How does rising atmospheric CO2 affect marine organisms?

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British Coastal Temperature Anomalies of the Last Millennium
Reference
Cage, A.G. and Austin, W.E.N. 2010. Marine climate variability during the last millennium: The Loch Sunart record, Scotland, UK. Quaternary Science Reviews: 10.1016/j.quascirev.2010.01.014.

What was done
From a broad sediment shelf at a water depth of 56 meters in the main basin of Loch Sunart -- a fjord on the northwest coast of Scotland (5640.20'N, 0552.22'W) -- the authors extracted several sediment cores from which they developed a continuous record of various physical and chemical properties of the sediment that spanned the last millennium and extended all the way up to AD 2006. Of most interest to us, in this regard, are the δ18O measurements made on the shells of the benthic foraminifer Ammonia beccarii, because prior such data -- when operated upon by the palaeotemperature equation of O'Neil et al. (1969) -- yielded bottom-water temperatures that had been judged by Cage and Austin (2008) to be "the most realistic water temperature values for infaunal benthic foraminifera from Loch Sunart."

What was learned
The results of the two researchers' most recent efforts revealed that the most distinctive feature of the Loch Sunart temperature record was an abrupt warming at AD 1540 that led to a temperature anomaly of 1.1C above the long-term mean from AD 1540-1600, which period was preceded within the interval AD 1445-1495 by some of the coldest temperatures of the past 1000 years.

What it means
Noting that "the rate and magnitude of the inferred warming at AD 1540 ... is similar to the rate of change and magnitude observed during the late twentieth century," Cage and Austin concluded that "changes in twentieth century marine climate cannot yet be resolved from a background of natural variability over the last millennium," which is another way of saying that late 20th-century warming -- which has not further manifested itself over the first decade of the 21st century -- was not unusual enough to validly ascribe it to the concomitant increase in the air's CO2 content.

References
Cage, A.G. and Austin, W.E.N. 2008. Seasonal dynamics of coastal water masses in a Scottish fjord and their potential influence on benthic foraminiferal shell geochemistry. In: Austin, W.E.N. and James, R.H. (Eds.) Biogeochemical Controls on Palaeoceanographic Environmental Proxies. Special Publication 303. Geological Society, London, UK, pp 155-172.

O'Neil, J.R., Clayton, R.N. and Mayeda, T. 1969. Oxygen isotope fractionation in divalent metal carbonates. Journal of Chemical Physics 51: 5547-5558.

Reviewed 23 June 2010