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Sir David Attenborough turns 99 on Thursday and the best birthday present we can collectively give the great man is by going to the cinema to see his powerful new feature-length documentary, Ocean.

The film has its world premiere tonight at the Royal Festival Hall: an apt venue in a week of festivities commemorating VE Day 80 years ago.

But while in every news bulletin his surviving contemporaries are being asked to talk about the past, Attenborough wants only to look ahead. The older he gets, the more invested he seems to be in the future.

In a way, Ocean unfolds like a party political broadcast on behalf of the planet. Attenborough stresses that we can still counter the effects of climate change and diminishing biodiversity – and the key lies below the waves.

Humanity’s potential salvation, improbable as it sounds, rests with the vast armies of ocean plankton, ‘the most abundant animal on earth’. They absorb about a third of all carbon dioxide emissions and produce more oxygen than all the world’s trees combined.

But as in all the best cinematic thrillers, Ocean dangles a lifeline while threatening to snatch it away. There’s a villain in this film.

It’s the deep-sea fishing industry, and in particular those enormous trawlers and dredgers that scrape all living things off the ocean bed. An entire eco-system is then boiled and processed to produce health supplements and pet food. And ‘all on board ship,’ adds Attenborough, with the nearest he ever gets to a sneer.

As always, his mighty age and experience, combined with that air of almost papal benevolence, imbue his every pronouncement with complete authority.

When he declares that our understanding of the sub-oceanic world has been transformed in his lifetime, there is supporting black-and-white footage of him scuba-diving off the Great Barrier Reef in 1957, when he was already in his 30s and so taken aback by the spectacle ‘that I forgot, momentarily, to breathe’.

Yet this film is not typical of his astonishing TV series, the likes of Planet Earth and the Blue Planet.

Ocean is full of spectacular underwater camerawork (co-directors Toby Nowlan, Colin Butfield and Keith Scholey all deserve a name-check, and Prince Albert of Monaco, eye-catchingly, is among the executive producers), but unlike most Attenborough documentaries it does not linger on pouncing predators or mysterious mating rituals.

Only occasionally, his customary enthusiasm undimmed, does he introduce us to fascinating creatures. For instance to the peacock mantis shrimp, with its fabulously complex eyes that can detect UV light and rotate independently like angle-poise lamps.

Or to boxer crabs, which for self-defence use clumps of venomous anemones as gloves, and look so much like pugilists limbering up for a fight that really there should be a promoter, a crustacean Frank Warren, standing just behind them.

On the whole, however, everything we see is to bolster Attenborough’s passionate message, namely that if the world’s nations, meeting next month at the UN Ocean Conference, can agree to create enough fully protected marine reserves, then humankind might be on to a winner.

What fills him with hope, he explains, is the increasing evidence that our seas and oceans and everything in them can recover more quickly than previously he dared to think.

The few areas where intensive trawler-fishing has been stopped now positively overflow with marine life, and there’s a tiny protected zone of the Mediterranean, the most over-fished of all waters, which suggests even that can be reclaimed.

None of this, he is keen to point out, imperils the fishing industry. It can continue to thrive, but must be better regulated.

In the meantime, populations of the giant black sea bass, once thought extinct, have rebounded. And nothing feeds Attenborough’s optimism like the fate of the whale.

Hunted virtually to extinction before commercial whaling was banned in 1986 – with almost three million killed in the 20th century alone - almost every species has made an astonishing comeback.

He vividly recalls thinking that it was curtains for the blue whale. But the largest animal ever to have existed has bounced back, and it’s a heck of a bounce.

There are blue whales being born today that will likely live for more than 100 years. Let us all hope that Attenborough matches and exceeds that tally. I fancy he still has plenty more to say.

Ocean with David Attenborough opens in cinemas Thursday