Ocean acidification is a result of an increased amount of carbon dioxide in the atmosphere. When the carbon dioxide is absorbed by sea water (H2O) it produces carbonic acid (H2CO3).  On average, a fourth of all the carbon dioxide in the atmosphere will be consumed by the ocean.  Since industrialization began, the ocean has absorbed 525 billion tons of carbon dioxide, averaging about 22 million tons a day.  Therefore, as carbon dioxide and other greenhouse gases start to increase in the atmosphere, so will the acidity of the Earth’s oceans. This process is changing the chemical make up the ocean and is greatly impacting the ecosystem.
Ocean pH is altered due to ocean acidification. Ocean pH levels represent the amount of hydrogen ions in the water. Therefore, when the ocean becomes more acidic, there is an increase in hydrogen ion concentration. A change of one pH unit represents a ten-fold change of hydrogen ions. This causes the scale to be very sensitive - for example, since the Industrial Revolution, ocean pH has decreased by 0.11 units, equating to a 30% increase of hydrogen ions. A change of one pH unit in human blood can result in health problems such as seizures and arrhythmias. Decreasing pH levels in the oceans are impacting biological processes in marine life such as photosynthesis, respiration, and calcification. 
Certain minerals are essential for life to thrive in the ocean, such as calcium carbonate minerals. Calcium carbonate is responsible for creating strong bones and shells for marine animals. Ocean acidification disrupts this development because large concentrations of carbon dioxide in ocean water results in the decay of certain minerals. Calcifying organisms that are being negatively affected by ocean acidification include shell fish, sea urchins, and many species of coral and plankton. This poses a threat to the entire food chain in Earth’s ocean especially the one million species who live within coral reef habitats.  Studies of acidification showed that a change in pH hurt the chemical balance in fish that allows their bodies to function properly. For example, clown fish become no longer able to hear or smell their way out of danger. 
Only a few species that thrive on carbon dioxide will be able to withstand this new environment, such as photosynthetic algae and sea grass. Loss of biodiversity in the ocean due to acidification will also impact humans because many communities rely on fish and shell fish as their main source of food. Another species being affected by ocean acidification are the zooplankton in the ocean. These small organisms are an important part of the food chain but are being impaired because their shells cannot withstand the increased acidity. Zooplankton and other similar organisms are a part of the carbon cycle. As the zooplankton die and sink to the sea floor, the calcium carbonate in their shells is stored in rocks, reducing the amount of carbon dioxide cycled back to the atmosphere.
Naturally occurring “buffers” once aided in regulating ocean acidification. For example, rivers emptying into the ocean can keep pH levels stable. However, the immense volume of carbon dioxide being absorbed by the ocean at rapid rates is cancelling these buffering processes.
Scientists are able to study what the atmosphere was like by extracting cores from rocks in the Earth's crust. These cores contain traces of what elements were in the atmosphere millions of years ago and in what abundance. By testing rock cores, scientists determined that the last time the ocean was this acidic was about 35 million years ago, and a result of a massive amount of carbon dioxide being introduced to the atmosphere. This period is known as the Paleocene-Eocene Thermal Maximum. Reasons for the increase in carbon dioxide range from increased volcanic activity, erosion of ocean sediment, and extensive wildfires, which burned trees, coal, and peat. The same processes that drove ocean acidification are present today, although carbon dioxide levels are rising more rapidly now than during the Paleocene-Eocene Thermal Maximum mainly due to human activity.  Laboratory experiments conducted with small ocean organisms demonstrate not only how they form in different pH levels, but also how they behave. Scientists look for how certain organisms adapt as opposed to others and how they grow through the stages of life in different conditions. Another area of investigation for scientists are deep sea vents, which leak carbon dioxide and contribute to ocean acidification.
If carbon dioxide continues to increase in the atmosphere at the current rate, buffering processes will be ineffective. Also, if carbon emissions are cut, it will take buffering processes thousands of years to create an equilibrium again. In 1988 measurement of carbon dioxide in the atmosphere passed 350 parts per million. Today, carbon dioxide averages over 400 ppm in the atmosphere. Since carbon dioxide can linger in the atmosphere for hundreds of years, reduced carbon emissions will only slow the process of climate change and ocean acidification. 
- National Oceanic and Atmospheric Administration Pacific Marine Environmental Laboratory (PMEL)
- NOAA Ocean Today - Ocean as a Lab: Ocean Acidification and The Acid Test (videos)
- Smithsonian National Museum of Natural History Ocean Portal
- United States Environmental Protection Agency - Ocean acidity
- Natural Resource Defense Council – Ocean acidification