Taking the so-called “People’s Seat” at the conference, Attenborough told global leaders that climate change is “our greatest threat in thousands of years.”

“The world’s people have spoken,” he said. “Their message is clear. Time is running out. They want you—the decision makers to act now.”

“Leaders of the world you must lead,” he added. “The continuation of our civilizations and the natural world on which we depend is in your hands.”

Calling climate change “a man-made disaster of global scale,” Attenborough added, “if we don’t take action, the collapse of our civilizations and the extinction of much of the natural world is on the horizon.”

According to a recent UN report, 2018 is shaping up to be the fourth hottest year on record, after the three previous years. COP, or the Conference of Parties, meetings are intended to bring the world’s leaders together to discuss tackling climate change. The Paris Agreement was created during COP21 in 2015.

While many countries are well on their way to meeting goals of that agreement, several larger countries are not in attendance at this year’s conference, including the U.S., which withdrew from the agreement under President Donald Trump.

The UN predicts that in order to contain global warming at 1.5°C, man-made global net carbon dioxide emissions will need to fall by about 45% by 2030 from 2010 levels and reach “net zero” by 2050.

Speaking to Thomson Reuters Foundation at the conference, Attenborough expressed disappointment in Trump’s decision to withdraw from the Paris Agreement, saying “it shouldn’t affect us and the whole of the rest of the world.” He nonetheless expressed hope that something will change.

“We have all been living beyond our means. It’s a perfectly simple thing,” Attenborough further explained. “We have let down the young generation, and they know it, and they are angry.”