If, however, there are significant underlying climate trends or cycles - or both - either known or unknown, that assumption is clearly invalid.  Consequently, Loehle, who is a Senior Research Scientist with the National Council for Air and Stream Improvement operating out of Naperville, Illinois, USA, uses a pair of 3,000-year-long proxy climate records that have minimal dating errors to characterize the pattern of climate change over the past three millennia in a new paper that provides the necessary context for properly evaluating the cause or causes of 20th century global warming.

The first of the two temperature series is the sea surface temperature (SST) record of the Sargasso Sea, which was derived by Keigwin (1996) from a study of the oxygen isotope ratios of foraminifera and other organisms contained in a sediment core retrieved from a deep-ocean drilling site on the Bermuda Rise.  This record provides SST data for about every 67th year from 1125 BC to 1975 AD.  The second temperature series is the ground surface temperature record derived by Holmgren et al. (1999, 2001) from studies of color variations of stalagmites found in a cave in South Africa, which variations are caused by changes in the concentrations of humic materials entering the region's ground water that have been reliably correlated with regional near-surface air temperature.

So why does Loehle use these two specific records?  And only these two records?  By way of explanation, he says that "most other long-term records have large dating errors, are based on tree rings, which are not reliable for this purpose (Broecker, 2001), or are too short for estimating long-term cyclic components of climate."  Also, in a repudiation of the approach employed by Mann et al. (1998, 1999) and Mann and Jones (2003), he reports that "synthetic series consisting of hemispheric or global mean temperatures are not suitable for such an analysis because of the inconsistent timescales in the various data sets," noting further, as a result of his own testing, that "when dating errors are present in a series, and several series are combined, the result is a smearing of the signal."

But can only two temperature series reveal the pattern of global temperature change?  Feeling a need to reassure us on this matter, Loehle reports that "a comparison of the Sargasso and South Africa series shows some remarkable similarities of pattern [our italics], especially considering the distance separating the two locations," and he says that this fact "suggests that the climate signal reflects some global pattern rather than being a regional signal only."  He also notes that a comparison of the mean record with the South Africa and Sargasso series from which it was derived "shows excellent agreement," and that "the patterns match closely," concluding that "this would not be the case if the two series were independent or random."

Proceeding with his plan of attack, which was to fit simple periodic models to the temperature data as functions of time, with no attempt to make the models functions of solar activity or any other physical variable, Loehle fit seven different time-series models to the two temperature series and to the average of the two series, using no data from the 20th century.  In all seven cases, he reports that good to excellent fits were obtained.  As an example, the three-cycle model he fit to the averaged temperature series had a simple correlation of 0.58 and an 83% correspondence of peaks when evaluated by a moving window count.

Comparing the forward projections of the seven models through the 20th century leads directly to the most important conclusions of Loehle's paper.  He notes, first of all, that six of the models "show a warming trend over the 20th century similar in timing and magnitude to the Northern Hemisphere instrumental series," and that "one of the models passes right through the 20th century data."  These results clearly suggest, in his words, "that 20th century warming trends are plausibly a continuation of past climate patterns" and, therefore, that "anywhere from a major portion to all of the warming of the 20th century could plausibly result from natural causes."

As dramatic and important as these observations are, they are not the entire story of Loehle's insightful paper.  His analyses also reveal a long-term linear cooling trend of 0.25°C per thousand years since the peak of the interglacial warm period that occurred some 7000 years ago, which result is essentially identical to the mean value of this trend that was derived from seven prior assessments of its magnitude and five prior climate reconstructions.  In addition, Loehle's analyses reveal the existence of the Medieval Warm Period of 800-1200 AD, which is shown to have been significantly warmer than the portion of the Modern Warm Period we have so far experienced, as well as the existence of the Little Ice Age of 1500-1850 AD, which is shown to have been the coldest period of the entire 3000-year record.

As corroborating evidence for the global nature of these major warm and cold intervals, Loehle cites sixteen peer-reviewed scientific journal articles that document the existence of the Medieval Warm Period in all parts of the world, as well as eighteen other articles that document the worldwide occurrence of the Little Ice Age.  And in one of the more intriguing aspects of his study - of which Loehle makes no mention, however - both the Sargasso Sea and South African temperature records reveal the existence of a major temperature spike that began sometime in the early 1400s.  This abrupt warming pushed temperatures considerably above the peak warmth of the 20th century before falling back to pre-spike levels in the mid 1500s, providing support for the similar finding of higher-than-current temperatures in that time interval by McIntyre and McKitrick (2003) in their reanalysis of the data employed by Mann et al. to create their controversial "hockeystick" temperature history, which gives no indication of the occurrence of this high-temperature regime.

In another accomplishment of note, the models developed by Loehle reveal the existence of three climate cycles previously identified by others.  In his culminating seventh model, for example, there is a 2388-year cycle that he describes as comparing "quite favorably to a cycle variously estimated as 2200, 2300, and 2500 years (Denton and Karlen, 1973; Karlen and Kuylenstierna, 1996; Magny, 1993; Mayewski et al., 1997)." There is also a 490-year cycle that likely "corresponds to a 500-year cycle found previously (e.g. Li et al., 1997; Magny, 1993; Mayewski et al., 1997)" and a 228-year cycle that "approximates the 210-year cycle found by Damon and Jirikowic (1992)."

The compatibility of these findings with those of several studies that have identified similar solar forcing signals caused Loehle to thus conclude that "solar forcing (and/or other natural cycles) is plausibly responsible for some portion of 20th century warming" or, as he indicates in his abstract, maybe even all of it.

References
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Damon, P.E. and Jirikowic, J.L.  1992.  Solar forcing of global climate change?  In: Taylor, R.E., Long A. and Kra, R.S. (Eds.), Radiocarbon After Four Decades.  Springer-Verlag, Berlin, Germany, pp. 117-129.

Denton, G.H. and Karlen, W.  1973.  Holocene climate variations - their pattern and possible cause.  Quaternary Research 3: 155-205.

Holmgren, K., Karlen, W., Lauritzen, S.E., Lee-Thorp, J.A., Partridge, T.C., Piketh, S., Repinski, P., Stevenson, C., Svanered, O. and Tyson, P.D.  1999.  A 3000-year high-resolution stalagmite-based record of paleoclimate for northeastern South Africa.  The Holocene 9: 295-309.

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