The ongoing decrease in the pH level of the Earht's oceans, caused by their uptake of anthropogenic carbon dioxide from the atmosphere.
An extensive article on ocean acidification can be found in the Wikipedia: Ocean acidification.
See also the literature review on the impacts of ocean stratification: Kroeker, K.J., Kordas, R.L., Crim, R., Hendriks, I.E., Ramajos, L., Singh, G,S, Duartes, C.M. and Gattusa, J-P. 2014. Impacts of ocean acidification on marine organisms: quantifying sensitivities and interaction with warming. Global Change Biology 19: 1884–1896, doi: 10.1111/gcb.12179. https://onlinelibrary.wiley.com/doi/epdf/10.1111/gcb.12179