NEW YORK — Of all the trends that emerged from fashion month, the four-week-long circuit of ready-to-wear shows in New York, London, Milan and Paris that ended last week, the one that trumped all others was neither a skirt length nor a colour nor a borrowed reference. It dominated runways in every single city; it became so ubiquitous that it was almost a cliché.

Forget streetwear. Sustainability was the hottest look of the day.

It started in New York on Sept 10, when Gabriela Hearst unveiled what she said was the first carbon-neutral fashion show, which also featured upcycled prints from old collections. That was the same day the British Fashion Council introduced its Institute of Positive Fashion, which aims to create a coalition that will set standards for green businesses.

Two days later Gucci announced that its show, too, would be carbon neutral, and it would also offset all carbon emissions from its operations and supply chain.

In Milan, Missoni handed out little solar powered sun lights by Olafur Eliasson during its show, and the Green Carpet Awards capped off the city’s shows on Sept 22, touting “the best in sustainable fashion”.

The next day Kering, Gucci’s parent group, committed to full carbon neutrality across all of its brands and to reduce its greenhouse gas emissions by 50 per cent by 2025.

The day after that, LVMH, Kering’s greatest rival, which had recently bought a stake in Stella McCartney (a company formerly a joint venture with Kering, which is famous for its green cred), held a news conference to discuss the progress of its LIFE program (LVMH initiatives for the environment, established in 2012) and achievements in the climate sphere.

It was the first such public-facing event the group had held, though it had established its sustainability department in 1992.

By Paris, seemingly almost every brand was trumpeting its commitment to addressing the climate crisis.

Courrèges used its show notes to talk up its use of a new algae-based vinyl, “which uses 10 times less plastic” than the classic material, and the fact that it had swapped exotic skins for the skin of the pirarucu fish, a staple of the Amazonian diet whose skin would be otherwise discarded.

Stella McCartney left a sheet (green, of course) detailing the brand’s very admirable history and achievements in recycled nylon, sustainable viscose and biodegradable shoe soles on every seat.

Alexander McQueen upcycled old laces and tulle from previous looks into its new collection. So did Alaïa. At Dior, the set was composed of more than 100 trees, all of which were bound for community gardens afterward. At Saint Laurent, the generators required to run the 414 spotlights that speckled the show were powered by biofuels. At Louis Vuitton, a note averred that the plywood set was bound for recycling.

“It’s turning into a competition,” said Mr Burak Cakmak, the dean of fashion at Parsons, during a party at the end of the whole shebang hosted by Business of Fashion.

To wit: During the LVMH conference, Mr Antoine Arnault, the head of communications for LVMH, had noted, “we prefer acts to pacts,” in what seemed a thinly disguised dig at the Fashion Pact, an initiative spearheaded by François-Henri Pinault, the chief executive of Kering, at the behest of French President Emmanuel Macron.

Introduced in late August, the pact brought together 32 groups from high fashion and fast fashion — though not LVMH — to commit to implementing changes (of their own choosing) in order to address their environmental impact.

To understand what a dramatic shift this is, consider the fact that the Copenhagen Fashion Summit was the first major conference devoted to fashion and sustainability, and when it was established, in 2009, people were still confusing the terms “vegan” and “eco” and “organic”.

Even a few years ago, brands were reluctant to even utter the word “sustainability” for fear of being accused of greenwashing, and thought nothing of sacrificing fields of flowers for Instagram impact. Now, all of a sudden, they can’t stop talking about it.

If real, systemic change is going to occur when it comes to fashion and sustainability (or fashion and inclusivity, or fashion and harassment) — and if it is to be trusted by people buying the products as more than marketing — then the industry as a whole needs to participate. Not with a variety of examples that ultimately obfuscate the goal more than illuminate it, but by agreeing on the best way forward.

By handing over the measurement of that to a consistent and disinterested third party, or group of third parties.

And with every fashion city at the table addressing the elephant in the room when comes to fashion weeks — the simple fact of their existence, in four countries, encouraging thousands of people to fly from country to country and then get into fleets of black cars and drive for many hours a day — rather than vying with one another to see who can snag the green crown.

It’s a complicated charge, unquestionably. Mr Arnault pointed out that it is difficult to apply the same measures to a sportswear brand like Nike and a couture brand like Dior, and that is true.

It is also true that brands are built on the premise of distinguishing themselves and their points of view from one another. And that cities have a vested economic interest in maintaining the fashion week status quo.

But, as Mr William McDonough, the co-author of “Cradle to Cradle: Remaking the Way We Make Things,” said, “If you wish to do ecosystem design, first you have to do ego-system management.” Then, maybe, everyone wins. THE NEW YORK TIMES