How does rising atmospheric CO2 affect marine organisms?

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The Medieval Warm Period in Western Norway
Reference
Mikalsen, G., Sejrup, H.P. and Aarseth, I. 2001. Late-Holocene changes in ocean circulation and climate: foraminiferal and isotopic evidence from Sulafjord, western Norway. The Holocene 11: 437-446.

What was done
The authors conducted detailed analyses of benthonic foraminfera, stable isotopes and other sedimentary material obtained from a core extracted from a fjord in western Norway, deriving a relative temperature history of the region that spanned the last 5500 years.

What was learned
Four cold periods characterized by 1.5-2C reductions in bottom-water temperature were identified: 2150 to 1800 BC, 850 to 600 BC, 150 BC to AD 150, AD 500 to 600, and "a cooling that may correspond to the 'Little Ice Age' (AD 1625)." The authors note "there is a good correlation between the cold periods and cold events recorded in other studies," which finding helps to strengthen their conclusions. Perhaps of even more importance, however, was their identification of a warm period from AD 1330 to 1600 that "had the highest bottom-water temperatures in Sulafjorden during the last 5000 years."

What it means
The authors' results clearly establish the reality and importance of the Medieval Warm Period in the North Atlantic region. Although certain climate alarmists continue to claim this most recent high-temperature excursion was not very striking, the data of this meticulous study strongly suggest it was warmer during this specific time period than it was during any other multi-century period of the last five millennia.