“Since then we’ve watched the summer Arctic disappear and the ocean turn steadily acidic,” Mr. McKibben said in a phone interview on Wednesday. “It’s not just that things are not getting better. They are getting horribly worse. Unlike any other issue we have faced, this one comes with a time limit. If we don’t get it right soon, we’ll never get it right.”

Organizers say it is impossible to predict how many people could show up. But 1,400 “partner organizations” have signed on, ranging from small groups to international coalitions. In addition, students have mobilized marchers at more than 300 college campuses, and more than 2,700 climate events in 158 countries are planned to coincide with the New York march, including rallies in Delhi, Jakarta, London, Melbourne and Rio de Janeiro.

In New York, organizers are expecting 496 buses from as far away as Minnesota and Kansas to bring marchers.

“The most useful gallon of gasoline anyone will ever burn is the one that gets them to the march,” Mr. McKibben said. (By contrast, all floats will be pulled by biodeisel-powered cars and trucks or by hand, organizers said.)

The forecast called for mostly sunny weather with a high temperature of 81, which would encourage a larger turnout. In February 2013, more than 40,000 protesters turned out in Washington to demand action on climate change and to challenge the contentious Keystone XL pipeline.