Since we started releasing Co2 in the air the sea has absorbed half of all the Co2 we let out. That is why it's called the planet's biggest carbon sink.
Now this is "good" in some way because it decreased the amount of co2 in the atmosphere. But as the ocean warms it takes in less and less Co2. But this is horrible for our marines friends and us. Why?
1. fish get less food and grow less. They are more easily exposed to having a desease. 2. 1 in 7 people only eat fish so around a billion people would be starving if our fish isn't safe to eat.