Unions and environmentalists are not traditionally thought of as allies. Labor, the story goes, supports job growth in unionized sectors despite the environmental consequences. The environmental movement, in turn, has been deaf to concerns that environmental regulations threaten working people’s jobs.

Yet unions and their members turned out in mass for the People’s Climate March, the largest climate march in history, held on September 21, 2014, days before the United Nations Climate Summit in New York. According to the organizers of the People’s Climate March, almost every labor union in New York City joined the march. Héctor Figueroa, president of 32BJ SEIU said, “We live in communities that get destroyed by storms like [Hurricane] Sandy. We work in the buildings that get flooded. We get hit by health epidemics like asthma that are rampant in our communities, and we care about the world that we will leave for our children and grandchildren.”

Unsurprisingly, Figueroa was not alone in invoking Hurricane Sandy when explaining labor’s support of the climate march. After all, unionized workers repaired infrastructure after the storm while unionized nurses and healthcare workers cared for patients in the blackouts and power shortages caused by Sandy.

But union activists’ support for the climate march went beyond the 2012 hurricane. “Let’s be clear, climate change is the most important issue facing all of us for the rest of our lives,” John Harrity, president of the Connecticut State Council of Machinists said. Christopher Erikson, business manager of IBEW, agreed. “This is your Woodstock,” Erikson said. “Today is the day your children and their children remember us. Today, our voices will be heard. This is not a moral issue. It’s an economic battle.”

Cross-organizing between labor and the environmental movement predate the march. To name a few examples, the 1999 World Trade Organization demonstrations saw the Teamster and Turtle alliance, when union and environmental activists found common cause protesting the ill effects of corporate capitalism. In 2012 AFL-CIO president Richard Trumka addressed the UN Investor Summit on Climate Change. The Blue Green Alliance brings major unions and environmental groups together to advocate for environmentally friendly jobs, while groups like the Los Angeles Alliance for a New Economy explicitly ties workers’ rights to environmental progress.

The relationship between unions and environmental activists is not always collaborative. The AFL-CIO issued a statement last year apparently supporting construction of the Keystone XL pipeline. Environmental groups have vigorously opposed the project, while some unions are eager for the jobs that the pipeline would create. Unions have also expressed concern that workers in economically depressed sectors will bear the brunt of transitioning to a green economy.

Union and community organizer Jane McAlevey has called for new strategies in building a sustainable labor and environmental alliance. Each side must mobilize in support of a “life or death issue” for the other; examples would be the Keystone XL pipeline for environmentalists and the struggle over collective bargaining rights in Wisconsin and Ohio for labor. McAlevey has also encouraged organizers to reach out to union’s membership base, building support from the bottom up.

Despite potential tension between the labor and environmental agendas, the numbers of union members and leaders at September’s march were impressive: 10,000 union members from over 70 unions joined the historic demonstration. “The People’s Climate March is labor’s Climate March,” stated a union flyer in support of the march. “A healthy planet equals good jobs.”