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Natural Climate Variability on Decade-to-Century Time Scales
The 14C age residuals from the polynomial trendline (A) and the approximately 200-year waveforms of temperature (solid curve) and 14C residuals (dashed curve) (B). The residuals have also been smoothed to highlight long-term 14C fluctuations particularly evident in the Medieval Warm Period and Little Ice Age. The waveforms are approximately in phase up to about A.D. 1050, after which the relationship breaks down until 1800.
respectively). Prior to A.D. 600, the series is very stable, with few significant long-term departures.
We applied SSA to the Δ14C residuals to isolate the waveform of the approximately 200-year oscillation. This waveform is also shown in Figure 11, with the 204-year temperature waveform superimposed on it for comparison. Not surprisingly, this comparison produced mixed results. There is some degree of phase-locking during the first 1300 years, especially considering the error in the radiocarbon dates of about ± 10 years (Stuiver and Becker, 1986), after which it completely breaks down during the Little Ice Age period until about A.D. 1800. There is also a lack of agreement in the amplitude modulation apparent in each series. The Δ14C oscillation has maximum amplitude through the period of the Spörer and Maunder Minima (also the Little Ice Age). In contrast, the temperature oscillation has maximum amplitude around the Medieval Solar Maximum (also the Medieval Warm Period).
The net result of this examination for a climate-sun link in Tasmanian temperatures at periods of about 80 and 200 years is mixed. If the relationship between solar cycle length and temperatures found by Friis-Christensen and Lassen (1991) is real, then there is some support for a climate-sun link in the temperature reconstruction at a period of 80 to 90 years. The link between the 204-year temperature oscillation and solar variability is more tenuous because of unstable phasing and the mismatch in amplitude modulation noted above. Given these somewhat contradictory results, the climate-sun link cannot be accepted at this time.
IMPLICATIONS FOR DETECTING THE GREENHOUSE EFFECT IN THE SOUTHERN HEMISPHERE
The decadal-scale natural oscillations found in warm-season Tasmanian temperatures indicate that the climate system in this part of the SH is to some degree internally, and perhaps also externally, forced. The level of forcing due to the four oscillations (as a percentage of the total yearly variance in temperatures) is comparatively small, amounting to only about 10 percent. However, when compared to the variance in the reconstruction due to decadal-scale temperature fluctuations alone (i.e., only variance at wavelengths >10 years), the oscillations account for approximately 46 percent of that low-frequency fraction. Given that only four oscillatory modes are necessary to explain a substantial fraction of the low-frequency variance, the following question is posed: To what extent have these natural oscillations contributed to the recent decadal-scale anomalous warming over Tasmania, as described in Cook et al. (1991)?
An examination of the waveform plots in Figure 6 suggests that their collective effect on Tasmanian temperatures since 1960 has been significant. Since that time, all four waveforms have either peaked or are still increasing. In terms of relative amplitude, the 204-year oscillation is also in its most active phase since the Medieval Warm Period. Therefore, it is plausible that the anomalous warming since 1960 in Tasmania is simply due to an unusual coincidence between the four oscillations and an increase in the amplitude of the 204-year term. It is also noteworthy that during