How does rising atmospheric CO2 affect marine organisms?

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Medieval vs. Current Warm Period: Iceberg Lake, Alaska
Reference
Loso, M.G., Anderson, R.S., Anderson, S.P. and Reimer, P.J. 2006. A 1500-year record of temperature and glacial response inferred from varved Iceberg Lake, southcentral Alaska. Quaternary Research 66: 12-24.

What was done
The authors "present a varve thickness chronology from glacier-dammed Iceberg Lake [6046'N, 14257'W] in the southern Alaska icefields," where "radiogenic evidence confirms that laminations are annual and record continuous sediment deposition from AD 442 to AD 1998," and where "varve thickness increases in warm summers because of higher melt, runoff, and sediment transport."

What was learned
Loso et al. report that temperatures implied by the varve chronology "were lowest around AD 600, warm between AD 1000 and AD 1300 [which they call "a clear manifestation of the Medieval Warm Period"], cooler between AD 1500 and AD 1850, and have increased dramatically since then."

What it means
The four scientists say their varve record "suggests that 20th century warming is more intense ... than the Medieval Warm Period or any other time in the last 1500 years." However, the intense warming of the 20th century peaks somewhere in the vicinity of 1965 to 1970 (as best we can determine from their graphical representation of varve thickness), after which it is followed by equally intense cooling, such that by 1998 (the supposedly warmest year of the past two millennia, according to the world's climate alarmists), temperatures are implied to be less than they were during the Medieval Warm Period.

The same story is also told by tree ring-width anomalies from the adjacent Wrangell Mountains, which Loso et al. portray as updated from Davi et al. (2003). Hence, it can be concluded from two different data bases that the region's current temperature is in fact lower than it was during the warmest part of the Medieval Warm Period, adding more weight to the growing mountain of evidence that indicates there is nothing unusual about the planet's current level of warmth (see our Medieval Warm Period Project).

Reference
Davi, N.K., Jacoby, G.C. and Wiles, G.C. 2003. Boreal temperature variability inferred from maximum latewood density and tree-ring width data, Wrangell Mountain region, Alaska. Quaternary Research 60: 252-262.

Reviewed 18 October 2006