How does rising atmospheric CO2 affect marine organisms?

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Northern Fennoscandia
Bjune, A.E., Seppa, H. and Birks, H.J.B. 2009. Quantitative summer-temperature reconstructions for the last 2000 years based on pollen-stratigraphical data from northern Fennoscandia. Journal of Paleolimnology 41: 43-56.

Working with mean July temperature reconstructions based on "pollen-stratigraphical data obtained from eleven small lakes located in the middle boreal, northern boreal, low-alpine, or low-arctic zones of northern Norway, northern Sweden, northern Finland and north-west Russia," the authors developed a mean quantitative temperature history of this Northern Fennoscandia region (6625'-7050'N, 1403'-3519'E) that spanned the past two millennia; and in describing their results, they say that "no consistent temperature peak is observed during the 'Medieval Warm Period'." Nevertheless, in viewing their final result (see figure below), it may readily be seen that what they describe as the present temperature (red vertical line) -- derived from the uppermost 1 cm of the sediment cores, which were collected at various times between AD 1994 and 2003 -- is colder than most all of the data points obtained by the authors over the past 2,000 years.

Focusing on the Medieval Warm Period, which most people generally define as the period of time between AD 800 and 1300, it is evident that temperatures were as much as 1.4C warmer than what they were over the most recent decade or so. And that is the key point. Irrespective of whether or not a "consistent temperature peak" was observed during this period, temperatures of the most recent decade or so in Northern Fennoscandia have not been higher than they were back in medieval times. In fact, they were exceeded for most of the entire two thousand-year record. If there is anything unusual about temperatures of the most recent decades, therefore, it is that they are unusually cold!

Figure 1. Reconstructed temperature history of Northern Fennoscandia. Adapted from Bjune et al. (2009).