Leonardo DiCaprio has said climate change "is fact" as he addressed the UN Climate Summit in New York.
Over 2,000 protests have taken place around the world, including in London.
"As an actor I pretend for a living - I play fictitious characters with fictitious problems. I believe that mankind has looked at climate change in that same way, as if it were a fiction," DiCaprio told the audience.
Actress Emma Thompson says people who deny that man-made climate change is taking place are "a little bit bonkers".
Speaking to ITV News, the Harry Potter and Love Actually star also described the reliance on polluting fossil fuels as "a form of collective suicide".
Environmental protesters reportedly left a trail of rubbish through the streets of New York after marching for climate change.
The 100 leaders did not agree on concrete proposals, but the meeting has raised hopes for concrete international agreement next December.
Over 2,000 People's Climate March protests have taken place in 166 countries ahead of a major UN conference on climate change.