Greenhouse gas emissions from the burning of fossil fuels have pushed the acidity of the world's oceans past a safe threshold, scientists warn, threatening their ability to sustain shellfish and corals and help us in the fight against climate change.

A new report says that ocean acidification is the latest "planetary boundary" to be crossed, a reference to a set of warning signs related to key planetary systems that keep the Earth safe for human civilization.

Other planetary boundaries that have already been crossed — including dangerous levels of chemical pollution, the warming atmosphere and changes to the nutrient cycle — have already signalled threats to people.

"Go outside of these boundaries and you first enter a danger zone, with higher risk of causing changes that would undermin

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