When CO2 reacts with water, the result is carbonic acid:
CO2 + H2O ⇌ H2CO3
Since the Industrial Revolution, the oceans have absorbed nearly 25% of the carbon dioxide produced by burning fuels.
As a result, the ocean acidity has increased 30% since then. If this trend continues, the ocean acidity will double by the end of the century, compared to pre-industrial times.
This will probably cause the extinction of numerous species, and will seriously harm the balance of the food chain all over the world.
Click here to watch a video that explains how it happens.