Most of what we hear about global warming concerns the atmosphere and its carbon dioxide levels. In the New Yorker a few weeks ago, Elizabeth Kolbert wrote about what's happening in the ocean (not online, unfortunately it is online (thx, tim)). It turns out that like all tightly coupled systems, the ocean and the atmosphere like to be in equilibrium with each other, which means that the chemistry of the ocean is affected by the chemistry of the atmosphere. Much of the extra carbon dioxide pumped into the atmosphere by humans over the past two hundred years is being absorbed into the ocean and slowly making the ocean more acidic.
The CO2 dissolves, it produces carbonic acid, which has the chemical formula H2CO3. As acids go, H2CO3 is relatively innocuous -- we drink it all the time in Coke and other carbonated beverages -- but in sufficient quantities it can change the water's pH. Already, humans have pumped enough carbon into the oceans -- some hundred and twenty billion tons -- to produce a .1 decline in surface pH. Since pH, like the Richter scale, is a logarithmic measure, a .1 drop represents a rise in acidity of about thirty per cent.
As Kolbert later states, "from the perspective of marine life, the drop in pH matters less that the string of chemical reactions that follow". The increased levels of carbonic acid in the water means there are less carbonate ions available in seawater for making shells, meaning that thousands of species that build shells or skeletons from calcium carbonate are in danger of extinction. As a particularly troubling example, coral use calcium carbonate taken from the seawater to construct themselves. Climate modeller Ken Caldeira believes that if humans keep emitting carbon dioxide into the atmosphere at the same rate as today, by 2075 the world's coral reefs will begin to disappear because their rate of natural erosion will surpass their ability to grow fast enough to keep up.
The truly worrisome thing about all this is that the ocean is an extremely slow moving machine and that once in motion, it's difficult to stop or change its course.