Like many of the animals he observes, David Attenborough is a creature of habit. For half a century he has marked out his territory in natural history films with a remit to explain what he calls “the glory of life”. Heavy sermonising is not his way. A leopard does not change its spots. Its cough is discreet.

Admiration for the veteran broadcaster, 80 earlier this month, has been tempered by chiding voices of late. An estimated 1 billion people have seen his programmes, so why, ask critics, can’t this most mesmerising of presenters use his platform to more outspoken effect? They thought he could have made the green message more explicit in his last series, Planet Earth.

This week we shall see a different Attenborough. He goes critical, assuming the mantle of a wrathful prophet as he enters the battle for the planet against climate change.

Attenborough had remained silent on the subject of global warming during the debate on its validity. “I was very sceptical,” he admits. His outlook changed when climatologists showed him graphs linking the increase of carbon dioxide in the atmosphere with rising temperatures.

“I was absolutely convinced this was no part of a normal climatic oscillation which the Earth has been going through and that it was something else,” he says.

The result of his conversion is a two-part BBC1 documentary starting on Wednesday as part of the corporation’s Climate Chaos season, in which he looks at the future impact of global warming and discovers what steps can save the planet from dramatic change. It is another luminous production by the BBC’s natural history unit, but this time infused with a stark warning.

Attenborough discovered a compelling reason for sounding the alarm. “How could I look my grandchildren in the eye and say I knew about this and I did nothing?” According to colleagues, he also feels a strong public obligation. “He’s very aware of the trust people hold in him,” says one.

I put this to Attenborough, described recently as the most trusted man in Britain after Rolf Harris. The label sends him into a paroxysm of laughter that leaves him gasping: “Quite so . . . thank you . . . I don’t think I need to say any more.”

But he does, veering off to blame himself for his part in the parlous state of the planet. “We are now realising the consequences of the things which we did: things that I did as a boy, things my parents did,” he begins. What can he mean? Yes, burning fires.

“The carbon from the open fire that my parents burnt is still up in the atmosphere and will remain there for 100 years. Absolutely innocently and unwittingly over my lifetime and my parents’ lifetimes, we have been stacking up and thickening the carbon dioxide layer. We didn’t know but now we do. No one could blame my parents for having a coal fire but they could blame me.”

Attenborough agrees there is little, “if anything”, we can do to reverse this backlog of carbon dioxide for the next 100 years. So what does he think of the assertions of Bjorn Lomborg, the Danish academic who says we should resign ourselves to a temperature increase of 2C over the next century, by which time a replacement will have been found for fossil fuel?

While acknowledging that a new energy source is “a real possibility”, Attenborough takes issue with Lomborg. “If we don’t take stock now, and even if we get to this paradisiacal situation of having consequence-free energy, the carbon dioxide ‘tanker’ will still go sailing on for another 100 years.”

The new BBC season is distinctive for the way it shows a whole range of climate indicators, from the examination of anaesthetised polar bears that are declining in numbers to climate modelling, all told by the top scientists in their field.