How does rising atmospheric CO2 affect marine organisms?

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The Medieval Warm Period in Fennoscandia
Seppa, H. and Birks, H.J.B. 2002. Holocene climate reconstructions from the Fennoscandian tree-line area based on pollen data from Toskaljavri. Quaternary Research 57: 191-199.

What was done
The authors used a recently developed pollen-climate reconstruction model and a new pollen stratigraphy from Toskaljavri, a tree-line lake in the continental sector of northern Fenoscandia (located just above 69N latitude), to derive quantitative estimates of annual precipitation and July mean temperature.

What was learned
Seppa and Birks say their reconstructions "agree with the traditional concept of a 'Medieval Warm Period' (MWP) and 'Little Ice Age' in the North Atlantic region (Dansgaard et al., 1975) and in northern Fennoscandia (Korhola et al., 2000)." In addition, they report there is "a clear correlation between our MWP reconstruction and several records from Greenland ice cores," and that "comparisons of a smoothed July temperature record from Toskaljavri with measured borehole temperatures of the GRIP and Dye 3 ice cores (Dahl-Jensen et al., 1998) and the 18O record from the Crete ice core (Dansgaard et al., 1975) show the strong similarity in timing of the MWP between the records." Last of all, they note that "July temperature values during the Medieval Warm Period (ca. 1400-1000 cal yr B.P.) were ca. 0.8C higher than at present," i.e., than the mean temperature of the last six decades of the 20th century.

What it means
Once again, another paleoclimate study confirms that the Medieval Warm Period of a thousand-plus years ago was significantly warmer than it has been recently, in spite of a much-reduced atmospheric CO2 concentration of only 280 ppm at that earlier time. The evidence thus continues to mount that the warming of the 20th century was totally unrelated to the concurrent rise in the air's CO2 content.

Dahl-Jensen, D., Monsegaard, K., Gundestrup, N., Clow, G.D., Johnsen, S.J., Hansen, A.W. and Balling, N. 1998. Past temperatures directly from the Greenland Ice Sheet. Science 282: 268-271.

Dansgaard, W., Johnsen, S.J., Gundestrup, N., Clausen, H.B. and Hammer, C.U. 1975. Climatic changes, Norsemen and modern man. Nature 255: 24-28.

Korhola, A., Weckstrom, J., Holmstrom, L. and Erasto, P. 2000. A quantitative Holocene climatic record from diatoms in northern Fennoscandia. Quaternary Research 54: 284-294.

Reviewed 7 April 2004