What was done
Extending the work of Mangini et al. (2005), who developed a 2000-year temperature history of the central European Alps based on an analysis of δ¹⁸O data obtained from stalagmite SPA 12 of Austria's Spannagel Cave, Vollweiler et al. (2006) used similarly-measured δ¹⁸O data obtained from two adjacent stalagmites (SPA 128 and SPA 70) within the same cave to create a master δ¹⁸O history covering the last 9000 years, which Mangini et al. (2007) compared with the Hematite-Stained-Grain (HSG) history of ice-rafted debris in North Atlantic Ocean sediments developed by Bond et al. (2001), who had reported that "over the last 12,000 years virtually every centennial time-scale increase in drift ice documented in our North Atlantic records was tied to a solar minimum."

What was learned
Mangini et al. (2007) found an almost unbelievably good correspondence between the peaks and valleys of their δ¹⁸O curve and the HSG curve of Bond et al., concluding that (1) "the excellent match between the curves obtained from these two independent data sets gives evidence that the δ¹⁸O signal recorded in Spannagel cave reflects the intensity of the warm North Atlantic drift, disproving the assumption that the Spannagel isotope record is merely a local phenomenon," and, therefore, that (2) their δ¹⁸O curve "can reasonably be assumed to reflect non-local conditions," implying it has wide regional applicability.

Having established this important point, Mangini et al. next focused on why their δ¹⁸O curve "displays larger variations for the last 2000 years than the multi-proxy record in Europe, which is mainly derived from tree-ring data" and "from low resolution archives (Mann et al., 1998, 1999; Mann and Jones, 2003)." The most probable answer, in their words, "is that tree-rings rather record the climate conditions during spring and summer," whereas both the HSG and δ¹⁸O curves "mirror winter-like conditions, which are only poorly recorded in tree-rings."

One important consequence of these differences is that whereas the Mann et al. and Mann and Jones data sets do not reflect the existence of the Medieval Warm Period and Little Ice Age, the Spannagel Cave data do. And applying the calibration curve derived for SPA 12 by Manginni et al. (2005) to the new δ¹⁸O curve, it can readily be determined that the peak temperature of the Medieval Warm Period was approximately 1.5°C higher than the peak temperature of the Current Warm Period.

What it means
Not only does the new data set of Manginni et al. (2007) confirm the inference of Bond et al.'s finding that over the last 12,000 years virtually every centennial-scale cooling of the North Atlantic region "was tied to a solar minimum," it also demonstrates that the data sets of Mann et al. and Mann and Jones fail to capture the full range of temperature variability over the past two millennia. As a result, the new data set clearly depicts the existence of both the Little Ice Age and Medieval Warm Period, the latter of which is seen to have been substantially warmer over periods of centuries than the warmest parts of the 20th century, almost certainly as a result of enhanced solar activity, and in spite of the fact that the air's CO2 concentration during the Medieval Warm Period was more than 100 ppm less than it is today. Consequently, there is every reason to believe that the global warming of the past century was neither unprecedented nor CO2-induced. Rather, it appears to have been nothing special and solar-induced.

References
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.

Mangini, A., Spotl, C. and Verdes, P. 2005. Reconstruction of temperature in the Central Alps during the past 2000 yr from a δ¹⁸O stalagmite record. Earth and Planetary Science Letters 235: 741-751.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814.

Vollweiler, N., Scholz, D., Muhlinghaus, C., Mangini, A. and Spotl, C. 2006. A precisely dated climate record for the last 9 kyr from three high alpine stalagmites, Spannagel Cave, Austria. Geophysical Research Letters 33: 10.1029/2006GL027662.