How does rising atmospheric CO2 affect marine organisms?

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Lake Skardtjorna, Western Spitsbergen, Svalbard, Norway
Velle, G., Kongshavn, K. and Birks, H.J.B. 2011. Minimizing the edge-effect in environmental reconstructions by trimming the calibration set: Chironomid-inferred temperatures from Spitsbergen. The Holocene 21: 417-430.

Working with two short gravity cores and two long piston cores of sediments obtained from the deepest part of Lake Skardtjorna (77°57.780'N, 13°48.799'E) in 2008, plus a long core obtained in 2003, Velle et al. reconstructed histories of chironomid types and concentrations over the past 2000 years, which they translated into mean July air temperatures based on a modern mean July air temperature calibration data set compiled by Brooks and Birks (2000, 2001), plus additional unpublished data for 2001-2009, utilizing new approaches they developed in their paper. [See: Brooks, S.J. and Birks, H.J.B. 2000. Chironomid-inferred late-glacial and early-Holocene mean July air temperatures for Krakenes Lake, Western Norway. Journal of Paleolimnology 23: 77-89; Brooks, S.J. and Birks, H.J.B. 2001. Chironomid-inferred air temperatures from Late-glacial and Holocene sites in north-west Europe: Progress and problems. Quaternary Science Reviews 20: 1723-1741.]

This work revealed, in the words of the two researchers, a "warming that occurred at 1000 to 830 BP," where BP = 2003, that "may correspond to what is known as the 'Medieval Warm Period'." And based on their graphical representation of that record, we estimate that the peak warmth of the MWP (~ AD 1000-1170) was about 0.3°C greater than the peak warmth of the Current Warm Period.