How does rising atmospheric CO2 affect marine organisms?

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Upper Fly Lake, Southwest Yukon Territory, Canada
Bunbury, J. and Gajewski, K. 2012. Temperatures of the past 2000 years inferred from lake sediments, southwest Yukon Territory, Canada. Quaternary Research 77: 355-367.

Bunbury and Gajewski obtained sediment cores from Upper Fly Lake (61.04°N, 138.09°W) that "yielded chironomid records that were used to provide quantitative estimates of mean July air temperature." As to how this was done, they write that "inference models were developed to estimate mean July air temperature from the fossil chironomid data using 145 modern sites from Barley et al. (2006), 39 sites from Wilson and Gajewski (2004), and an additional two sites from Bunbury and Gajewski (2008)," which exercise, in their words, "resulted in a new chironomid calibration dataset containing 186 samples, 74 species, and 17 environmental variables." This effort revealed the existence of "relatively warm conditions during medieval times, centered on AD 1200, followed by a cool Little Ice Age, and warming temperatures over the past 100 years." And from the graphical representation of their data (the running mean presented in their Figure 8) we estimate the peak warmth of the Medieval Warm Period was equivalent to the peak warmth of the Current Warm Period at Upper Fly Lake.