You have probably heard that as the global climate changes due to human influence the sea surface is going to rise and the oceans will get warmer and more acidic. The bit about the oceans increasing in acidity is particularly troubling because it implies calcium carbonate based organisms (oysters, snails, corals, etc.) will simply dissolve in this future dystopian acid-ocean (that is a bit of an oversimplification, but you get the idea).
Ocean acidity is determined by measuring the pH, which relates acidity based on the number of hydrogen ions found in the water. So long story short, as more and more carbon dioxide (CO2) is emitted into the atmosphere, it in-turn fluxes into the oceans forming carbonic acid which results in the release of hydrogen ions lowering the pH. Simple logic would suggest that this spells bad news for calcifying organism (poor Mr. Snail).
Healthy Pretopod shell (left) and degraded Pteropod shell due to ocean acidification (right). (Photo credit: NOAA [climate.gov])