The world’s oceans are in worse health than realised, scientists have said today, as they warn that a key measurement shows we are “running out of time” to protect marine ecosystems.

Ocean acidification, often called the “evil twin” of the climate crisis, is caused when carbon dioxide is rapidly absorbed by the ocean, where it reacts with water molecules leading to a fall in the pH level of the seawater. It damages coral reefs and other ocean habitats and, in extreme cases, can dissolve the shells of marine creatures.

However, a new study by the UK’s Plymouth Marine Laboratory (PML), the Washington-based National Oceanic and Atmospheric Administration and Oregon State University’s Co-operative Institute for Marine Resources Studies found that ocean acidification’s “boundary” was also reached about five years ago.

The study drew on new and historical physical and chemical measurements from ice cores, combined with advanced computer models and studies of marine life, which gave the scientists an overall assessment of the past 150 years.

It found that by 2020 the average ocean condition worldwide was already very close to – and in some regions beyond – the planetary boundary for ocean acidification. This is defined as when the concentration of calcium carbonate in seawater is more than 20% below preindustrial levels.

The deeper in the ocean they looked, the worse the findings were, the scientists said. At 200 metres below the surface, 60% of global waters had breached the “safe” limit for acidification.

“Most ocean life doesn’t just live at the surface,” said PML’s Prof Helen Findlay. “The waters below are home to many more different types of plants and animals. Since these deeper waters are changing so much, the impacts of ocean acidification could be far worse than we thought.”