Over the weekend, 19 G20 member states reaffirmed their commitment to the Paris climate accord - except for the U.S., which pulled out of the landmark agreement a year ago.
Meanwhile, the presidents of at-risk states such as Fiji, Nigeria and Nepal are expected at COP24 talks, which aim to flesh out the promises agreed to in the 2015 Paris climate accord.
The conference, attended by 30,000 delegates from 196 countries including Ireland, is being held in Katowice, in the heart of Poland's coal-mining region.
"America is more than just Washington or one leader", he said, adding that he wished he could travel back in time - like the cyborg he portrayed in "The Terminator" - to stop fossil fuels from being used.
Recent studies have shown that 20 of the past 22 years have been the warmest in recorded history, and climate change action needs to be increased fivefold if we want to have a chance to avoid catastrophic warming, which would cause permanent and irremediable damage to both human and natural environments.
"Katowice (Poland) may show us if there will be any domino effect", from the USA withdrawal, Laurence Tubiana, CEO of the European Climate Foundation and an architect of the Paris agreement, said in the AFP report.
"Right now, we are facing a man-made disaster of global scale".
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"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.
The UN chief asked decision makers to focus on four key things: stepping up climate action, according to a solid plan, with more funding, as a smart investment in future of the planet.
But Polish President Andrzej Duda later told a news conference the coal-rich country would reduce its reliance on coal but would never entirely give up its "strategic fossil fuel".
If the world is to truly prevent climate change-and the extreme weather disasters and rising seas that accompanies it-the attendees of the United Nations climate conference will have to disagree with Duda. Attenborough was there to represent the public, by taking the "People's Seat" at the conference.
Stating that "we have no time for limitless negotiations", the Secretary-General insisted on the need to operationalise the Paris Agreement, and reminded Member States that 2018 is the deadline that they set for themselves to finalise the guidelines for implementation.
Delegates from almost 200 countries now have two weeks of negotiations to finalize how those goals work in practice, even as science suggests the pace of climate change is rapidly outstripping mankind's response.
"We achieved success in Paris because negotiators were working toward a common goal", and called for urgent collaboration to "ensure that the bonds of trust established in Paris will endure".
Of the 10 countries in the world considered most threatened by climate change, seven are in Africa - Central African Republic, Chad, Eritrea, Ethiopia, Nigeria, Sierra Leone and South Sudan.