Rigozo, N.R., Echer, E., Vieira, L.E.A. and Nordemann, D.J.R. 2001. Reconstruction of Wolf sunspot numbers on the basis of spectral characteristics and estimates of associated radio flux and solar wind parameters for the last millennium. Solar Physics 203: 179-191.
What was done
In the words of the authors, "a reconstruction of sunspot numbers for the last 1000 years was obtained using a sum of sine waves derived from spectral analysis of the time series of sunspot number RZ for the period 1700-1999." From this record they then derived the strengths of a number of parameters related to various aspects of solar variability over the past millennium.
What was learned
The authors say "the 1000-year reconstructed sunspot number reproduces well the great maximums and minimums in solar activity, identified in cosmonuclides variation records, and, specifically, the epochs of the Oort, Wolf, Sporer, Maunder, and Dalton Minimums, as well [as] the Medieval and Modern Maximums," the latter of which they describe as "starting near 1900." The mean sunspot number for the Wolf, Sporer and Maunder Minimums was 1.36. For the Oort and Dalton Minimums it was 25.05; while for the Medieval Maximum it was 53.00, and for the Modern Maximum it was 57.54. Compared to the average of the Wolf, Sporer and Maunder Minimums, therefore, the mean sunspot number of the Oort and Dalton Minimums was 18.42 times greater; while that of the Medieval Maximum was 38.97 times greater, and that of the Modern Maximum was 42.31 times greater. Similar strength ratios for the solar radio flux were 1.41, 1.89 and 1.97, respectively. For the solar wind velocity the corresponding ratios were 1.05, 1.10 and 1.11; while for the southward component of the interplanetary magnetic field they were 1.70, 2.54 and 2.67.
What it means
Both the Medieval and Modern Maximums in sunspot number and solar variability parameters stand out head and shoulders above all other periods of the past thousand years, with the Modern Maximum slightly besting the Medieval Maximum. Due to the many empirical evidences for climate modulation by solar variability, therefore, it is only to be expected, on this basis, that current temperatures may well be higher than at any other time during the past millennium. Since other factors come into play too, however, and since the Medieval and Modern Maximums were not all that different, this conclusion may not be precisely correct. In any event, the observations of this study suggest no need whatsoever for invoking variations in the air's CO2 content as a cause of temperature variations during any period of the past thousand years.