How does rising atmospheric CO2 affect marine organisms?

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Cueva del Cobra, Cantabrian Range, Northern Spain
Martin-Chivelet, J., Muņoz-Garcia, M.B., Edwards, R.L., Turrero, M.J. and Ortega, A.I. 2011. Land surface temperature changes in Northern Iberia since 4000 yr BP, based on δ13C of speleothems. Global and Planetary Change 77: 1-12.

The authors demonstrate that well-dated δ13C data derived from a stalagmite removed in 2002 from a cave in northern Spain -- Cueva del Cobre (~43°N, 4.4°W) -- were "strongly sensitive to surface temperature changes." And, therefore, it can be seen from their plot of the δ13C data that the peak warmth of the Medieval Warm Period (which they assign to the time period 1350-750 yr BP) was slightly greater than the peak warmth of the Current Warm Period (i.e., the last decade or so of the cave's δ13C history).