How does rising atmospheric CO2 affect marine organisms?

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Southern Canadian Tundra, Southwestern Keewatin, Nunavut, Canada (Level 3 Study)
Bryson, R.A., Irving, W.N. and Larsen, J.A. 1965. Radiocarbon and soil evidence of former forest in the Southern Canadian tundra. Science 147: 46-48.

Starting from the northern edge of continuous forest, which in the early 1960s crossed Canada's Ennadai Lake at about 6045'N, 101W, the authors presented evidence -- based on buried soil and charcoal characteristics and their radiocarbon dating -- "of forests at least 280 km north of the present tree line about 3500 years ago and at least 90 km north about 900 years ago," as well as evidence that forests were "farther south than at present" during the intervening interval of time, going on to state that they "correlate the first advance with the Climatic Optimum and the second with the Little Climatic Optimum," which is otherwise known as the Medieval Warm Period. Based on these findings, the three researchers concluded that the ancient more northerly forests "were associated with periods of relatively mild climate." Hence, we conclude that the warmth of the MWP in this part of Canada was more significant than it was at the time of their study; and, based on the uncertainties associated with their radiocarbon dates, extended at least from about AD 1000 to AD 1200.