Dissolved carbon dioxide (CO2) makes water more acidic. Natural carbon dioxide vents on the sea floor are showing scientists how carbon emissions will affect marine life. Around the vents, researchers saw a fall in species numbers, and snails with their shells disintegrating.
Writing in the journal Nature, the UK scientists suggest these impacts are likely to be seen across the world as CO2 levels rise in the atmosphere.
Some of the extra CO2 emitted enters the oceans, acidifying waters globally.
Studies show that the seas have become more acidic since the industrial revolution.
Research leader Jason Hall-Spencer from the University of Plymouth said that atmospheric CO2 concentrations were now so high that even a sharp fall in emissions would not prevent some further acidification.
"It's clear that marine food webs as we know them are going to alter, and biodiversity will decrease," he told BBC News.
"Those impacts are inevitable because acidification is inevitable - we've started it, and we can't stop it."