How does rising atmospheric CO2 affect marine organisms?

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Helama, S., Timonen, M., Holopainen, J., Ogurtsov, M.G., Mielikainen, K., Eronen, M., Lindholm, M. and Merilainen, J. 2009. Summer temperature variations in Lapland during the Medieval Warm Period and the Little Ice Age relative to natural instability of thermohaline circulation on multi-decadal and multi-centennial scales. Journal of Quaternary Science 24: 450-456.

The authors employed tree-ring data obtained from living and subfossil wood of Scots pine (Pinus sylvestris L.) trees found in the northern limits of forests in Finland and Norway (68-70N, 20-30E), which they alternately calibrated and tested using mid-summer (July) temperature data from northern Norway for the two periods AD 1876-1937 and 1937-1998. This work revealed that the warmest 250-year period of their record, which began in AD 750, "occurred at AD 931-1180," which they describe as sharing "significant temporal overlap with the general hemispheric climate variability due to the Medieval Warm Period (MWP)."

In terms of decadal variability, the researchers' data (shown below) indicate that the peak warmth of the Current Warm Period (CWP) occurred between 1920 and 1930. Since that time, temperatures have declined, such that the peak warmth of the MWP is about 0.8C higher than that recorded for the past two decades of the 20th century, which latter period is claimed by the world's climate alarmists to have been the warmest of the past thousand or more years. Hence, it is this latter period to which we compare the peak warmth of the MWP.