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 color nor a borrowed reference. It dominated runways in every single city; it became so ubiquitous that it was almost a cliché.

Forget street wear. 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.”