanding action for a greater cause. In a video message, Christiana Figueres, the former executive secretary of the United Nations Framework Convention on Climate Change, compared the civil disobedience in London to the civil-rights movement of the sixties and the suffragettes of a century ago. “It is not the first time in history we have seen angry people take to the streets when the injustice has been great enough,” she said.

On Tuesday, as members of Parliament returned to work, Greta Thunberg, the sixteen-year-old Swedish environmental activist, was in Westminster to address them. Last August, Thunberg stopped attending school in Stockholm and began a protest outside the Swedish Parliament to draw political attention to climate change. Since then, Thunberg’s tactic of going on strike from school—inspired by the response to the Parkland shooting in Florida last year—has been taken up by children in a hundred countries around the world. In deference to her international celebrity, Thunberg was given a nauseatingly polite welcome in England. John Bercow, the speaker of the House of Commons, briefly held up proceedings to mark her arrival in the viewing gallery. Some M.P.s applauded, breaching the custom of not clapping in the chamber. When Thunberg spoke to a meeting of some hundred and fifty journalists, activists, and political staffers, in Portcullis House, where M.P.s have their offices, she was flanked by Ed Miliband, the former Labour Party leader; Michael Gove, the Environment Secretary and a prominent Brexiteer; and Caroline Lucas, Britain’s sole Green Party M.P., who had invited her.

Thunberg, who wore purple jeans, blue sneakers, and a pale plaid shirt, did not seem remotely fazed. Carefully unsmiling, she checked that her microphone was on. “Can you hear me?” she asked. “Around the year 2030, ten years, two hundred and fifty-two days, and ten hours away from now, we will be in a position where we set off an irreversible chain reaction beyond human control that will most likely lead to the end of our civilization as we know it.”

Thunberg—along with her younger sister—has been given a diagnosis of autism and A.D.H.D. In interviews, she sometimes ascribes her unusual focus, and her absolute intolerance of adult bullshit on the subject of climate change, to her neurological condition. “I see the world a bit different, from another perspective,” she told my colleague Masha Gessen. In 2015, the year Thunberg turned twelve, she gave up flying. She travelled to London by train, which took two days. Her voice, which is young and Scandinavian, has a discordant, analytical clarity. Since 2006, when David Cameron, as a reforming Conservative Party-leadership contender, visited the Arctic Circle, Britain’s political establishment has congratulated itself on its commitment to combatting climate change. Thunberg challenged this record, pointing out that, while the United Kingdom’s carbon-dioxide emissions have fallen by thirty-seven per cent since 1990, this figure does not include the effects of aviation, shipping, or trade. “If these numbers are included, the reduction is around ten per cent since 1990—or an average of 0.4 per cent a year,” she said. She described Britain’s eagerness to frack for shale gas, to expand its airports, and to search for dwindling oil and gas reserves in the North Sea as absurd. “You don’t listen to the science because you are only interested in solutions that will enable you to carry on like before,” she said. “Like now. And those answers don’t exist anymore. Because you did not act in time.”

The climate-change movement feels powerful today because it is politicians—not the people gluing themselves to trucks—who seem deluded about reality. Thunberg says that all she wants is for adults to behave like adults, and to act on the terrifying information that is all around us. But the impact of her message does not come only from her regard for the facts. Thunberg is an uncanny, gifted orator. Last week, the day after the fire at Notre-Dame, she told the European Parliament that “cathedral thinking” would be necessary to confront climate change.

Yesterday, Thunberg repeated the phrase. “Avoiding climate breakdown will require cathedral thinking,” she said. “We must lay the foundation while we may not know exactly how to build the ceiling.” In Westminster, Thunberg’s words were shaming. Brexit is pretty much the opposite of cathedral thinking. It is a process in which a formerly great country is tearing itself apart over the best way to belittle itself. No one knew what to say to Thunberg, or how to respond to her exhortations. Her microphone check was another rhetorical device. “Did you hear what I just said?” she asked, in the middle of her speech. The room bellowed, “Yes!” “Is my English O.K.?” The audience laughed. Thunberg’s face flickered, but she did not smile. “Because I’m beginning to wonder.”