Robert Redford told the United Nations on Monday that negotiations on a global deal to tackle climate change could be the world's "last chance" to save the planet.
"This December, the world must unite behind a common goal," said the American actor and producer.
"Because look, this is it. This is our only planet, our only life source.
"This may be our last chance."
World governments will try to forge a new global accord to address climate change at a UN climate conference in Paris in December, with both developed and developing countries committing to cutting greenhouse gas emissions.
Read: The climate change disaster
The landmark agreement would limit global warming to 3.6 degrees Fahrenheit (two degrees Celsius) from pre-industrial revolution levels as of 2020.
"We are all responsible for this crisis," Redford told the gathering. "Your mission is as simple as it is daunting: save the world before it's too late."
Redford pointed to global warming fueling extreme weather such as the deadly heatwaves in India and Pakistan that have claimed thousands of lives.
"Everywhere we look, moderate weather seems to be going extinct," said Redford, who starred in such classic films as "Butch Cassidy and the Sundance Kid" and founded the Sundance Film Festival.
"Unless we move quickly away from fossil fuels, we're going to destroy the air we breathe, the water we drink, and the health of our children, grandchildren and future generations."
Read: Climate change: Global warming behind erratic weather patterns
Redford told the gathering that he was an "actor by trade" but an "activist by nature" and that his convictions on the need for action to save the environment had grown over the past 40 years.
The 78-year-old celebrity recalled that he had invited mayors from key cities to Sundance to galvanize action to stop global warming and that the local leaders were ready "for action now."
Redford urged world governments to "assume a leading role" to combat climate change at the Paris conference, adding that the "citizens of the world will be watching."
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