Blue Planet II is an environmental emergency disguised as a nature documentary

 “The health of our oceans is under threat. They’re changing at a faster rate than ever before in human history.”

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A year on from the successes of Planet Earth II, the BBC’s naturalist-in-chief Sir David Attenborough returns with Blue Planet II – a series four years in the making that dares to take its viewers into the uncharted frontiers of the oceans.

Significant advances in technology since 2001 when The Blue Planet aired, Attenborough tells us, means “we can enter new worlds and shine a light on behaviours in ways that were impossible just a generation ago.”

Low-light cameras are deployed to film mobula rays in a fantasia-like dance with bioluminescent-blue plankton in the Mexican Sea of Cortez. Suction cameras offer a “giant’s-eye-view” of the ocean. Armed with this technology, the filming crew's 6,000 hours spent submerged have successfully unveiled phenomena of the seas previously unseen.

Although the series has been accused of dishonesty for creating a few scenes under lab conditions, the vast majority of what is seen onscreen is the result of tireless work by the production team to bring us - the viewers - closer to marine life than we have ever been. This is of course something that the Into The Blue behind-the-scenes clips at the tail end of each episode serve to remind us of (the first instalment of this segment sees the production team ride 20ft waves on jet skis to swim and film alongside dolphins on the coast of South Africa).

However, the more pressing concern for Attenborough, the “uncomfortable fact”, recognised and raised in the opening moments of episode one is this: “The health of our oceans is under threat. They’re changing at a faster rate than ever before in human history.”

The first three episodes of Blue Planet II take us across oceans, sea floors and coral reefs, while offering glimpses into the lives of sea creatures both familiar and alien. On Australia’s Great Barrier Reef, we meet a determined little tusk fish that has learned to use coral as a tool; in the underwater forests of northern Japan, we witness a female kobudai change gender; in the twilight zone, a sea of “eternal gloom” 200m down in the Pacific, strange creatures appear such as the barreleye – a deep-sea fish with a transparent head filled with jelly so that it can look above.

But it is here, in the furthest reaches of the ocean, that Attenborough’s concerns become apparent. The second episode introduces us to Venus’s flower basket, a delicate, translucent white sponge that lives at the bottom of the sea. It hosts shrimps that use the sponge as a protective nest, hatching and dispersing their larvae through the flower basket’s walls. It’s a symbiotic marvel which is soon overshadowed by Attenborough’s commentary: “But today, their timeless world is being reduced to rubble.”

Overfishing has emptied the surface seas, leading trawlers to cast their nets far into the deep. According to the United Nations, fishing subsidies are accelerating the depletion of many of the ocean’s species, with “as much as 40 per cent of the world oceans” being devastated by human activities.

Blue Planet II documents the life of the deep blue greyed out, as a despondent Attenborough tells us “countless numbers of the reefs that have flourished for millennia lie in ruins”. When the very bedrock of the planet is tainted by the excessiveness of human consumption and activity, a moment of pause is needed.

In the latest episode to be aired, which focuses on coral reefs, a family of saddleback clownfish are searching for a sturdy item for their female leader to lay her eggs when they encounter an old plastic water bottle. It’s a fleeting moment in the series - the clownfish dismiss the solitary bottle as an inanimate, throwaway part of the reef debris. But with estimates of more than 5 trillion plastic pieces, “weighing over 250000 tons” afloat at sea, it isn’t difficult to see plastic minefields disrupting hundreds of thousands of underwater species.

The seas have not escaped the impact of global warming either. We are told in the same episode that half the world’s coral reefs have been affected by bleaching in recent years, including since 2016 “two thirds of the shallow water corals” on Australia’s Great Barrier Reef. Sustained increases in temperature cause coral polyps to eject plant-like cells that lead to the discolouring.

Lab footage of coral bleaching, which could only be filmed with careful lighting and cinematography, give us a close look at the future we could be facing – one in which coral reef life is left homeless after these “crowded submarine cities are reduced to bleak ruins” by human-driven climate change.

In the Arctic, ice has been reduced by 40 per cent in the last 30 years, with the walruses starring in Blue Planet II taking the hardest hit. In need of a safe retreat, these walruses can no longer rely on sheets of ice as sturdy resting spots and instead must fight for space on land or floating, diminutive rafts of ice.

These short moments interrupt the magic with serious reminders of the state of the planet’s waters. Last year’s Planet Earth II drew in viewers who marvelled at the beauty of animals living in and around our jungles, mountains and deserts. Blue Planet II’s first episode surpassed more than 14 million views, and the remaining episodes expected to follow suit. Yet its reflections on the relationship between humans and sealife subtly transform it from a classic nature documentary to an environmental one.

Of course, the series will bring light and wonder to its audience, with a Hans Zimmer score threading together stunning footage of surreal sealife. But when it is our very own actions that are damaging our oceans, the call to action is apparent.

Blue Planet II is on BBC1, Sundays 8pm.