The existence of a narrow spectral line in the internal oscillations of the climate system would be a physical discovery of the first order. Thus the appearance of such a spectral peak, with a Q>20 near 1500 years period, in the physical properties of the Greenland ice core is intriguing (bandwidth less than two cycles in 110,000 years). Apparently similar phenomena exist in some deep sea cores. It is shown, however, that the peak appears at precisely the period predicted as a simple alias of the seasonal cycle inadequately (under the Nyquist criterion) sampled at integer multiples of the common year. If this peak is therefore discounted, climate variability appears, as expected, to be a continuum process in the millennial band. More generally, however, aliasing of high frequencies must be a concern in any core analysis. ¿ 2000 American Geophysical Union