Ocean Acidification takes place when the ocean absorbs carbon dioxide from the atmosphere, making the ocean more acidic (lowering the pH, the measure of acidity).  Scientists estimate that the average pH of surface waters of the ocean has fallen by about 0.1 pH unit since the Industrial Revolution and will continue to fall in the future.  Since the pH scale is logarithmic, a 0.1 unit pH drop means about 30 percent increase in the ocean hydrogen ion concentration [H+] and therefore a large increase in acidity.

From Smokestacks To Seashells

Source: Ocean Conservancy