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.
Global emissions of carbon dioxide are set to rise again this year to reach a new record high, scientists have warned.
Despite increasingly urgent warnings over the need to curb greenhouse gases to avoid "dangerous" climate change, emissions are estimated to be rising by 2.5% this year, to a record 40 billion tonnes.
The stark warning that time is running out to tackle greenhouse gas emissions comes ahead of a major UN conference on climate change on Tuesday.
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.