The oceans are one of the sinks for Carbon Dioxide, CO2.  The sink is where the CO2 is removed from the atmosphere.  Right now we are producing more CO2 then can be removed from the atmosphere.  Not only can the earth not remove the Carbon Dioxide fast enough, the sinks that do exist for CO2 are saturated and cannot recover, slowing the removal process even further.

How the oceans are becoming more acidic
CO2 is released into the atmosphere by the burning of fossil fuels as one example.  (This is Cars Gone Green hence the example of cars burning fossil fuels.)  This CO2 in the right amounts actually helps maintain a livable environment for people, plants and animals.  Society has been producing CO2 at greater amounts of the past 200 years.  CO2 can be removed from the atmosphere by the ocean, up to a ¼ of CO2 output a year is removed by the oceans making it one of the largest sinks.  What is happening is that the large amounts that the ocean has been removing is causing a rise in acid levels with in the ocean.

How does the ocean become acidic with CO2
The CO2 is removed from the atmosphere by the ocean.  As CO2 is dissolved into the ocean waters it is turned into carbonic acid.

How much acidic is the oceans becoming
The oceans are about 30% more acidic now then they were at the beginning of the industrial revolution (according to Unesco).  CO2 and acidic levels in the ocean waters have been monitored for the past 25 years continuously.  What the data shows is that the acidic levels and CO2 levels rise at the same pace.  CO2 levels are rising at about a 3% rate a year.  At this rate if CO2 emissions are not reduced in as little as 10 to 15 years many coastal see animals that have shells and skeletons will begining to corrode.

Solutions
The reduction of CO2 is the obvious solution.  That does not seem likely with an ever increasing population and the ever increasing need for energy from all parts of the world the conumption rates that produce CO2 will only increase with time.
One solution is to spread limestone to help netueralize the acid levels by releasing caronate ions.  The cost estimates are in the billions of dollars per year and that is in todays dollars.  This of course would create more CO2 in getting the limestone and then transporting the limestone across the oceans waters.