Ever since we started using fossil fuels to make energy, the ocean has absorbed about half of the CO2 that has been produced by burning them.
Although it has been helpful for the ocean to do this because it means that less CO2 goes into the atmosphere, it has caused some big problems for sea creatures because they need carbon to grow.
When CO2 and water (which is H20) meet, hydrogen is eventually released. People often talk about the ocean becoming more acidified - and this is a part of that effect.
But the hydrogen ion that is released by the water and that attaches to the CO2 prevents the sea creatures from getting all the carbon they need to grow and to grow hard shells. (If you want to learn more then look for information on carbonic acid and carbonate later).
It all means that it takes sea creatures longer to grow and some may not be able to grow at all.
Because other fish rely on these sea creatures for food and there is less food around, it can also lead to there being less fish for us to eat as well as less fish for other animals that need fish to survive.
What makes this even more complicated is that, as the ocean has filled up with CO2 from all our fossil fuels, it means that it can't take up as much as it used to.
This has meant that more CO2 is left in the atmosphere because it has no-where to go. This makes the planet (and the ocean) warmer ! This changes the environment that the sea creatures are used to and this also causes serious problems for them.
All of this means that we need to produce less CO2 by burning less fossil fuels.