How does rising atmospheric CO2 affect marine organisms?

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Corals at the Southern Extreme of China Reveal Temperatures of the Roman Warm Period
Wei, G., Yu, K. and Zhao, J.  2004.  Sea surface temperature variations recorded on coralline Sr/Ca ratios during Mid-Late Holocene in Leizhou Peninsula.  Chinese Science Bulletin 49: 1876-1881.

What was done
High-resolution Sr/Ca ratios of two Porites corals from the coast of Leizhou Peninsula (2012'N, 10955'E) in the northern South China Sea were measured using inductively coupled plasma atomic spectrometry, while their ages were determined via U-Th dating.  The transfer function relating the Sr/Ca ratio to temperature was established on a modern Porites lutea coral by calibrating against sea surface temperatures (SSTs) measured from 1989 to 2000 at the nearby Haikou Meteorological Station.

What was learned
One of the two coral sections was dated to AD 489-500 in the middle of the Dark Ages Cold Period, while the other was dated to 539-530 BC in the middle of the Roman Warm Period, both of which multi-century climatic epochs are clearly delineated in the high-resolution oxygen isotope time series of a well-dated stalagmite from southwestern Ireland's Crag Cave (McDermott et al., 2001).

From the Dark Ages Cold Period portion of the coral record, Wei et al. determined that the average annual SST was approximately 2.0C colder than that of the last decade of the 20th century (1989-2000), while from the Roman Warm Period portion of the record they obtained a mean annual temperature that was identical to that of the 1989-2000 period as measured at the Haikou Meteorological Station.

What it means
As has been demonstrated many times before in a number of places throughout the world, these data indicate that the last decade of the 20th century did not display unusual or unprecedented warmth.  Indeed, there were prior times within both the Medieval Warm Period and Roman Warm Period when temperatures were equally as warm as, or actually warmer than, what they have been recently; and all of these earlier warm periods occurred at times when the air's CO2 concentration was fully 100 ppm less than what it is today.  Hence, there is no compelling reason to believe that any of the planet's current warmth is due to the extra CO2 that the burning of fossil fuels has added to earth's atmosphere.

McDermott, F., Mattey, D.P. and Hawkesworth, C.  2001.  Centennial-scale Holocene climate variability revealed by a high-resolution speleothem δ18O record from SW Ireland.  Science 294: 1328-1331.

Reviewed 2 March 2005