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A Novel Proxy for Continental Mean Annual Air Temperature
Reference
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.

Background
As so many have indicated before them, the authors write that "the assessment of climate variations in Earth's history is of paramount importance for our comprehension of recent and future climate variability," and they state that for this important purpose "geological archives containing climate-sensitive proxy indicators are used to reconstruct paleoclimate."

What was done
In taking this approach to the problem, Niemann et al. employ what they describe as "a novel proxy for continental mean annual air temperature (MAAT) and soil pH" that is "based on the temperature (T) and pH-dependent distribution of specific bacterial membrane lipids (branched glycerol dialkyl glycerol tetraethers - GDGTs) in soil organic matter," which technique derives from the fact that "microorganisms can modify the composition of their cellular membrane lipids to adapt membrane functionality to specific environmental parameters such as T and pH," as described by Hazel and Williams (1990) and Weijers et al. (2007), the latter of whom devised "transfer functions that relate the degree of the GDGT methylation (expressed in the Methylation index - MBT) and cyclisation (expressed in the cyclisation ratio - CBT) to mean annual air temperature." This they did 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.

What was learned
The nine Dutch and Swiss researchers report that "major climate anomalies recorded by the MBT/CBT-paleothermometer" were "the Little Ice Age (~14th to 19th century) and the Medieval Warm Period (MWP, ~9th to 14th century)," which they say experienced "temperatures similar to the present-day values." And "in addition to the MWP," they state that their "lacustrine paleo T record indicates Holocene warm phases at about 3, 5, 7 and 11 kyr before present, which agrees in timing with other records from both the Alps and the sub-polar North-East Atlantic Ocean."

What it means
As we have written so many times before, Niemann et al.'s study once again indicates that there is nothing unusual, unnatural or unprecedented about earth's current climate! And, therefore, there is no need to invoke anthropogenic-induced increases in the air's CO2 content as the cause of the planet's current level of non-unique warmth.

References
Hazel, J.R. and Williams, E.E. 1990. The role of alterations in membrane lipid composition in enabling physiological adaptation of organisms to their physical environment. Progress in Lipid Research 29: 167-227.

Weijers, J.W.H., Schouten, S., van den Donker, J.C., Hopmans, E.C. and Damste, J.S.S. 2007. Environmental controls on bacterial tetraether membrane lipid distribution in soils. Geochimica et Cosmochimica Acta 71: 703-713.

Reviewed 2 January 2013