He warned world leaders: "We have left the stable and secure climatic period that gave birth to our civilisations."
"There is no going back - no matter what we do now, it's too late to avoid climate change and the poorest, the most vulnerable, those with the least security, are now certain to suffer."
He said wealthy nations have a duty to do all they can to help those in the most immediate danger.
Despite saying it is already too late to tackle some climate change, Attenborough told leaders "I think there are grounds for hope".
"While it’s true we can never go back to the stable benign climate that enabled us to flourish for the past 10,000 years, I do believe that if we act fast enough, we can reach a new stable state."
Boris Johnson opened the first UN leader-level summit, and described the threat climate change posed to global security as of "paramount importance".
He added: "Whether you like it or not, it is a matter of when, not if, your country and your people will have to deal with the security impacts of climate change."