Sir David Attenborough dispels any doubts on global warming - Telegraph

It is likely to be a persuasive development, for until quite recently the old magician was a global warming sceptic, waiting, as he put it, “until the proof [like what, specifically?] was conclusive that it was humanity changing the climate”. Now the man who easily topped a poll to find Britain’s most trusted public figure (Peter Mandelson came bottom, so it must have been accurate) says he has “no doubt” that it is “man-made”.
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But during his next appearance – in the last episode, with something of a personal testimony – there will be no mistaking the message. Standing at the North Pole on ice just a couple of yards thick, he will warn that it is likely that “within the next few decades there will be open water here for the first time in human recorded history”. ...

The main explanation, however, seems to be the BBC’s desire to maximise sales by avoiding controversy. At the time, it boasted that Planet Earth ground “no political axe” because it was wanting “to sell to 100 countries”, and Attenborough’s episode on global warming is being marketed separately from the rest of Frozen Planet.

2000: A Warning From The North Pole

"For the first time in 50 million years, visitors to the North Pole can see something extraordinary: water.

"The thick ice that covers the Arctic Ocean at the North Pole has melted, leaving a mile-wide stretch of water at the top of the world, The New York Times reported Saturday.

"The water could be the result of global warming, although there is debate among experts about the case."