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Just How Icy was the Little Ice Age?
Reference
Osborn, G., Menounos, B., Ryane, C., Riedel, J., Clague, J.J., Koch, J., Clark, D., Scott, K. and Davis, P.T. 2012. Latest Pleistocene and Holocene glacier fluctuations on Mount Baker Washington. Quaternary Science Reviews 49: 33-51.

Background
The authors write that "glaciers on stratovolcanoes of the Pacific Northwest of North America offer opportunities for dating late Pleistocene and Holocene glacier advances because tephra and fossil wood are common in lateral moraines and in glacier forefields."

What was done
Using "dated tephras (Bacon, 1983; Hallet et al., 1997, 2001; Foit et al., 2004 Tucker et al., 2007) and radiocarbon ages on fossil wood and vegetation mats within several lateral moraines to constrain the magnitude and times of past glacier advances," as they describe it, Osborn et al. developed "a latest Pleistocene and Holocene glacial chronology for Mount Baker, a stratovolcano in northwest Washington [USA]."

What was learned
The Canadian and U.S. team of nine researchers determined that "glaciers on Mount Baker were of minimal extent during the early Holocene, began to advance by ca 6 ka, and continued to advance to successively greater limits ... with intervening periods of retreat," achieving "their maximum Holocene extents in the 1800s at the end of a two-part Little Ice [Age] advance punctuated by retreat before ca 0.4 ka." And they add that "the similarity between glacier fluctuations here and those in British Columbia [Canada] and elsewhere in the Cascades suggests a common history of Holocene climate change over a broad area of the northern Cordillera."

What it means
These findings for a vast region of North America suggest that the coldest portion of the entire Holocene, or current interglacial period, occurred during the latter part of the Little Ice Age; and it is thus not surprising that there would be a significant subsequent increase in the planet's air temperature, as the most recent upswing of a well-documented millennial-scale oscillation of temperature has made itself felt, which oscillation - going back in time from the Current Warm Period to the Little Ice Age to the Medieval Warm Period to the Dark Ages Cold Period to the Roman Warm Period and etc. - has been responsible for bringing the planet into and out of these several-century-long alternating warm and cold periods that (1) continue back in time throughout interglacial and glacial periods alike (Oppo et al., 1998; Raymo et al., 1998; Bond et al., 2001), and that (2) occur totally independently of any changes in the atmosphere's CO2 concentration. And there is thus no reason to attribute earth's most recent warming to the latter phenomenon.

References
Bacon, C.R. 1983. Eruptive history of Mount Mazama and Crater Lake caldera, Cascade Range, U.S.A. Journal of Volcanology and Geothermal Research 18: 57-115.

Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.

Foit, F.F., Gavin, D.G. and Feng, S.H. 2004. The tephra stratigraphy of two lakes in south central British Columbia, Canada, and its implications for mid-late Holocene activity at Glacier Peak and Mount St. Helens, Washington. Canadian Journal of Earth Sciences 41: 1401-1410.

Hallet B., Hills, L.V. and Clague, J. 1997. New accelerator mass spectrometry ages for the Mazama ash layer from Kootenay National Park, British Colombia, Canada. Canadian Journal of Earth Sciences 34: 1202-1209.

Hallet B., Mathewes, R.W. and Foit, F.F. 2001. Mid-Holocene Glacier Peak and Mount St. Helens We tephra layers detected in lake sediments from southern British Columbia using high-resolution techniques. Quaternary Research 55: 284-293.

Oppo, D.W., McManus, J.F. and Cullen, J.L. 1998. Abrupt climate events 500,000 to 340,000 years ago: Evidence from subpolar North Atlantic sediments. Science 279: 1335-1338.

Raymo, M.E., Ganley, K., Carter, S., Oppo, D.W. and McManus, J. 1998. Millennial-scale climate instability during the early Pleistocene epoch. Nature 392: 699-702.

Tucker, D.S., Scott, K.M., Foit Jr., F.F. and Mierendorf, R.R. 2007. Age, distribution and composition of Holocene tephras from Mount Baker, Cascade arc, Washington, USA. Geological Society of America Abstracts with Programs 39 (4), 66.

Reviewed 19 December 2012