Much before industrial era. ^
During industrial era and cow mass production. ^^
Based off of these two comparison graphs. It shows that the pH level started to drop in the 2000's (8.2-8.1) ("Ocean Acidification -- National Geographic." National Geographic. N.p., n.d. Web. 27 May 2015.) and is going to continue to drop at the rate we are emitting CO2 into the atmosphere. This predicts that the pH level will be at a record low at 7.8. Where, during the 1000's, there was no evidence of the pH level dropping significantly without it coming back up. According to David Littschwager/National Geographic Society.
Shocking fact of the page: Over the past 200 years the ocean's acidity has risen 30%. Faster than the previous 50 million years.
(Littschwager, David. "Ocean Acidification." Smithsonian Ocean Portal. N.p., n.d. Web. 03 June 2015.)