How does rising atmospheric CO2 affect marine organisms?

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Two Thousand Years of Icelandic Climate
Axford, Y., Geirsdottir, A., Miller, G.H. and Langdon, P.G. 2009. Climate of the Little Ice Age and the past 2000 years in northeast Iceland inferred from chironomids and other lake sediment proxies. Journal of Paleolimnology 41: 7-24.

The authors write that "the idea of a widespread and spatially coherent 'Medieval Warm Period' (MWP) has come under scrutiny in recent years," but that "it remains a viable hypothesis that a period of relative warmth in northwestern Europe and the northern North Atlantic region helped facilitate Norse expansion across the North Atlantic from the ninth to thirteenth centuries, including settlement of Iceland and Greenland," and that "subsequent cooling contributed to the demise of the Norse settlements on Greenland."

What was done
In further exploring this subject, Axford et al. developed a regional climatic record from a sediment core retrieved from lake Stora Vioarvatn in northeast Iceland (6614.232'N, 1550.083'W) in the summer of 2005, based on chironomid assemblage data -- which were well correlated with nearby measured temperatures over the 170-year instrumental record -- and total organic carbon, nitrogen and biogenic silica content.

What was learned
With respect to the Medieval Warm Period, the four researchers report that their data indicate "warm temperatures in the tenth and eleventh centuries, with one data point suggesting temperatures slightly warmer than present [our italics]." Even more striking in this regard, they found that "temperatures were higher overall and more consistently high through much of the first millennium AD [our italics]."

What it means
In discussing their findings, the Icelandic, UK and US scientists say "the historical perception of a significant medieval climate anomaly in Iceland may be primarily a reflection of the human perspective," in that "Iceland was settled ca. AD 870, during a period of relative warmth that was followed by many centuries of progressively colder and less hospitable climate," and that "had the Norse settled Iceland 1000 years earlier, the MWP might be viewed only as a brief period of climatic amelioration, a respite from a shift to colder temperatures that began in the eighth century," near the end of several centuries of even greater warmth. In any event, and viewed from either perspective, it is clear there is nothing unusual or unnatural about the region's present-day temperatures, which the researchers say "do not show much recent warming."

Reviewed 22 April 2009