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The Medieval Warm Period at Lake Silvaplana in Switzerland
Larocque-Tobler, I., Grosjean, M., Heiri, O., Trachsel, M. and Kamenik, C. 2010. Thousand years of climate change reconstructed from chironomid subfossils preserved in varved lake Silvaplana, Engadine, Switzerland. Quaternary Science Reviews 29: 1940-1949.

The authors write that to better describe the amplitude of temperature change during the last millennium, "new records to increase the geographic coverage of paleoclimatic information are needed," and that "only by obtaining numerous high-resolution temperature records will it be possible to determine if the 20th century climate change exceeded the natural pre-industrial variability of European climate." Thus, to help achieve this important goal, they proceeded to obtain another such temperature record.

What was done
Based on their analysis of fossil chironomids (non-biting midges) -- which were identified and quantified in four sediment cores extracted from the bed of Lake Silvaplana (46°26'56"N, 9°47'33"E) in the Upper Engadine (a high-elevation valley in the eastern Swiss Alps) -- the authors constructed a detailed history of that region's mean July air temperature over the last millennium.

What was learned
Larocque-Tobler et al. report that "at the beginning of the record, corresponding to the last part of the 'Medieval Climate Anomaly' (here the period between ca. AD 1032 and 1262), the chironomid-inferred mean July air temperatures were 1°C warmer than the climate reference period (1961-1990)," which would also make them warmer than most subsequent temperatures as well. And in looking at their graphs of 20- and 50-year running means, it can be seen that the peak mean warmth of the Medieval Warm Period exceeded that of the Current Warm Period by approximately 0.5°C in the case of 20-year averages and 1.2°C in the case of 50-year averages.

What it means
The five researchers conclude that "based on the chironomid-inferred temperatures, there is no evidence that mean-July air temperature exceeded the natural variability recorded during the Medieval Climate Anomaly in the 20th century at Lake Silvaplana," while noting that similar results "were also obtained in northern Sweden (Grudd, 2008), in Western Europe (Guiot et al., 2005), in a composite of Northern Hemisphere tree-ring reconstructions (Esper et al., 2002) and a composite of tree rings and other archives (Moberg et al., 2005)." In addition, there are many other studies that could also be thus cited, as may be seen by perusing the materials we have assembled in our website's Medieval Warm Period Project. And in light of the story told by all of these many palaeoclimatic datasets, it is clear that there has been nothing unprecedented, unusual or unnatural about the level of warmth that has been reached during the Current Warm Period. Consequently, there is no compelling reason to attribute even the warmest of the temperatures that have occurred to this point in time to the historical increases that have occurred in the concentrations of the atmosphere's several trace greenhouse gases.

Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.

Grudd, H. 2008. Tornetrask tree-ring width and density AD 500-2004: a test of climatic sensitivity and a new 1500-year reconstruction of north Fennoscandian summers. Climate Dynamics 31: 843-857.

Guiot, J., Nicault, A., Rathgeber, C., Edouard, J.L., Guibal, F., Pichard, G. and Till, C. 2005. Last-Millennium summer-temperature variations in Western Europe based on proxy data. The Holocene 15: 489-500.

Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko, N.M. and Karlen, W. 2005. Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433: 613-617.

Reviewed 22 September 2010