Human emissions of carbon dioxide are increasing the levels of acidity in the oceans at rates not seen for millions of years, scientists say.
The world's oceans are becoming acidic at an "unprecedented rate" and may be souring more rapidly than at any time in the past 300 million years.
In their strongest statement yet on this issue, scientists say acidification could increase by 170% by 2100.
They say that some 30% of ocean species are unlikely to survive in these conditions.
The researchers conclude that human emissions of CO2 are clearly to blame.
The study will be presented at global climate talks in Poland next week.
In 2012, over 500 of the world's leading experts on ocean acidification gathered in California. Led by the International Biosphere-Geosphere Programme, a review of the state of the science has now been published.
This Summary for Policymakers states with "very high confidence" that increasing acidification is caused by human activities which are adding 24 million tonnes of CO2 to oceans every day.