By Richard Black
Environment correspondent, BBC News website
Natural carbon dioxide vents on the sea floor are showing scientists how carbon emissions will affect marine life.
Dissolved CO2 makes water more acidic, and 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.
The only way of reducing the impact of ocean acidification is the urgent reduction in CO2 emissions
Plymouth Marine Laboratory
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."
Corals construct their external skeletons by extracting dissolved calcium carbonate from seawater and using it to form two minerals, calcite and aragonite. Molluscs use the same process to make their shells.
As water becomes more acidic, the concentration of calcium carbonate falls. Eventually there is so little that shells or skeletons cannot form.
The oceans are thought to have absorbed about half of the extra CO2 put into the atmosphere in the industrial age
This has lowered its pH by 0.1
pH is the measure of acidity and alkalinity
The vast majority of liquids lie between pH 0 (very acidic) and pH 14 (very alkaline); 7 is neutral
Seawater is mildly alkaline with a "natural" pH of about 8.2
The IPCC forecasts that ocean pH will fall by "between 0.14 and 0.35 units over the 21st Century, adding to the present decrease of 0.1 units since pre-industrial times"
Around the vents which Dr Hall-Spencer's team investigated, in the Mediterranean Sea near the Italian coast, CO2 bubbling into the water forms a sort of natural laboratory for studying the impacts of acidified water on marine life.
Globally, the seas now have an average pH of about 8.1 - down about 0.1 since the dawn of the industrial age.
Around the vents, it fell as low as 7.4 in some places. But even at 7.8 to 7.9, the number of species present was 30% down compared with neighbouring areas.
Coral was absent, and species of algae that use calcium carbonate were displaced in favour of species that do not use it.
Snails were seen with their shells dissolving. There were no snails at all in zones with a pH of 7.4.
Meanwhile, seagrasses thrived, perhaps because they benefit from the extra carbon in the water.
These observations confirm that some of the processes seen in laboratory experiments and some of the predictions made by computer models of ocean ecosystems do also happen in the real world.
"I can't count the number of times that scientific talks end with 'responses have not yet been documented in the field'," said Elliott Norse, president of the Marine Conservation Biology Institute (MCBI).
"This paper puts that to rest for several ecologically important marine groups."
The Intergovernmental Panel on Climate Change (IPCC) suggests that without measures to restrain carbon dioxide emissions, ocean pH is likely to fall to about 7.8 by 2100.
Seagrasses were among the few beneficiaries of more acid waters
This suggests that some of the impacts seen around the Mediterranean vents might be widespread.
"I think we will see the same pattern in other parts of the world, because we're talking about keystone species such as mussels and limpets and barnacles being lost as pH drops," said Dr Hall-Spencer.
The IPCC suggests that some areas, notably the Southern Ocean, might feel the impacts at lower concentrations of CO2.
Last month, scientists reported that water with CO2 levels high enough to be "corrosive" to marine life was rising up off the western US coast.
Bottom water naturally contains more CO2 than at shallower depths. This scientific team argues that human emissions have pushed these levels even higher, contributing to pH values as low 7.5 in waters heavily used by US fishermen.
"If [pH 7.8] is a universal 'tipping point', then it indicates that sections of the western coast waters off North America may have passed this threshold during periods when this upwelling of waters high in CO2 occurs," commented Carol Turley from Plymouth Marine Laboratory (PML), who was not involved in the Mediterranean Sea study (PML is not affiliated with Plymouth University).
Organisms such as coral are also damaged by rising temperatures, and studies are ongoing into the combined effect of a warming and acidifying ocean.
There is much to learn. And during the coming week, scientists will announce the inauguration of the European Project on Ocean Acidification (Epoca), a four-year, 16m euro (£12.5m) initiative aiming to find some answers.
Studying the impacts may prove easier than doing anything about them.
"The reason that the oceans are becoming more acidic is because of the CO2 emissions that we are producing from burning fossil fuels," observed Dr Turley.
"Add CO2 to seawater and you get carbonic acid; it's simple chemistry, and therefore certain.
"This means that the only way of reducing the future impact of ocean acidification is the urgent, substantial reduction in CO2 emissions."