How does rising atmospheric CO2 affect marine organisms?

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The Late Medieval Warm Period at Switzerland's Seebergsee
Larocque-Tobler, I., Stewart, M.M., Quinlan, R., Trachsel, M., Kamenik, C. and Grosjean, M. 2012. A last millennium temperature reconstruction using chironomids preserved in sediments of anoxic Seebergsee (Switzerland): consensus at local, regional and Central European scales. Quaternary Science Reviews 41: 49-56.

In presenting the rationale for their study, the authors rhetorically ask: "Does the amplitude of climate change of the last century exceed the natural variability?" And in describing the first step in answering this question for themselves, they state that "high-temporally resolved paleo-climate reconstructions are needed to place the last century into a broader spatial and temporal context," which they thus proceed to do.

What was done
Working at Seebergsee (46°37'N, 7°28'E) - a small two-basin lake of surface area 0.06 km2 in the northern Swiss Alps - Larocque-Tobler et al. extracted a 3-m sediment core from the lake's deepest point back in AD 2005, which they scrutinized for head capsules of various chironomid species at near-annual resolution over the upper 36 cm of the core and at approximately decadal resolution throughout the rest of the core. These species assemblage data were then calibrated against instrumental temperature data from the closest meteorological station and compared with regional instrumental records stretching as far back as AD 1760, after which the result was used to reconstruct mean July air temperatures for the period AD 1073-2005.

What was learned
First of all, the six scientists are careful to note that the temperature reconstruction they derived "starts at the end of the previously defined 'Medieval Climate Anomaly'," or MCA. This fact is very important, for it suggests that their temperature reconstruction may not include the warmest temperatures of the MCA. Not needing to do so, however, they state in the conclusion section of their report that even with their highly-truncated temperature record, "the MCA period has been shown to be warmer than the last century by about 1.2°C." However, they hasten to add that "the temperatures of the last century increased by 0.8°C," but following - in the very same sentence - they state that even with this increase, the latest temperatures of their record "did not exceed the MCA chironomid-inferred temperatures."

What it means
Once again, we have another carefully conducted study (see our Medieval Warm Period Project for more) that strongly suggests that the peak warmth of the Medieval Warm Period was likely significantly greater than that of the Current Warm Period has been to date.

Reviewed 31 October 2012