The great outdoors is also reflected in McCartney’s state-of-the-sustainable-arts London flagship store—which she designed herself, with a soundtrack that includes a three-hour loop of her father, Paul’s, demo tapes along with a Bob Roth meditation in the changing rooms. “The audio is important for me,” she says as she proudly walks me round it, “because it’s obviously such a big part of my upbringing.” There are papier-mâché walls made from “all of the shredded paper from the office,” along with a silver birch grove and a moss-covered rockery of giant granite rocks brought from the 1,100-acre McCartney family farm on Scotland’s Mull of Kintyre. “My personality is this sort of contrast between the hard and the soft, the masculine and feminine,” says McCartney. “I wanted to have life in the store—to bring nature into the experience of shopping,” she explains as she takes me up in the Stellevator to the floor where she fitted the Duchess of Sussex for the glamorous halter-neck dress she wore for the wedding reception following her marriage to Prince Harry. There are also pieces from McCartney’s “All Together Now” Beatles collaboration, inspired by a friends-and-family screening of Yellow Submarine that her father staged on the film’s 50th anniversary. “It just blew my brains because I hadn’t seen it since I was a kid,” she recalls. “It’s astonishing—just mental and so trippy and so childlike and so innocent and so heavy and so meaningful.”

Since McCartney’s 1995 Central Saint Martins graduation show, her brand has been defined by the urgent desire to do away with animal cruelty in the fashion industry. And while, 20 years ago, there were fake furs on the market, the only glues available were animal-based. “I imagine Vikings sitting around a pot, boiling down the last bones of the elk that they skinned for the fur,” says McCartney. “And I think, Wow—we’re still there.” Today McCartney uses renewable energy where it’s available for both her stores and offices; the eyewear she shows me in her store is bi-acetate, and her sneakers are made with biodegradable Loop technology; she uses regenerated nylon, polyester, and cashmere but also works with producers making innovative fashion fibers—building fake fur from sustainable corn fiber, for instance, producing vegan microsilk, and growing mycelium-based “leather.”

“I was always a bit of a freak in the house of fashion,” McCartney says. “My regime, my culture, has been different from day one.” In Paris, where she was appointed creative director of Chloé in 1997, she struggled with the perception that at 26 she was too young and unqualified for the job (“The Beatles wrote Sgt. Pepper when they were 26,” she told Vogue tartly), and her working practice was “totally at odds with the rest of the industry,” as she recalls. Even now, she says, “every single day in our office is this sort of daily challenge—a way of trying to perfect and persist and find realistic solutions within the luxury-fashion sector—and even in a more broadstream way with the collaborations with Adidas [initiated in 2004]. Each day,” she says, “there are questions that I ask that we try to find an answer for. And if we can’t, we’ll try again tomorrow.”

Despite what she refers to as “a lot of resistance,” McCartney turned the Chloé gig (which lasted through the launch of her self-titled brand in 2001) into a triumph, tripling sales. Today, as we march inexorably to global Armageddon, her commitment to cruelty-free fashion and sustainability is fast becoming the industry norm. In recent years, for instance, luxury brands including Gucci, Prada, Michael Kors, Armani, and Chanel have declared themselves fur-free. “I’m hugely relieved,” says McCartney, “but I’m actually astounded that it’s taken so long.”

McCartney now gives scholarships at Central Saint Martins, her alma mater, for students who “adhere to our ethical charter,” and helps young designers navigate the complicated terrain of sustainability. “We’re in the farming industry in fashion,” she says. “We look at the biodiversity and the soil. It’s crazy. It’s basically exhausting. It’s much easier not to do it. So I kind of understand why the world hasn’t quite followed.”

BUT McCartney has far more ambitious goals for expanding her global industry reach. Last year, she bought back full ownership of her label from Kering, 17 years after the group’s then–creative director Tom Ford had urged the company to invest in McCartney’s fledgling brand. Following her move, “people began to show an interest quite quickly,” as McCartney recalls. “I was fortunate enough that Mr. Arnault was one of the people.” She’s speaking, of course, of Bernard Arnault, the all-powerful chairman and chief executive of LVMH, which acquired a minority share in Stella McCartney in July. “I think it’s incredibly exciting. It sends a big, big message to the industry if Mr. Arnault is asking me to be his personal adviser on sustainability at LVMH. I think that was one of the attractions for me—it is a big, timely statement, and hopefully game-changing for all of us.”

McCartney points out that the fashion brands with the biggest environmental impact in terms of scale are “the high-end luxury houses, and then the fast-fashion sector. They have massive impact in a negative way, and they can have a massive impact in a positive way.” These fast-fashion retailers, as she observes, turned from fur far earlier than luxury brands. “They’re more in touch with the youth,” she says, “and what the next generation of consumers actually wants. It’s a given for my children,” she notes, “that you have to show some kind of mindfulness or awareness.” (In recognition of the next generation’s activists, McCartney has launched the Stella McCartney Today for Tomorrow Award—video nominations via Instagram—“to celebrate,” as she says, “a new generation of change agents and eco-warriors under 25 who are kicking ass for Mother Earth.”)

She may have her work cut out for her. A week after our coffee klatch and four days before presenting her spring-summer 2020 show in Paris (“our most sustainable collection ever”), Arnault, addressing an LVMH sustainability event in Paris, called out 16-year-old activist Greta Thunberg for “indulging in an absolute catastrophism about the evolution of the world” in her electrifying appearance at the United Nations summit on climate change. “I find it demoralizing,” he added. It was perhaps no accident that McCartney raced to put together a sustainability panel (no questions, no photographs) of her own on the eve of her show at the Opéra Garnier—a panel that included Extinction Rebellion activist Clare Farrell, the legendary environmentalist and activist Yann Arthus-Bertrand, and author Dana Thomas (Fashionopolis: The Price of Fast Fashion and the Future of Clothes), who noted that “we wear our clothes seven times on average before throwing them away . . . we’re perpetuating this bulimia of buying, using, and throwing away.”

“What we’ve seen over the last few weeks and months,” McCartney said, pointedly, “is children and young people taking action.” The designer also addressed the issue of young activists’ rejecting the idea of consumerism. “If the youth of today stop buying into it,” McCartney added, “then obviously, the people at the top have got to deliver on that.”

Rayon, or viscose, an indispensable fashion fiber, for instance, is created from wood pulp. “This year alone,” McCartney says, “up to 150 million trees have been cut down just for viscose.” McCartney now sources hers from sustainable forests in Sweden. “I’m trying to create something that’s still sexy and desirable and luxurious that isn’t landfill,” she tells me. “Every single second, fast fashion is landfill.”

Does McCartney feel that she’s had an impact on the practices of other brands? “That’s not for me to say,” she demurs. “That would be so unchic of us. But we are a kind of incubator. I have sympathy for how hard it is to shift the massive Titanic ship away from the iceberg,” she says. “We’re a little agile sailboat, and we built the ship. And I think that’s easier than changing something that’s been going in one direction for so long.”