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You're probably used to hearing Sir David Attenborough's sonorous, British voice describe the miracles of pufferfish courtship and blooming stink flowers in nature documentaries like "Planet Earth" and "Blue Planet." But today (Dec. 3), the naturalist and filmmaker delivered a far more somber monologue at the United Nations Climate Summit in Katowice, Poland.

"Right now, we're facing a man-made disaster of global scale," Attenborough told delegates from almost 200 nations. "Our greatest threat in thousands of years: 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 was chosen to speak at the summit as part of the U.N.'s new "people's seat" initiative, which encouraged citizens of the world to share their personal messages and videos explaining how climate change has already affected their lives. Several of these messages were shared as part of Attenborough's speech today; they included footage of people standing in front of the ashen remains of their homes, which had been incinerated by wildfires. [6 Spectacular Species Named for David Attenborough]

"The world's people have spoken," Attenborough said. "Their message is clear. Time is running out. They want you, the decision-makers, to act now."

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This meeting of the U.N. was convened so that leaders of the world could negotiate ways to turn their pledges made at the 2015 Paris climate accord into a reality. Per the Paris accord, 184 countries agreed to implement emissions-reduction policies to help limit global warming to less than 2 degrees Celsius (3.6 degrees Fahrenheit) above preindustrial levels over the next century. Most of the world's nations are not on track to meet this goal; in fact, a global temperature rise of 4 degrees C (7.2 degrees F) seems far more likely right now.

According to a recent U.N. climate report, even limiting warming to 1.5 degrees C (2.7 degrees F) this century could result in serious consequences for the planet's cities and ecosystems. Those effects include increased flooding and severe weather around the world, the destruction of up to 90 percent of the ocean's coral reefs, mass animal extinctions, and food shortages brought on by regular droughts. A recent U.S. climate assessment, released quietly over Thanksgiving weekend by President Donald Trump's White House, affirmed these findings and the impending danger of climate change.

"Leaders of the world, you must lead," Attenborough concluded. "The continuation of our civilizations and the natural world upon which we depend is in your hands."

Originally published on Live Science.