Increasing ocean acidification

Although it has gained less mainstream media attention the effects of increasing greenhouse emissions, in particular carbon dioxide, on the oceans may well be significant.
Short overview of ocean acidification: Ocean Acidification, ABC World News Webcast, June 7, 2008
Scientists are finding that on the one hand oceans have been able to absorb some of the excess CO2 released by human activity. This has helped keep the planet cooler than it otherwise could have been had these gases remained in the atmosphere.
However, the additional CO2 being absorbed is also resulting in the acidification of the oceans (when CO2 reacts with water it produces a weak acid called carbonic acid, changing the sea water chemistry).
This change is also occurring rapidly, so some marine life may not have the chance to adapt. Some marine creatures are growing thinner shells or skeletons, for example. Some of these creatures play a crucial role in the food chain, and in ecosystem biodiversity.
Clay animation by school children: The other CO2 problem, March 23, 2009 (commissioned by EPOCA)
Some species may benefit from the extra carbon dioxide, and a few years ago scientists and organizations, such as the European Project on OCean Acidification, formed to try to understand and assess the impacts further.
One example of recent findings is a tiny sand grain-sized plankton responsible for the sequestration of 25–50% of the carbon the oceans absorb is affected by increasing ocean acidification. This tiny plankton plays a major role in keeping atmospheric carbon dioxide (CO2) concentrations at much lower levels than they would be otherwise so large effects on them could be quite serious.
Other related problems reported by the Inter Press Service include more oceanic dead zones (areas where there is too little oxygen in the sea to support life) and the decline of important coastal plants and forests, such as mangrove forests that play an important role in carbon absorption. This is on top of the already declining ocean biodiversity that has been happening for a few decades, now.