How does rising atmospheric CO2 affect marine organisms?

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Tornetrask Area of Northern Sweden
Grudd, H. 2008. Tornetrask tree-ring width and density AD 500-2004: a test of climatic sensitivity and a new 1500-year reconstruction of north Fennoscandian summers. Climate Dynamics: 10.1007/s00382-0358-2.

Working with an extensive pre-existing set of Scots pine (Pinus sylvestris L.) tree-ring maximum density (MXD) data from the Tornetrask area of northern Sweden (68.21-68.31N, 19.45-19.80E), which originally covered the period AD 441-1980, Grudd extended the record an additional 24 years to 2004 using new samples obtained from 35 relatively young trees, which had the effect of reducing the mean cambial age of the MXD data in the 20th century and thus eliminating, as he described it, a "loss of sensitivity to temperature, apparent in earlier versions of the Tornetrask MXD chronology." This work revealed, as shown in the figure below, and as stated in Grudd's own words, that "the warmest summers in this new reconstruction occur in a 200-year period centered on AD 1000," which he specifically calls the Medieval Warm Period.

Based on Grudd's findings, we estimate that the peak temperature of the MWP was approximately 1.2C higher than the peak temperature of the CWP, which occurs at the very end of the record, and that the entire MWP, as we have defined it (AD 880-1110) was warmer than the peak warmth of the CWP.