With the direst environmental warnings yet still ringing in their ears, nations gathered in Poland Sunday for a UN summit aimed at heading off the "urgent threat" of runaway climate change.
Delegates from almost 200 countries now have two weeks of negotiations to finalise how those goals work in practice, even as science suggests the pace of climate change is rapidly outstripping mankind's response.
Mr Attenborough was speaking on behalf of the UN's "people's seat" initiative to give ordinary people a voice at the worldwide talks by gathering their thoughts, ideas and concerns through social media and polling in the past two weeks.
"Right now we are facing a man-made disaster of global scale, our greatest threat in thousands of years: Climate change", Attenborough said as the worldwide climate conference got underway with talks on how countries will implement the 2015 Paris Agreement limiting carbon emissions.
Human civilisations could collapse and many species be driven to extinction unless there is more action to tackle climate change, Sir David Attenborough has warned.
As part of the initiative, people were asked to speak up on climate change using the hashtag #TakeYourSeat. "One of the things I don't want to do is to look at my grandchildren and hear them say: 'Grandfather, you knew it was happening - and you did nothing'".
While the Polish government claims Katowice is in the process of transforming into a green city, power plant chimneys pumped plumes of smoke into a boring December sky and monitoring sites showed elevated levels of air pollution. Mr Guterres's comments underline what we are seeking to do and highlight the urgency of this work.
He added the Paris agreement was not enough and that more must be done.
Attenborough, known for hosting nature broadcasts including the popular BBC series "Planet Earth", was chosen for the UN's "people's seat", representing those populations most affected by climate change.
At his address at the climate talks yesterday, Mr Attenborough said: "The people have spoken: leaders of the world, you must lead, the continuation of our civilisations and the natural world upon which we depend are in your hands".
United Nations chief Antonio Guterres.
"Climate change is running faster than we are and we must catch up sooner rather than later, before it's too late", he said. "For many people, regions and even countries, this is already a matter of life or death".
"This is the challenge on which this generation's leaders will be judged", he said. Our greatest threat in thousands of years.
Frank Bainimarama, prime minister of Fiji and president of last year's COP, said developed nations must act now to save the planet. "This must be powered by multilateral co-operation". Simply put, three years after Paris, the world has few reasons for optimism - and this is why COP24 is so crucial.
A process to enable countries to announce efforts by 2020 to ramp up their domestic ambition on cutting greenhouse gas emissions must be launched, they said, as current efforts are not enough to prevent unsafe temperature rises.
"We are in trouble, we are in deep trouble with climate change".
It is a meeting where lots of governments are coming together to discuss what to do after a climate change report came out in October saying global temperature rises should be limited to 1.5°C.
Yet political and United Nations leaders have been struggling to inject urgency into two weeks of haggling on how to move on from fossil fuels to give practical effect to the Paris accord.