How does rising atmospheric CO2 affect marine organisms?

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Summer Temperatures in the Northern French Alps
Millet, L., Arnaud, F., Heiri, O., Magny, M., Verneaux, V. and Desmet, M. 2009. Late-Holocene summer temperature reconstruction from chironomid assemblages of Lake Anterne, northern French Alps. The Holocene 19: 317-328.

The authors write that "among biological proxies from lake sediments, chironomid [non-biting midge] assemblages are viewed as one of the most promising climatic indicators," and that "the accuracy of chironomid assemblages for the reconstruction of Lateglacial temperatures is now broadly demonstrated."

What was done
Millet et al. (1) "present a new chironomid-based temperature record from Lake Anterne (northern French Alps) covering the past two millennia," (2) "compare this reconstruction with other late-Holocene temperature records from Central Europe," and (3) "address the question of whether previously described centennial-scale climate events such as the 'Medieval Warm Period' [MWP] or the 'Little Ice Age' [LIA] can be detected in this new summer temperature record," noting that "at a hemispheric or global scale the existence of the LIA and MWP have been questioned."

What was learned
The six scientists report that "evidence was found of a cold phase at Lake Anterne between AD 400 and 680, a warm episode between AD 680 and 1350, and another cold phase between AD 1350 and 1900," and they say that these events were "correlated to the so-called 'Dark Age Cold Period' (DACP), the 'Medieval Warm Period' and the 'Little Ice Age'." In addition, they say that "many other climate reconstructions across western Europe confirm the existence of several significant climatic changes during the last 1800 years in Central Europe and more specifically the DACP, the MWP and the LIA." Last of all, they report that the reconstructed temperatures of the 20th century failed to show a return to MWP levels of warmth, which failure they attributed to a breakdown of the chironomid-temperature relationship over the final century of their 1800-year history.

What it means
Ever more evidence continues to indicate that the Medieval Warm Period was a real and global phenomenon (see our Medieval Warm Period Project). It also continues to indicate that the MWP was likely warmer than the Current Warm Period (CWP) has been to date. Such could also be said about the new evidence provided by the study of Millet et al., although we tend to agree that there was indeed a breakdown of their chironomid-temperature relationship when it mattered most and disallowed a valid (apples-to-apples) comparison to be made between the warmth of the MWP and the CWP. However, the fact that Millet et al.'s reconstructed summer temperature dropped by about 1.3C during the MWP to LIA transition would indeed suggest that the MWP was warmer than the CWP has yet been, since post-LIA warming is generally considered to have been somewhat less than 1.3C ... even when comparing apples to oranges!

Reviewed 10 June 2009