The Service Employees International Union marching with the Sierra Club, the United Federation of Teachers protesting alongside 350.org.

Some observers of the massive People’s Climate March on Sept. 21 may have thought nothing of seeing so many unions participating in environmental activism, but it really was noteworthy.

The labor and environmental movements have often been at odds in recent decades, with unions frequently viewing environmentalists’ goals as a danger to industries that employ union members and environmentalists often casting unions as defenders of an ecologically unsustainable system.

But in recent years, the two have formed an unlikely — though in some cases a still uneasy — partnership. That new alliance was on display at the protest for action against global warming.

An estimated 300,000 people came from across the United States and other countries to join the march through Midtown that was timed to coincide with a United Nations summit of world leaders on the climate crisis.

“Global warming is wreaking havoc on the environment and will have real impacts on the economy and people regardless of the work they do,” said Lenore Friedlaender of 32BJ SEIU, which helped to coordinate labor’s support for the march.

The UFT was among the unions that sponsored the event, with the UFT Executive Board passing a resolution in support.

“Hurricane Sandy gave our union and many other people across our region painful, firsthand knowledge of the devastation caused by the kind of severe weather events that scientists say are becoming more common with global warming,” said Janella Hinds, the UFT vice president for academic high schools and the secretary-treasurer of the New York City Central Labor Council. “Our members — and the students, patients and others they serve — need for world leaders to take action on the climate crisis.”

The protest drew wide and diverse support in part because of its broad aims. Tying concerns about the environment to growing economic inequality, it called for “a world with an economy that works for people” and “with good jobs.”

For unions like the United Steel Workers that have seen their core industries decline, “good jobs” today mean jobs in environmentally sustainable industries. A steelworkers local is among the unions listed as a partner in the climate march.

The steelworkers have been at the forefront of labor on many environmental issues and the founding union and main force behind the BlueGreen Alliance, which since 2006 has united unions and environmental organizations to push for massive infrastructure modernization to both create jobs and protect the environment.

But industrial-sector unions generally have struggled with how combating climate change will affect their members.

“Everyone understands global warming is a crisis, but the challenge in moving away from fossil fuels is what happens to those workers,” said Friedlaender, the 32BJ officer.

Sean Sweeney, the co-director of Cornell’s Global Labor Institute, points out that the AFL-CIO as a whole has only reluctantly backed cuts to carbon emissions, in part due to fears that they will accelerate outsourcing of industry to China and other developing nations that have not been asked to make such steep reductions.

But with climate change increasingly leading to dramatic weather events like Hurricane Sandy, Sweeney said the pressure is on.

“What we’re seeing in the last two or three years is that weather is now playing a part in politics,” he said. Unions that have taken a lead in the climate change movement “recognize it is their moral and political responsibility to fight for climate protection.”