sentence2_structure: Ocean acidification<>occurs<>when excess carbon dioxide in the atmosphere causes the oceans to become acidic$$$excess carbon dioxide in the atmosphere<>causes<>the oceans to become acidic$$$the oceans<>to become<>acidic
sentence1: Measurements of carbon dioxide in the atmosphere and in the upper ocean suggest that the Southern Ocean is one of the primary sinks for atmospheric carbon dioxide.
sentence2: Ocean acidification occurs when excess carbon dioxide in the atmosphere causes the oceans to become acidic.
question: What occurs when excess carbon dioxide in the atmosphere causes the oceans to become acidic?