The more carbon waters absorb, the more acid they get
It's common knowledge that global warming is heating up our planet, because of ever-increasing quantities of carbon dioxide emitted in the atmosphere by heavy industrial activities worldwide. And, though some people still don't believe this is happening, nature doesn't really care what they believe, nor will it stop the changes it’s going through because a few individuals are pretending nothing is going on. A major part in reducing the effects of global warming is played by oceans, which are now beginning to reach their limits.
That's not to say they won't keep on absorbing excess carbon from the atmosphere. However, increasing carbon amounts stocked inside the waters means that, over time, the ocean's pH will change dramatically. It has already decreased from 8.2 to 8.1, and predictions say that this trend will accelerate over the next decades. Naturally, changes in the ocean's acidity levels will not occur without someone, or something, paying the price.
In the case of planetary waters, both humans and marine populations are at a very high risk of dying. That is to say, people who are entirely dependent on oceans or seas for their very survival. Thousands of marine species will face extinction once the pH in the water passes alkaline and moves on to acidic. Creatures that use carbon to create hard shells, like clams and oysters, will no longer have the means to construct their “homes.”
Also, coral reefs, which are already endangered, will be left with no means of extending their habitats. Furthermore, carbonic acid, a byproduct of carbon in the atmosphere reacting to water, has the ability to destroy the shells of living organisms, as well as the existing coral reefs. The situation is somehow similar to when you place your hand in water and then sulphuric acid is gradually added. Over time, you'll notice the tissue begins to peel off and the hand disintegrates. This will happen, on a planetary level, to animals requiring carbon processing to survive.