How does rising atmospheric CO2 affect marine organisms?

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The "Little" Medieval Warm Period in the Bahamas
Saenger, C., Cohen, A.L., Oppo, D.W., Halley, R.B. and Carilli, J.E. 2009. Surface-temperature trends and variability in the low-latitude North Atlantic since 1552. Nature Geoscience 2: 492-495.

Near the tail-end of the Medieval Warm Period, there was a brief spike in temperature at various sites around the world that appears to rival the temperatures of the latter part of the Current Warm Period. We here describe the findings of the most recent evidence for this mini-warm period, which comes to us from the Bahamas.

What was done
The authors developed what they describe as "an absolutely dated and annually resolved record of sea surface temperature [SST] from the Bahamas [25.84N, 78.62W], based on a 440-year time series [1552-1991] of coral [Siderastrea siderea] growth rates," which they found to possess "an inverse correlation with instrumental SST," which was verified by "applying it to an S. siderea colony from Belize (17.50N, 87.76W)."

What was learned
Saenger et al. report that "the reconstruction indicates that temperatures were as warm as today from about 1552 [where their record begins, somewhere in the midst of the mini-warm period] to 1570, then cooled by about 1C from 1650 to 1730 before warming until the present [italics added]," which for their record was 1991. In comparing 1991 warmth with that of the true present (2009), however, we find that the HadCRUT3 and Global Historical Climatology Network databases depict about a 0.3C increase in temperature between 1991 and today; but the graph of Saenger et al.'s data shows their temperature history ending about 0.3C short of its peak mini-warm period value. Hence, their conclusion that "SSTs were as warm as present from 1552 to 1570" indeed appears to be correct.

What it means
The fact that way back in the mid-1500's, when the atmosphere's CO2 concentration was 100 ppm less than it is today, temperatures in the Bahamas - as well as many other parts of the planet (see Little Medieval Warm Period in our Subject Index) - were about the same as (or even greater than) they are today, certainly suggests that there is nothing unusual, unnatural or unprecedented about today's temperatures, and that there is thus no need (as well as no valid reason) to blame current CO2 concentrations for our current (and actually welcome) warmth.

Reviewed 16 December 2009