The 93-year-old veteran TV presenter said “life-or-death decisions” now needed to be made to save the planet.
The renowned naturalist said action has been put off “year after year” but there is no more time because “human beings have overrun the world”.
His comments came as the BBC launched a year of special coverage of climate issues.
Sir David said: “We have been putting things off year after year, we have been raising targets and saying ‘Oh well, if we do it within the next 20 years…’
“The moment of crisis has come, we can no longer prevaricate.
“We can’t go on saying ‘but there’s hope’ and leave it to next year – we have to change and we have to change not by appeals to different kinds of optimism but to deliberate, compelling, life-or-death decisions.”
Sir David hailed what he called a huge change in public opinion towards tackling climate change, in particular among young people which he hoped would force governments to take action.
He added: “We have to realise that this is not playing games, this is not just having nice little debates and arguments, then coming away with a compromise.
“This is an urgent problem that has to be solved.
“And, what is more, we know how to do it – that’s the paradoxical thing, that we are refusing to take steps that we know have to be taken, and every year that passes makes those steps more and more difficult to achieve.”
The BBC’s “most ambitious year” of covering the impact of climate change will include broadcasting documentaries, podcasts and debates under the Our Planet Matters banner.
As part of the project, BBC News will introduce new programmes across its channels, including Climate Check for BBC Weather, a weekly global climate podcast from BBC World Service and a show with presenter Simon Reeve exploring the Amazon rainforest.
Sir David used a trailer for his new film, David Attenborough: A Life On Our Planet, to warn that “human beings have overrun the world”. But he also explores potential solutions to climate change.