(CNN) Renowned nature broadcaster David Attenborough has told world leaders that climate change could lead to collapse of civilizations, and much of the natural world. Among the world leaders speaking at the opening ceremony, Arnold Schwarzenegger unexpectedly took to the podium, invited by Austrian President Alexander Van der Bellen. COP21, in 2015, led to the landmark Paris Agreement, a legally binding agreement to fight climate change, where countries committed to keep global warming well below two degrees Celsius above pre-industrial levels and, if possible, below 1.5 degrees. Speaking at the opening ceremony of the COP24 UN climate conference, in Katowice, Poland, Attenborough called climate change "our greatest threat in thousands of years." In the weeks leading up to the event, the UN asked people to send their thoughts on climate change. Sir David Attenborough at #COP24: "If we don't take #ClimateAction, the collapse of our civilisations and the extinction of much of the natural world is on the horizon." Attenborough was there to represent the public, by taking the "People's Seat" at the conference. He said: "The world's people have spoken. Their message is clear. Time is running out. They want you, the decision-makers, to act now," he said. Earlier in the day, UN Secretary-General Antonio Guterres called the event "the most important gathering on climate change since the Paris Agreement was signed." This Conference of the Parties (COP) is the first to be held since the landmark Intergovernmental Panel on Climate Change (IPCC) report on limiting global temperature rise to 1.5C came out in October. "This is the challenge on which this generation's leaders will be judged." "What we need is more political will and more far-sighted leadership," he added. Asked about the United States, which President Trump has said will leave the Paris Agreement, Attenborough pleaded for the US to remain committed to fighting climate change.
– Sir David Attenborough was holding no punches during an address Monday at a summit hosted by the United Nations. Per BBC, the beloved British naturalist used his time at the podium to issue a dire warning to humanity regarding climate change: "If we don't take action, the collapse of our civilizations and the extinction of much of the natural world is on the horizon." Attenborough, known worldwide for his uniquely passionate nature documentary narrations, was invited to the COP24 UN climate conference in the city of Katowice as a representative of the people of the world, many of who sent in messages that Attenborough reportedly used to inform his speech. "The world's people have spoken," Attenborough said, per CNN. "Their message is clear. Time is running out." Attenborough went on to issue a call to world leaders to take decisive action now. "The continuation of our civilizations and the natural world on which we depend is in your hands," he said. The 92-year-old specifically addressed the US after he was questioned about the Trump administration's decision to withdraw from the Paris Agreement. "Join the rest of the world," he urged the US. (Meanwhile, several US cities and at least one state have opted themselves into the worldwide accord regardless of the president's intentions.)