Ocean acidifcation is taking place at a faster rate than has been seen in the last 65 million years - and it's still accelerating.
It could potentially exceed the rate at which plankton can adapt, say researchers from the University of Bristol.
The team applied a model that compared current rates of ocean acidification with the greenhouse event at the Paleocene-Eocene boundary, about 55 million years ago.
During this period, surface ocean temperatures rose by around 5-6°C over a few thousand years. Surface ecosystems, such as plankton, survived, but bottom-dwelling organisms in the deep ocean experienced a major extinction.
Dr Andy Ridgwell, lead author on the paper, said: "The widespread extinction of these ocean floor organisms during the Paleocene-Eocene greenhouse warming and acidification event tells us that similar extinctions in the future are possible."
The oceans are currently absorbing about a quarter of the CO2 released into the atmosphere, forcing the pH of the surface ocean lower.
Lab experiments suggest that if this continues, effects could include the dissolution of carbonate shells of marine organisms, slower growth, muscle wastage and dwarfism, with knock-on effects throughout the ecosystem.
On the basis of their simulations, the authors believe plankton will be unable to adapt.
TThe occurrence of widespread extinction of deep-sea organisms during the Paleocene-Eocenegreenhouse warming and acidification event raises the possibility of a similar extinction in the future, they say.