H2O + CO2 = H2CO3

That means water plus carbon dioxide equals carbonic acid, which disolves sea shells and coral.

the Independent:

A world dying, but can we unite to save it?

Pollution in the seas is now speeding global warming, says a devastating new climate report.

'IoS' Environment Editor Geoffrey Lean reports from Valencia

Published: 18 November 2007

Humanity is rapidly turning the seas acid through the same pollution that causes global warming, the world's governments and top scientists agreed yesterday. The process – thought to be the most profound change in the chemistry of the oceans for 20 million years – is expected both to disrupt the entire web of life of the oceans and to make climate change worse.

The warning is just one of a whole series of alarming conclusions in a new report published by the official Intergovernmental Panel on Climate Change (IPCC), which last month shared the Nobel Peace Prize with former US vice president Al Gore.

[. . .]

Scientists have found that the seas have already absorbed about half of all the carbon dioxide emitted by humanity since the start of the industrial revolution, a staggering 500 billion tons of it. This has so far helped slow global warming – which would have accelerated even faster if all this pollution had stayed in the atmosphere, already causing catastrophe – but at an increasingly severe cost.

The gas dissolves in the oceans to make dilute carbonic acid, which is increasingly souring the naturally alkali seawater. This, in turn, mops up calcium carbonate, a substance normally plentiful in the seas, which corals use to build their reefs, and marine creatures use to make the protective shells they need to survive. These include many of the plankton that form the base of the food chain on which all fish and other marine animals depend.

As the waters are growing more acid this process is decreasing, with incalculable consequences for the life of the seas, and for the fisheries on which a billion of the world's people depend for protein. Every single species that uses calcium in this way, that has so far been studied, has been found to be affected. And the seas are most acid near the surface, where most of their life is concentrated.

A report by the Royal Society, Britain's premier scientific body, concludes that, as a result, of the pollution, the world's oceans are probably now more acidic that they have ever been in "hundreds of millennia", and that even if emissions stopped now, the waters would take "tens of thousands of years to return to normal".

Professor Ulf Reibesell of the Leibnitz Institute of Marine Sciences in Kiel, Germany's leading expert on the process, concludes in an issue of UNEP's magazine Our Planet, to be published next month, that, if it continues to the levels predicted by yesterday's report for the end of the century, the seas will reach a condition unprecedented in the last 20 million years.