Burning fossil fuels increases Carbon Dioxide concentrations in our atmosphere. These higher levels force CO2 to dissolve into our oceans. The mixture of CO2 and water yields carbonic acid, which drives pH levels lower: Ocean Acidification.

Watch NRDC and leading scientists explain this process clearly here

The National Academy of Sciences and the National Research Council has issued a release:

CO2 Emissions Causing Ocean Acidification to Progress at Unprecedented Rate