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On the Pervasiveness of Millennial-Scale Climatic Cyclicity
Elrick M. and Hinnov, L.A. 2007. Millennial-scale paleoclimate cycles recorded in widespread Palaeozoic deeper water rhythmites of North America. Palaeogeography, Palaeoclimatology, Palaeoecology 243: 348-372.

What was done
In a major study of rhythmically-interbedded limestone and shale or limestone and chert known as rhythmites, the authors, as they describe it, "(1) review the persistent and widespread occurrence of Palaeozoic rhythmites across North America, (2) demonstrate their primary depositional origin at millennial time scales, (3) summarize the range of paleoenvironmental conditions that prevailed during rhythmite accumulation, and (4) briefly discus the implications primary Palaeozoic rhythmites have on understanding the origin of pervasive late Neogene-Quaternary millennial-scale climate variability."

What was learned
The two U.S. researchers determined that the various rhythmic alternations they studied "accumulated under dramatically different paleoenvironment and paleogeographic conditions including active to passive tectonic settings, equatorial to subtropical latitudes, long-term icehouse through greenhouse climatic conditions, calcite versus aragonitic seas, variable atmospheric CO2 concentrations, before and after land plant and animal evolution, and across widely varying ocean basin configurations."

What it means
On the basis of their many and diverse observations, Elrick and Hinnov conclude "it is apparent that millennial-scale climate changes occurred over a very wide spectrum of paleoceanographic, paleogeographic, paleoclimatic, tectonic, and biologic conditions and over time periods from the Cambrian to the Quaternary," and that given this suite of observations, "it is difficult to invoke models of internally driven thermohaline oceanic oscillations or continental ice sheet instabilities to explain their origin." Hence, they suggest that "millennial-scale paleoclimate variability is a more permanent feature of the earth's ocean-atmosphere system, which points to an external driver such as solar forcing [our italics]."

We would merely add, in this regard, that the 20th-century global warming that led to the establishment of the Current Warm Period appears to have occurred at a time that is not inconsistent with the times of occurrence of other externally-driven century-scale warm intervals of the current interglacial or Holocene - such as the Medieval and Roman Warm Periods that preceded the Current Warm Period - and that there is therefore no need to invoke the anthropogenic-augmented CO2-induced greenhouse effect as an explanation for our present level of warmth. What we experienced over the course of the 20th-century was likely just another of the "run of the mill" millennial-scale climatic transitions that has been a pervasive characteristic of earth's climatic behavior for as long as the earth has been a satellite of the sun.

Reviewed 16 May 2007