Pollution in our oceans has reached crisis point.

According to renowned naturalist Sir David Attenborough, the wildlife in our seas is facing its greatest threat in human history as it struggles to survive against warming temperatures and unprecedented quantities of plastic waste.

Attenborough makes this warning in the final episode of the BBC documentary series Blue Planet II, which is the UK’s most watched TV programme this year.

Warming oceans and waste threaten sea creatures’ survival. Image: BBC

It is also proving popular around the world. An estimated 80 million people in China watched the first two episodes of the series.

So many Chinese tried to stream the first episode via the online Tencent Video service that it reportedly temporarily slowed China’s internet.

The final episode focuses on the damage being done to the world’s oceans by climate change, plastic waste, overfishing and even noise.

Sir David Attenborough warns the oceans are at crisis point. Image: REUTERS/Hannah McKay

Attenborough says: “For years we thought the oceans were so vast and the inhabitants so infinitely numerous that nothing we could do could have an effect upon them. But now we know that was wrong.

“It is now clear our actions are having a significant impact on the world’s oceans. [They] are under threat now as never before in human history. Many people believe the oceans have reached a crisis point.”

Climate threat

Coral reefs could be killed off by rising sea temperatures. Image: BBC

One of the environmental challenges highlighted by the programme is how global warming is changing the very nature of the oceans.

Recent research suggests that more than half the world’s oceans could suffer these multiple effects of rising carbon dioxide levels over the next 15 years. By mid-century it is possible that more than 80% of oceans could be affected, forcing their inhabitants to migrate, adapt, or in some cases, become extinct.

The final episode focuses on the phenomenon of coral bleaching.

Global warming is starving coral of their food. Image: National Ocean Service

The algae that act as a food source for corals, and give them colour, leave the corals when water temperatures rise.

The corals turn white and are more susceptible to disease and death.

Many corals around the world, including two-thirds of Australia’s Great Barrier Reef, are already affected by bleaching.

Other effects of warming sea beds include the destruction of vital seagrass meadows, kelp beds and mangrove forests, which can store vast amounts of carbon. In 2015 and 2016 the worst instance of mangrove forest die-off ever recorded occurred off the Australian coast.

Walrus mothers struggle with melting ice in the Arctic.

And the first episode of Blue Planet II highlighted how warming seas are affecting wildlife in the Arctic.

It showed how there was less and less ice to go around for walrus mothers struggling to find a safe haven for their pups.

More plastic than fish

Another major threat to the oceans is the sheer amount of plastic waste clogging up our seas.

Previous research by the World Economic Forum and Ellen MacArthur Foundation estimates that by 2050 there could be more plastic than fish in the sea.

Some footage of albatross chicks choking and dying after their mother feeds them plastic by mistake was deemed too stressful and shocking to air on the final episode of Blue Planet II.

It is a widespread problem. According to one study, 71% of seabirds and 30% of turtles have been found with plastics in their stomachs.

When they eat plastic marine animals have a 50% chance of dying.