How does rising atmospheric CO2 affect marine organisms?

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Duck Pond, Yangmingshan National Park, Northern Taiwan
Chen, S.-H., Wu, J.-T., Yang, T.-N., Chuang, P.-P., Huang, S.-Y. and Wang, Y.-S. 2009. Lat Holocene paleoenvironmental changes in subtropical Taiwan inferred from pollen and diatoms in lake sediments. Journal of Paleolimnology 41: 315-327.

Working with a sediment core that "represents deposition from AD 650 to present" that was taken from a mountain lake (Duck Pond, 2510.441'N, 12133.013'E) in Northern Taiwan, the authors identified, measured and analyzed "pollen, spores, diatoms, organic carbon, nitrogen, and δ13C of organic matter in lake sediments to infer climate changes and reconstruct the paleo-environment of subtropical Taiwan over the past ~1300 years," temporally delineating five different climate zones in the process. Zone III (AD 1050-1250), as they describe it, was "wet and warmer; ~MWP [Medieval Warm Period]", Zone IV (AD 1250-1790) was "wetter and colder than in Zone III; corresponding to LIA [Little Ice Age]," and Zone V (AD 1790-2000) was "drier and warmer than in Zone IV." In commenting further on these delineations, they say that "in Europe and other regions, there was a short warm period (the medieval warm period, or MWP) prior to the LIA," and that their "Zones III and IV likely correspond to such warm and cold periods." In addition, they report that the ratio of arboreal pollen (AP) to non-arboreal pollen (NAP) "showed a positive correlation with temperature," and on the basis of this relationship, plus the fact that the peak AP/NAP ratio of the MWP was about three times greater than the peak AP/NAP ratio of the CWP, the peak warmth of the former period must have been considerably greater than the peak warmth of the latter period.