How does rising atmospheric CO2 affect marine organisms?

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The Medieval Warm Period in Greenland
Wagner, B. and Melles, M.  2001.  A Holocene seabird record from Raffles So sediments, East Greenland, in response to climatic and oceanic changes.  Boreas 30: 228-239.

What was done
The authors analyzed a 3.5-m-long sediment core taken from a lake (Raffels So) on an island (Raffles O) situated just off Liverpool Land on the east coast of Greenland for a number of properties related to the past presence of seabirds there, obtaining a 10,000-year record that tells us much about the region's climatic history.  Key to the study were biogeochemical data that, in the words of the authors, reflect "variations in seabird breeding colonies in the catchment which influence nutrient and cadmium supply to the lake."  Previously-derived proxy records of temperature from two other locations were also employed in the study.

What was learned
The authors' data reveal sharp increases in the values of the parameters they measured between about 1100 and 700 years before present (BP), indicative of the summer presence of significant numbers of seabirds during that "medieval warm period," which had been preceded by a several-hundred-year period of little to no (inferred) bird presence.  Thereafter, their data suggested another absence of birds during "a subsequent Little Ice Age," which they note was "the coldest period since the early Holocene in East Greenland."  The data also show signs of a "resettlement of seabirds during the last 100 years, indicated by an increase of organic matter in the lake sediment and confirmed by bird observations."  However, values of the most recent biogeochemical measurements are not as great as those obtained from the earlier Medieval Warm Period.  Reconstructed proxy temperature histories from two Greenland ice cores lead to the same conclusion, indicating higher temperatures during the period from 1100 to 700 years BP than what has been observed over the most recent hundred years.

What it means
As with many other paleoclimate investigations - and contrary to the repeated claims of climate alarmists - the results of this "paleobird" study suggest that the global warming of the last century has not yet returned the planet to temperatures as high as those it experienced during the Medieval Warm Period.  Hence, there is no compelling reason to invoke the historical rise in the air's CO2 content as the cause of any portion of the most recent increase in the globe's near-surface air temperature.