The veteran broadcaster, 95, says in a short video message it is "crucial" global leaders find a solution to environmental issues, which will become "greater" in the next few years.
The UN's Cop26 climate conference is due to be held in Glasgow this November, aiming to bring together world leaders to agree a plan to tackle climate change.
Sir David has been named the summit's 'People's Advocate' and will make an address to political leaders about why immediate climate action is important.
In the video message, Sir David said: "I am greatly honoured to be given the role of People's Advocate. There could not be a more important moment that we should have international agreement.
"The epidemic has shown us how crucial it is to find agreement among nations if we are to solve such worldwide problems. But the problems that await us within the next five-10 years are even greater.
"It is crucial that these meetings in Glasgow, Cop26, have success, and that at last the nations will come together to solve the crippling problems that the world now faces."
He will also make a speech at the G7 leaders' summit in Cornwall next month, where environmental issues will be at the heart of discussions.
Prime Minister Boris Johnson said: "Sir David Attenborough has already inspired millions of people in the UK and around the world with his passion and knowledge to act on climate change and protect the planet for future generations.
"There is no better person to build momentum for further change as we approach the Cop26 climate summit in November. I am hugely grateful to Sir David for agreeing to be our People's Advocate."
Cop26 president-designate, Alok Sharma, said: "Climate change is the greatest threat facing humanity and the stakes could not be higher for our planet.
"The next decade will be make, or break, for cutting global emissions sufficiently to avoid the worst effects of climate change.
"That is why I am delighted to be working with Sir David, a hero for our country and our planet, to inspire action ahead of Cop26."