How does rising atmospheric CO2 affect marine organisms?

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Hallet Lake, Alaska, USA
McKay, N.P., Kaufman, D.S. and Michelutti, N. 2008. Biogenic silica concentration as a high-resolution, quantitative temperature proxy at Hallet Lake, south-central Alaska. Geophysical Research Letters 35: 10.1029/2007GL032876.

McKay et al. used biogenic silica concentrations preserved in lacustrine sediments from oligotrophic Hallet Lake (61.5N, 146.2W) in south-central Alaska (USA) to quantitatively reconstruct June-July-August air temperature there over the past 2000 years. Their work revealed a broad period of nearly constant substantial warmth over the interval AD 1280-1450, which was about 1.2C cooler than the peak warmth at the end of what they call "the past 20 years." Prior to this almost vertical spike in modern summer air temperature, however, the Medieval Warm Period was consistently warmer; and they note that the magnitude and rate of change they recorded over the past 20 years "are greater than in other quantitative temperature proxies (Hu et al., 2001; Wilson et al., 2007)." Nevertheless, we report MWP-CWP = -1.2C.