Besides ocean warming, marine calcifying organisms have to deal with another CO2 problem: Ocean acidification. Increasing atmospheric CO2 gets absorbed by the ocean and alters seawater carbonate chemistry. Surface ocean pH already dropped about 0.1 units since the pre-industrial values and the consequent decrease in calcium carbonate saturation potentially threatens shell formation. Oceans pH is expected to become another 0.3 - 0.4 units lower until the end of this century. Synergistic effects of temperature and CO2 on carbonate shell properties will be tested on bivalve mollusks in controlled perturbation experiments.