How does rising atmospheric CO2 affect marine organisms?

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Peruvian Shelf
Rein B., Luckge, A. and Sirocko, F. 2004. A major Holocene ENSO anomaly during the Medieval period. Geophysical Research Letters 31: 10.1029/2004GL020161.

Rein et al. analyzed a high-resolution sediment core retrieved from a sheltered basin situated on the edge of the Peruvian shelf about 80 km west of Lima, Peru (12.05°S, 77.66°W) to produce a proxy record of El Niņo flooding over the past 12,000 years. Results indicated the presence of a significant dry episode during the late Medieval period in which lithic concentrations - a proxy for El Niņo events - were "very low for about 450 years during the Medieval climatic anomaly from A.D. 800 to 1250." Because heavy winter rainfalls along and off coastal Peru only occur during times of maximum El Niņo strength, and because El Niņos are typically much more prevalent and stronger during cooler as opposed to warmer periods [see El Niņo (Relationship to Global Warming) in our Subject Index], the implied lack of strong El Niņos during the period of time from A.D. 800-1250 suggests that this period was truly a Medieval Warm Period.