What was done
A δ13C profile of Globigerinoides rubber was extracted from a shallow-water core in the Gulf of Taranto (39°45'53"N, 17°53'33"E) to produce a high-precision record of climate variability over the past two millennia. This high-precision record was then statistically analyzed, together with a second two-millennia-long tree-ring record obtained from Japanese cedars (Kitagawa and Matsumoto, 1995), for evidence of recurring cycles using Singular Spectrum Analysis and Wavelet Transform, after which both records were compared with a 300-year record of sunspots.
What was learned
Plots of the two two-thousand-year series reveal the existence of the Dark Ages Cold Period (~400-800 AD), Medieval Warm Period (~800-1200 AD), Little Ice Age (~1500-1800 AD), and Current Warm Period, the roots of which can be traced to an upswing in temperature that began in the depths of the Little Ice Age "about 1700 AD." Results of the statistical analyses showed a common 11-year oscillation in phase with the Schwabe cycle of solar activity, plus a second multi-decadal oscillation (of about 93 years for the shallow-water G. rubber series and 87 years for the tree-ring series) in phase with the amplitude modulation of the sunspot number series over the last 300 years.
What it means
According to the authors, the overall phase agreement between the two climate reconstructions and the variations in the sunspot number series "favors the hypothesis that the [multi-decadal] oscillation revealed in δ13C from the two different environments is connected to the solar activity," which further suggests that a solar forcing was at work in both terrestrial and oceanic domains over the past two millennia. Thus, and once again, we have additional evidence for solar forcing of climate at decadal and multi-decadal time scales, as well as for the millennial-scale oscillation of climate that likely has been responsible for the 20th-century warming of the globe that led to the demise of the Little Ice Age and ushered in the Current Warm Period.
Kitagawa, H. and Matsumoto, E. 1995. Climatic implications of δ13C variations in a Japanese cedar (Cryptomeria japonica) during the last two millennia. Geophysical Research Letters 22: 2155-2158.