How does rising atmospheric CO2 affect marine organisms?

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Khibiny Mountains, Kola Peninsula, Northwest Russia
Hiller, A., Boettger, T. and Kremenetski, C. 2001. Medieval climatic warming recorded by radiocarbon dated alpine tree-line shift on the Kola Peninsula, Russia. The Holocene 11: 491-497.

Based upon age determinations of subfossil wood samples found in the Khibiny Mountains of Russia's Kola Peninsula (67-68°N, 33-34°E) and the altitudes at which they were found, Hiller et al. determined that between AD 1000 and 1300 the upper tree-line within that region was located at least 100-140 m above its current elevation, which suggests, in their words, that mean summer temperatures during this "Medieval climatic optimum" were "at least 0.8°C higher than today."