How does rising atmospheric CO2 affect marine organisms?

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A Hot Time in the Medieval Southwestern Baltic Sea
Andren, E., Andren, T. and Sohlenius, G.  2000.  The Holocene history of the southwestern Baltic Sea as reflected in a sediment core from the Bornholm Basin.  Boreas 29: 233-250.

What was done
The authors conducted an extensive analysis of changes in siliceous microfossil assemblages and chemical characteristics of various materials found in a well-dated sediment core obtained from the Bornholm Basin in the southwestern Baltic Sea.

What was learned
The data revealed the existence of a period of high primary production at approximately AD 1050.  Contemporaneous diatoms were warm water species such as Pseudosolenia calcar-avis, which the authors indicate is "a common tropical and subtropical marine planktonic species" that "cannot be found in the present Baltic Sea."  They also note that what they call the Recent Baltic Sea Stage, which begins at about AD 1200, starts "at a point where there is a major decrease in warm water taxa in the diatom assemblage and an increase in cold water taxa, indicating a shift towards a colder climate," which they associate with the Little Ice Age.

What it means
The data clearly indicate there was a period of time in the early part of the past millennium when the climate in the area of the southwestern Baltic Sea was warmer than it is today, as the sediment record of that time and vicinity contained several warm water species of diatoms, some of which can no longer be found there.  This period of higher temperatures, the authors say, falls within "a period of early Medieval warmth dated to AD 1000-1100," which they make a point of noting "corresponds to the time when the Vikings succeeded in colonizing Iceland and Greenland."

Once again, we thus have another research study that confirms that the Medieval Warm Period was significantly warmer than it is today.  We also note that this period was one of strikingly high primary productivity, demonstrating what seems to be becoming our mantra, i.e., that warmer is better.