A network of ocean sanctuaries protected from human activity is needed to help the seas store carbon and preserve nature, campaigners have urged.

A report from Greenpeace highlights the role of the oceans as the world's largest "carbon sink", storing large amounts of the carbon dioxide put into the atmosphere by human activity, as well as heat as the planet warms.

But the continued reliance on burning fossil fuels and the carbon dioxide emissions it causes has led to ocean heating, sea level rises and increasingly acidic seawater - with rapid and large-scale impacts on people and wildlife.

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The changing climate and loss of marine wildlife and habitats are threatening the processes that allow the oceans to store carbon, the report warns.

There is a need to drastically cut emissions to curb climate change, Greenpeace said.

At the same time, a network of ocean sanctuaries, off limits to human activity, would help marine areas be more resilient to the changing climate and protect "blue carbon" stores.

These stores of carbon in the seas are the largest on Earth, containing 50 times the amount of carbon in the atmosphere and 10 times the amount in land-based vegetation, soils and microbes combined, Greenpeace said.

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The campaign group is calling on governments to agree a global ocean treaty in 2020 to safeguard the world's oceans, and to commit to protecting at least 30% of the oceans by 2030 when they meet at a key nature conference next year, the UN Convention on Biological Diversity in Beijing.

The poles, whale hotspots, coral reefs, mangroves and seagrass meadows should be among the priorities for protection, the report said.

Louisa Casson, an oceans campaigner at Greenpeace UK, said: "The climate crisis is an ocean crisis.

"The ocean is fundamental to keeping our planet healthy, but for too long we have treated marine protection as a separate issue to the climate crisis.

"We must protect our ocean, the biggest carbon sink on earth, and stop burning fossil fuels."

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And she said: "From mangroves and seagrass meadows, to whales and krill, the ocean's biology is one of our best allies in the fight against climate change.

"But over-exploitation and our addiction to fossil fuels has pushed our ocean to the brink of collapse.

"We need urgent, concerted action on a global scale to properly protect at least 30% of our ocean and safeguard its vast stores of blue carbon.

"Ocean protection is climate action, and if we can save our ocean, it can save us."