The world's oceans absorb about a quarter of all carbon dioxide emitted by humans each year and it is making the water so acidic it could start dissolving some cold water corals, according to scientists.
The oceans are acting as a giant storage locker for the main gas causing global warming, but at a cost to all marine life, said a report from the European Project on Ocean Acidification.
More acidic seas could weaken shells and damage creatures that build them, block chemicals that fish use to find their homes and make life noisier for dolphins as some sounds travel better in water that has soaked up carbon - among other effects researched by scientists in recent years.
"It is a global phenomenon that will be felt hardest and first in the polar regions but this doesn't mean that warm water (regions) will not be affected," Carol Turley from the Plymouth Marine Laboratory told a news conference on the sidelines of climate talks in Copenhagen.
Small island nations that rely on tourism for much of their income fear reefs that draw divers and snorkelers will deteriorate or die as oceans become too acidic for corals, already suffering from warmer water temperatures.
Seas are already about one third more acidic than they were at the start of the Industrial Revolution and will become more so as emissions increase. The changes are believed to be the fastest for 55 million years, the report said.
Dr Turley said world leaders should keep carbon dioxide concentrations in the atmosphere at no more than 450 parts per million to ward off the most dramatic changes.
"A substantial and urgent reduction in carbon dioxide emissions is the only solution. There is no geo-engineering that will help," she said, referring to large-scale projects proposed to limit warming without capping carbon dioxide emissions.
Oceans are often left out of climate talks, or have a lower profile, because the science is less well-known and perhaps because humans live on land and focus on their immediate surroundings, said Carl Gustaf Lundin, head of the marine programme at the International Union for Conservation of Nature.
But changes now cannot be undone for generations.
"It will take tens of thousands of years for the carbon dioxide to disappear, essentially to be buffered by the ocean chemistry and sediments. So it is not a short-term problem," Dr Turley said.
Scientists say increasingly acidic oceans are disrupting the process of calcification used by sea creatures to build shells as well as coral reefs.
For example, tiny amoeba-like animals called foraminifera, which live on the ocean's surface, play a major role in trapping CO2 and transporting it to the ocean depths where it can be locked away for decades or centuries.
The Southern Ocean between Australia and Antarctica is the largest of the ocean carbon sinks and disruption of the shell-building process could have a major impact on the ability of oceans to soak up CO2, scientists say.