Two hundred actors, musicians, artists and scientists have banded together to issue a fervent plea to world leaders to act “firmly and immediately” to address climate change.

Without prompt and courageous political action, the famous group warned in an open letter published Monday in the French daily Le Monde, “global catastrophe” will be imminent.

“It is time to get serious,” the letter’s signatories declared, according to a France24 translation. “The sixth mass extinction is taking place at unprecedented speed. But it is not too late to avert the worst.”