How Unusual Was 20th-Century Global Warming?
Pak, D.K., Lea, D.W. and Kennett, J.P. 2012. Millennial scale changes in sea surface temperature and ocean circulation in the northeast Pacific, 10-60 kyr BP. Paleoceanography 27: 10.1029/2011PA002238.
We know that 20th-century global warming represents the recovery of the world from the global chill of the Little Ice Age, which had been preceded by the Medieval Warm Period, which in turn had come about as a result of the warming that followed the Dark Ages Cold Period, which had been preceded by the Roman Warm Period; and so has Earth's climate periodically shifted - from cold to warm, and from warm to cold - all the way back through the entire Holocene, as a result of some oscillating forcing factor that alternately warmed and cooled the planet on a millennial timescale. And we know that each of the between-period transitions was of approximately the same absolute magnitude. But what was the situation prior to the Holocene?
Perhaps the most recent study to provide evidence pertinent to this question is that of Pak et al. (2012), who sampled the upper 12.5 meters of a sediment core recovered from the Santa Lucia Slope off Point Arguello, California (USA) at 3 to 5 cm intervals, and who developed from the data thereby obtained a centennial-scale sea surface temperature (SST) record of the California margin that covers the past 10 to 60 thousand years, based on the Mg/Ca ratio of fossil shells of the planktic foraminifer Globigerina bulloides. So what did they find?
"On millennial scales," as Pak et al. write, "the last glacial episode (60 to 25 kyr) is marked by large abrupt oscillations in temperature of ~4-5°C in amplitude," and they note that "these oscillations are synchronous with previously described Dansgaard-Oeschger [D/O] type oscillations in δ18O," adding that D/O-type events have been recognized in high-resolution climate records throughout the Northern Hemisphere, citing the studies of Kotilainen and Shackleton (1995), Porter and An (1995), Hughen et al. (1996), Cannariato and Kennett (1999), Hendy and Kennett (2000), Peterson et al. (2000), Seki et al. (2002) and Asmerom et al. (2010)."
In light of this wealth of information, which continues to be discovered, published and thereby augmented on a regular basis, it should be clear to all that 20th-century-type global warmings occurred multiple times over the course of the Holocene, and that it was significantly exceeded many more times during the glacial period that preceded it. And, of course, none of those earlier "regime changes" was associated with atmospheric CO2 concentration changes anywhere near what occurred over the 20th century. It should therefore be readily evident to most people that anthropogenic CO2 emissions have likely had next to nothing to do with the global warming of the 20th-century, which the world's climate alarmists irrationally continue to attribute to them.
Asmerom, Y., Polyak, V. and Burns, S. 2010. Variable winter moisture in the southwestern United States linked to rapid glacial climate shifts. Nature Geoscience 3: 114-117.
Cannariato, K.G. and Kennett, J.P. 1999. Climatically related millennial-scale fluctuations in strength of California margin oxygen-minimum zone during the past 60 k.y. Geology 27: 975-978.
Hendy, I.L. and Kennett, J.P. 2000. Dansgaard-Oeschger cycles and the California Current System: Planktonic foraminiferal response to rapid climate change in Santa Barbara Basin, Ocean Drilling Program hole 893A. Paleoceanography 15: 30-42.
Hughen, K.A., Overpeck, J.T., Peterson, L.C. and Trumbore, S. 1996. Rapid climate changes in the tropical Atlantic region during the last deglaciation. Nature 380: 51-54.
Kotilainen, A.T. and Shackleton, N.J. 1995. Rapid climate variability in the North Pacific Ocean during the past 95,000 years. Nature 377: 323-326.
Peterson, L.C., Haug, G.H., Hughen, K.A. and Rohl, U. 2000. Rapid changes in the hydrologic cycle of the tropical Atlantic during the last glacial. Science 290: 1947-1951.
Porter, S. and An, Z.S. 1995. Correlation between climate events in the North Atlantic and China during the last glaciation. Nature 375: 305-308.
Seki, O., Ishiwatari, R. and Matsumoto, K. 2002. Millennial climate oscillations in NE Pacific surface waters over the last 82 kyr: New evidence from alkenones. Geophysical Research Letters 29: 10.1029/2002GL015200.