From CO2 Science:
conducted a multi-proxy study of several short sediment cores they recovered from Lake Estanya (42°02'N, 0°32'E) in the Pre-Pyrenean Ranges of northeast Spain. Their studies suggest that there is no compelling need to attribute 20th-century global warming to the concomitant increase in the air's CO2 content.
Morellon, M., Valero-Garces, B., Gonzalez-Samperiz, P., Vegas-Vilarrubia, T., Rubio, E., Rieradevall, M., Delgado-Huertas, A., Mata, P., Romero, O., Engstrom, D.R., Lopez-Vicente, M., Navas, A. and Soto, J. 2011. Climate changes and human activities recorded in the sediments of Lake Estanya (NE Spain) during the Medieval Warm Period and Little Ice Age. Journal of Paleolimnology 46: 423-452.
The authors write that "in the context of present-day global warming, there is increased interest in documenting climate variability during the last millennium," since "it is crucial to reconstruct pre-industrial conditions to discriminate anthropogenic components (i.e., greenhouse gases, land-use changes) from natural forcings (i.e., solar variability, volcanic emissions)."
What it means
In light of these several observations it would appear that the multi-centennial climate oscillation uncovered by Morellon et al. has been driven by a similar oscillation in solar activity, as well as by multi-decadal solar activity fluctuations superimposed upon that longer-period oscillation. And these relationships suggest that there is no compelling need to attribute 20th-century global warming to the concomitant increase in the air's CO2 content. Natural variability appears quite capable of explaining it all.
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