How does rising atmospheric CO2 affect marine organisms?

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Yamal Peninsula, Western Siberia, Russia
Reference
Hantemirov, R.M. and Shiyatov, S.G. 2002. A continuous multimillennial ring-width chronology in Yamal, northwestern Siberia. The Holocene 12: 717-716.

Description
The authors and others collected remains of subfossil Siberian larch trees in Holocene deposits of the Yamal Peninsula over an area stretching from approximately 67°15'N to 67°40'N and from 69°50'E to 71°E, which they used to develop a 4000-year tree-ring width chronology covering the period 2000 BC to AD 1996. Then, based on a transfer function that yielded mean June-July air temperatures -- which they developed from temperatures measured at a meteorological station 150 km southwest of their research area over the period AD 1883-1996 -- the two researchers transformed their tree-ring width chronology into a June-July summer air temperature history.

Upon examining the history, Hantemirov and Shiyatov write that "relatively favorable conditions existed in 1200-900 BC, 100 BC-AD 200, and during the 'Medieval Optimum' (AD 700-1400)," the latter of which periods they indicate was most pronounced over the time interval AD 1100-1350. From their Figure 11, which presents the reconstruction after applying a 20-year low-pass filter, it can be seen that temperatures peaked during this interval about 0.5C above those experienced at the end of the 20th century.