How does rising atmospheric CO2 affect marine organisms?

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Oberer Landschitzsee, an Alpine Lake in the Southern Austrian Alps
Schmidt, R., Kamenik, C. and Roth, M. 2007. Siliceous algae-based seasonal temperature inference and indicator pollen tracking ca. 4,000 years of climate/land use dependency in the southern Austrian Alps. Journal of Paleolimnology 38: 541-554.

The authors combined spring and autumn temperature anomaly reconstructions based on siliceous algae and pollen tracers found in a sediment core extracted from an Alpine lake (Oberer Landschitzsee; 4714'52" N, 1351'40" E) located at the southern slopes of the Austrian Central Alps just slightly above the present tree-line. This work revealed that "spring-temperature anomalies during Roman and Medieval times equaled or slightly exceeded [our italics] the modern values and paralleled tree-line and glacier fluctuations," indicative of their broad range of applicability. As for the timing of the Medieval Warm Period, Schmidt et al. identified "warm phases similar to present between ca. 850-1000 AD and 1200-1300 AD," which they say were "followed by climate deterioration at ca. 1300 AD, which culminated during the Little Ice Age." Hence, their data place the Medieval Warm Period as occurring between AD 850 and 1300.