Clues to past climatic conditions may be found in a wide variety of proxy data pertaining to many more things than simply temperature and precipitation. In the paragraphs that follow, we review the findings of some recent studies of such data that tell us much about the genesis and demise of the Medieval Warm Period, along with what these things imply about the planet's current warmth.
Maasch et al. (2005) examined changes in eight well-dated high-resolution non-temperature histories covering the past two millennia: (1) K+ concentrations obtained from the GISP2 ice core in Greenland, (2) Na+ concentrations derived from the Siple Dome ice core in Antarctica, (3) percent Ti present in an ocean sediment core retrieved from the Cariaco Basin off the coast of Venezuela, (4) Fe intensity from a marine sediment core extracted near the coast of mid-latitude Chile, (5) oxygen isotope fractions from Punta Laguna near the Yucatan, (6) carbon isotope data from a speleothem in Makapansgat, South Africa, (7) percent of shallow water diatoms found in a sediment core taken from Lake Victoria, and (8) past levels of Lake Naivasha in equatorial Africa. They then compared these histories with a similar history of atmospheric 14C to ascertain if any solar influence might have operated on these parameters. This comparison revealed that over the past 2000 years there had been, in the words of the researchers, a "strong association between solar variability and globally distributed climate change [our italics]," and they say that this "remarkable coherence" among the data sets was particularly noticeable in the Medieval Warm Period to Little Ice Age transition.
In a similar vein, but covering the entire period of the current interglacial or Holocene, Mayewski et al. (2005) examined some fifty globally distributed paleoclimate records in search of evidence for what they call rapid climate change (RCC). This terminology is not to be confused with the rapid climate changes typical of glacial periods, but is used in the place of what the sixteen researchers call the "more geographically or temporally restrictive terminology such as 'Little Ice Age' and 'Medieval Warm Period'." Hence, RCC events, as they also call them, are multi-century periods characterized by extremes of various climatic properties, rather than the much shorter periods during which the changes that produced them took place.
Mayewski et al. identified six RCCs during the Holocene: 9000-8000, 6000-5000, 4200-3800, 3500-2500, 1200-1000 and 600-150 cal yr BP, the last two of which intervals are the "globally distributed" Medieval Warm Period and Little Ice Age, respectively. In speaking further of these two periods, they say that "the short-lived 1200-1000 cal yr BP RCC event coincided with the drought-related collapse of Maya civilization and was accompanied by a loss of several million lives (Hodell et al., 2001; Gill, 2000), while the collapse of Greenland's Norse colonies at ~600 cal yr BP (Buckland et al., 1995) coincides with a period of polar cooling."
With respect to the causes of these and other Holocene RCCs, the international team of scientists says that "of all the potential climate forcing mechanisms, solar variability superimposed on long-term changes in insolation (Bond et al., 2001; Denton and Karlen, 1973; Mayewski et al., 1997; O'Brien et al., 1995) seems to be the most likely important forcing mechanism." In addition, they note that "negligible forcing roles are played by CH4 and CO2," and that "changes in the concentrations of CO2 and CH4 appear to have been more the result than the cause of the RCCs."
In another study that shows the pervasiveness of the Medieval Warm Period-Little Ice Age type of cyclical climate change, de Garidel-Thoron and Beaufort (2001) reconstructed a 200,000-year history of primary productivity in the Sulu Sea north of Borneo, based on measured abundances of the coccolithophore Florisphaera profunda in a giant piston core. Three time-slices of this core were explored in detail in order to determine high-frequency cycles in the primary production record: one from 160 to 130 ka, one from 60 to 30 ka, and one from 22 to 4.1 ka. The finest-scale repeatable feature observed in all three time-slices was a climate-driven primary production oscillation that had a mean period of approximately 1500 years. With respect to this cycle, they say that its occurrence in the three different time-slices is suggestive of "a common origin and an almost stationary signal across different climatic conditions." They also point out the primary production cycle's similarity to the 1470-year temperature cycle observed by Dansgaard et al. (1984) in the Camp Century δ18O ice core record, the ~1500-year δ18O and chemical markers cycles observed by Mayewski et al. (1997) in the Summit ice core, the 1470-year climate cycle found by Bond et al. (1997) in North Atlantic deep-sea cores, and the 1500-year climate cycle found by Campbell et al. (1998) in an Alaskan lake. These and other observations led them to suggest that there is also "a common origin" for the documented cyclicity in the climates of both high and low latitudes, which Bond et al. (2001) associated with variable solar activity.
In light of the findings of these many multi-parameter analyses, it is becoming ever more clear that the millennial-scale oscillation of climate that has reverberated throughout glacial and interglacial periods alike is indeed the result of similar-scale oscillations in solar activity. Consequently, Mayewski et al. (2005) suggest that "significantly more research into the potential role of solar variability is warranted, involving new assessments of potential transmission mechanisms to induce climate change and potential enhancement of natural feedbacks that may amplify the relatively weak forcing related to fluctuations in solar output." We couldn't agree more, for until these mechanisms have been elucidated to everyone's satisfaction, the world's climate alarmists will continue to ignore the mountains of evidence that link millennial-scale climate cycles with similar solar cycles, and they will push ever harder for the adoption of wrong-headed energy policies to restrict anthropogenic CO2 emissions to the serious detriment of man and nature alike.
References
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Bond, G., Showers, W., Chezebiet, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I. and Bonani, G. 1997. A pervasive millennial scale cycle in North-Atlantic Holocene and glacial climates. Science 278: 1257-1266.
Buckland, P.C., Amorosi, T., Barlow, L.K., Dugmore, A.J., Mayewski, P.A., McGovern, T.H., Ogilvie, A.E.J., Sadler, J.P. and Skidmore, P. 1995. Bioarchaeological evidence and climatological evidence for the fate of Norse farmers in medieval Greenland. Antiquity 70: 88-96.
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Gill, R.B. 2000. The Great Maya Droughts: Water, Life, and Death. University of New Mexico Press, Albuquerque, New Mexico, USA.
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Mayewski, P.A., Rohling, E.E., Stager, J.C., Karlen, W., Maasch, K.A., Meeker, L.D., Meyerson, E.A., Gasse, F., van Kreveld, S., Holmgren, K., Lee-Thorp, J., Rosqvist, G. Rack, F., Staubwasser, M., Schneider, R.R. and Steig, E.J. 2004. Holocene climate variability. Quaternary Research 62: 243-255.
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Last updated 14 June 2006