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.