The pH of the ocean has dropped by about 1/10 of a pH unit in the last century. Since pH is a logarithmic scale, that means the acidity of the ocean has moved by about 25% toward the acid side. There are no natural causes that could possibly account for such a large change in the acidity.

That means we must look to other causes to explain that increase in acidity. Increasing the amount of carbon dioxide in the atmosphere increases the partial pressure and according to Henry's law, the amount that dissolves in the oceans is directly proportional to the partial pressure in the atmosphere. Since the partial pressure of carbon dioxide in the atmosphere as gone up by about 40%, then we might expect that the amount that dissolves in the upper levels of the oceans to be about 40% greater.

Carbon dioxide dissolved in water produces carbonic acid, which would lower the pH of the ocean. Mankind is responsible for most of the 40% increase in carbon dioxide in the atmosphere and we are now releasing about 30 billion tons of carbon dioxide into the air each year. Mankind is a part of nature so I suppose you can say that man might be the natural cause.

Heavy volcanic activity. Unfortunately, denialists seem to think that because something natural could influence such things as Earth's temperature or oceanic pH, that this natural cause is what is influencing whatever. Just because volcanoes could cause ocean acidification does not mean they are and there is no evidence of any spike in volcanic activity.

It's not natural in the sense due to a biogeochemical process, but the Rothschilds secret ocean acidification plan would also explain the pH decrease.

Lizard people going for a swim.

Try and scare us so you can collect more fix-it taxes.