Sir David Attenborough may be 91, but he is a busy 91. As we set up in his Richmond home, he is upstairs studying footage of orcas and humpback whales on a herring hunt. He has just come back from Edinburgh. And last night he was up late writing the latest programme for his new series: Blue Planet Two.

Sixteen years on from the first Blue Planet series, Attenborough is both delighted and saddened by his return to the oceans.

“It wasn’t until the 50s that I first got put on an aqualung, but when you do – here is the richest, the most diverse, the most beautiful, the most exciting, the least known of all earth’s ecosystems.”

The programme he has been writing is about how the oceans are changing. One change he has noticed is the plastic. Lots of it.

There’s a shot of the young being fed, and what comes out of the beak of the adult? Plastic. It’s heart-breaking.

“Plastics are of crucial importance. It’s heart-breaking of course. Which example do you choose as being the most heart-breaking? There are so many of them.”

“The one I would choose because I feel most strongly for them…is the albatross. Such marvellous birds! They form partnerships for 50 years, they circle Antarctica searching for food, they come back to their mates in the same place, but they also feed their young.”

“There’s a shot of the young being fed, and what comes out of the mouth of the beak of the adult? Not sand eels, not fish, not squid…it’s plastic. It’s heart-breaking. Heart-breaking.”

It’s a topic Attenborough was keen to return to at the documentary’s premiere in London on Wednesday.

The prequel is out now – with a soundtrack from Hans Zimmer and Radiohead.

The naturalist used the opportunity of the premiere to call for action.

Asked what concerned him the most with regard to the health of the oceans, Attenborough chose to highlight two challenges: climate change and plastic pollution.

He said: “We could actually do something about plastics right now….internationally, tomorrow. I just wish we would….

“We have a responsibility, every one of us. We may think we live a long way from the oceans …but what we actually do here – and in the middle of Asia – has direct effect on the oceans and what the oceans do affects back on us. It is one world and it is in our care. For the first time in 500 million years, one species has the future in the palm of its hands. I just hope we realise that that is the case.”

The series, which has been four years in the making, visits every continent and ocean. It promises new filming techniques, from probe cameras that can capture life in miniature to suction cameras that sit on the backs of sharks.

“I’m going to have to say we’ve got new techniques and new technologies and we’re going to places we’ve never been before. It’s true to an extent but it’s not what it’s about. What it’s about is that life underwater is amazing.”