How does rising atmospheric CO2 affect marine organisms?

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Lake Laihalampi, Southern Boreal Zone of Finland
Heikkila, M. and Seppa, H. 2003. A 11,000 yr palaeotemperature reconstruction from the southern boreal zone in Finland. Quaternary Science Reviews 22: 541-554.

Working with sediment cores extracted from a small shallow lake (Laihalampi: 61°29'18"N, 26°04'51"E), and employing a pollen-climate transfer function derived from data on pollen types and abundances obtained from the surface sediments of 113 small to medium size Finnish lakes and adjacent meteorological stations in 1997, the authors were able to reconstruct annual mean regional temperatures for the past eleven millennia. This work revealed that the mean annual temperature of the 200-year period stretching from AD 1000 to 1200 -- which we will designate the Medieval Warm Period -- was approximately 0.8°C warmer than the most recent 200-year period. In a more refined plot of the data, however, it can be seen that the most recent century of the MWP was about 2.2°C warmer than the modern reconstructed mean value for the period 1961-1990. Thus, we employed the HadCRUT3 Land- and Sea-Surface temperature dataset to determine that the latter 2.2°C temperature differential of the MWP-CWP needed to be reduced by approximately 0.8°C to account for subsequent warming after 1990. Hence, we assign the MWP to the period AD 1000-1200; and we calculate MWP-CWP to be 1.4°C.