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More than 500 organizations are planning a historic event for Sept. 21 in New York City, what they say will be the largest rally for climate action ever. Organizers and ralliers will be calling on world leaders to craft a new international climate treaty, two days before those leaders will convene at a Climate Summit at the United Nations headquarters. Jamie Henn, spokesperson for 350.org, the main convener of the event, declined to offer a precise target for turnout, but the current holder of the largest-climate-rally title, a February 2012 march on the White House, drew around 50,000 people, so organizers are expecting more than that – possibly significantly more.

However many people show up, though, this march will likely be historic for another reason: its diversity and its focus on climate justice. More than 20 labor unions are among the organizations leading in the planning and turnout efforts. On Wednesday morning, representatives of a handful of them gathered in the Midtown Manhattan office of 1199, the local chapter of the Service Employees International Union (SEIU), for a press conference, and then they were joined in Times Square by more unions for a small pep rally to promote the September event. Other groups present included locals from the International Brotherhood of Electrical Workers, the Transport Workers Union of Greater New York (TWU), and local social- and environmental-justice organizations such as UPROSE and the NYC Environmental Justice Alliance.