How does rising atmospheric CO2 affect marine organisms?

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Lake Cadagno, Piora Valley, Southern Switzerland
Niemann, H., Stadnitskaia, A., Wirth, S.B., Gilli, A., Anselmetti, F.S., Damste, J.S.S., Schouten, S., Hopmans, E.C. and Lehmann, M.F. 2012. Bacterial GDGTs in Holocene sediments and catchment soils of a high Alpine lake: application of the MBT/CBT-paleothermometer. Climate of the Past 8: 889-906.

Using sediment cores that were collected in September 2009 and May 2010 from a small alpine lake (Cadagno) in the Piora Valley of south-central Switzerland, as well as soil samples taken from the surrounding catchment area, Niemann et al. measured the amounts of branched glycerol dialkyl glycerol tetraethers (GDGTs) of certain bacterial cellular membrane lipids and their degree of methylation and cyclisation; and using transfer functions that relate the degree of the GDGT methylation and cyclisation to mean annual air temperature (MAAT), they created a Holocene history of MAAT for the region that revealed the existence of the Medieval Warm Period "(MWP, ~9th to 14th century)," which they say experienced "temperatures similar to the present-day values."