How does rising atmospheric CO2 affect marine organisms?

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Tornetrask Area, Swedish Lapland
Grudd, H., Briffa, K.R., Karlen, W., Bartholin, T.S., Jones, P.D. and Kromer, B. 2002. A 7400-year tree-ring chronology in northern Swedish Lapland: natural climatic variability expressed on annual to millennial timescales. The Holocene 12: 657-665.

The authors used tree-ring widths from 880 living, dry dead, and subfossil northern Swedish pines to assemble a continuous and precisely dated chronology (the Tornetrask chronology: ~68N, 20E) covering the period 5407 BC to AD 1997. A "strong association with summer mean temperature (June-August)" then enabled them to create "a temperature reconstruction for the past 7400 years." This history indicated, in their words, that "the relatively warm conditions of the late twentieth century do not exceed those reconstructed for several [our italics] earlier time intervals," one of which was the Medieval Warm Period, centered on about AD 1000-1125, when MWP - CWP ~ 0C.