How does rising atmospheric CO2 affect marine organisms?

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Peruvian Shelf
Rein B., Lckge, A., Reinhardt, L., Sirocko, F., Wolf, A. and Dullo, W.-C. 2005. El Nio variability off Peru during the last 20,000 years. Paleoceanography 20: 10.1029/2004PA001099.

The authors derived sea surface temperatures from alkenones extracted from a high-resolution marine sediment core retrieved off the coast of Peru (12.05S, 77.66W), spanning the past 20,000 years and ending in the 1960s. From their Figure 11, adapted below, it can be seen that the warmest temperatures of this 20,000 year period (~23.2C) occurred during the late Medieval time (AD 800-1250). Taking this value, 23.2C, and comparing it with the modern monthly long-term means in sea surface temperature, which the authors characterize as between 15C and 22C, we estimate the peak warmth of the Medieval Warm Period for this region was about 1.2C above that of the Current Warm Period.