The 20-yr cycle in Greenland ice core records

Abstract

CLIMATIC oscillations with a period of ∼20 yr have been reported in North American winter temperature records^1,2 midwestern US drought indicators (ref. 3 and J. M. Mitchell, Jr personal communication), dendrochronological data⁴, and sea-surface temperature anomalies⁵. These oscillations have mostly been attributed to double sunspot cycles, which seem to occur at about 22-yr intervals. Other possible causes include an 18.6-yr modulation of the diurnal and semidiurnal lunar tidal forces⁵. Critics^6,7 of these observations argue that such oscillations may only be an artefact of a red noise record, and are not uniformly present in a wide range of climatic data. It has also been argued that even if the oscillations are significant, their small amplitude renders them relatively unsuitable for climatic prediction¹. However, if such oscillations prove to be real, identifying their nature and possible causes, even though their amplitude is small, can enhance our understanding of climatic change. To gain more insight into such 20-yr oscillations, we have examined characteristic oscillations in Greenland ice core oxygen isotope records. The results show that the strongest spectral peak occurs at a period of 20±0.5 yr with a statistically significant amplitude. This period is essentially the same as that found in eastern North American winter temperatures¹. Moreover this 20-yr oscillation is found to be coherent in phase with the dominant variation in the Sun's motion about the centre of mass of the Solar System. This motion has been hypothesised⁸ to be closely linked with sunspots. On the basis of these observations we speculate here that a 20-yr oscillation may be a more fundamental solar and/or climatic oscillation than the nominal 22-yr cycle often mentioned.