Man-made carbon emissions since the industrial revolution have acidified the oceans far beyond natural levels, research suggests.
Ocean acidity makes it harder for organisms such as molluscs and coral to construct the shells they need to survive.
In some regions, acidity rose faster in the last two centuries than it did in the previous 21,000 years, a study has shown.
Measuring changes in ocean acidity is difficult because it varies naturally between seasons, years and regions. Direct observations only date back 30 years, not long enough to reveal a meaningful trend.
The new research used simulations of ocean and climate conditions going back 21,000 years to the Last Glacial Maximum and forward in time to the end of the 21st century.
Scientists looked at changes in the saturation level of aragonite, a form of calcium carbonate used to measure ocean acidification. As seawater acidity rises, the saturation level of aragonite falls.
In several key coral reef regions aragonite saturation is already five times below its lowest pre-industrial range, according to the model.
This translates to a decrease in overall calcification rates of corals and other shell-forming organisms of 15%, scientists believe.
They fear calcification rates of some marine organisms could drop by more than 40% of their pre-industrial levels within the next 90 years.
"Any significant drop below the minimum level of aragonite to which the organisms have been exposed to for thousands of years and have successfully adapted will very likely stress them and their associated ecosystems," said Dr Tobias Friedrich, from the University of Hawaii, who led the study published in the journal Nature Climate Change.