The rate at which the oceans are becoming more acidic is greater today than at any time in tens of millions of years, according to a new study.

Rapidly rising concentrations of carbon dioxide in the atmosphere mean that the rate of ocean acidification is the fastest since the age of the dinosaurs, which became extinct 65m years ago, scientists believe.

The oceans are likely to become so acidic in coming centuries that they will become uninhabitable for vast swathes of life, especially the little-studied organisms on the deep-sea floor which are a vital link in the marine food chain.

Scientists have concluded, in a study published today in the journal Nature Geoscience, that the current rate of ocean acidification is up to 10 times faster than 55m years ago – the last time the deep oceans became so acidic.

This is because of the speed at which carbon-dioxide concentrations are rising in the atmosphere. This carbon dioxide dissolves in seawater at the sea surface to form carbonic acid. The increased acidity of the water affects the amount of dissolved carbonate minerals that are available for marine organisms to use in forming their shells and hard skeletons.

When the oceans became acidified in a similar way about 55m years ago, it resulted in a mass extinction of deep-sea marine organisms, especially those living in the sediments of the sea floor, which can be studied geologically through changes to rock formations, said Dr Andy Ridgwell of the University of Bristol.

"Unlike surface plankton dwelling in a variable habitat, organisms living deep down on the ocean floor are adapted to much more stable conditions. A rapid and severe geochemical change in their environment would make their survival precarious," he said.

Studies also suggest that temperatures of the surface ocean rose, and carbon-dioxide levels increased over a period of a few thousand years.