About 400,000 people packed the streets of New York City for the People's Climate March and organizers say it's the biggest climate change demonstration in history.
Marchers are pushing for world leaders and corporations to cut fossil fuel and reduce greenhouse gas emissions.
The march precedes the United Nations summit in New York where 125 world leaders, including President Obama, will meet at the U.N. to talk about climate change, It's the first time in five years that they've gotten together to do so.
Celebrities are part of the movement, too.
Leonardo DiCaprio was named U.N. messenger of peace for his environmental work.
Between DiCaprio, activists, and world leaders, sounds like one big traffic jam in the Big Apple.