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.
The Prime Minister is joining more than 120 world leaders, including US president Barack Obama, today at a summit convened by United Nations secretary general Ban Ki-Moon with the intention of restoring momentum to the international battle against climate change.
Downing Street said Mr Cameron will press for strong and early commitments from the international community ahead of next year's Paris conference, when a new global deal on action to limit climate change is planned.
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.