A crowd of about 200 black-clad members of the climate activist group Extinction Rebellion gathered Monday morning at the southern end of New York City’s financial district. Some held banners painted with ghostly white animals or cardboard cutouts of trees and waves. In the background, a New Orleans-style jazz funeral band warmed up tubas, and one of the march’s emcees instructed people on the proper way to wail. (“Dig down and pull out your grief — because you gotta cry!”)