Acidification - acid seas
Much of the CO2 released into the atmosphere by natural processes is absorbed by the sea. However, as humans have added ever more CO2 to the air, some of it is also absorbed by the oceans, combining with water to create carbonic acid. This is the same thing that makes fizzy drinks taste sharp. In the sea though, excess acid is very destructive and is dissolving coral reefs in many parts of the world and preventing sea animals from creating shells by removing calcium from the water.
Rising sea levels