What was done
One of the most difficult challenges facing today's climate community is to differentiate natural climate change from change due to anthropogenic forcing.  A key aspect of this challenge is to document and understand cycles of natural climate variability, because without an accurate understanding of nature's role in climate variability, it is impossible to separate out any anthropogenic component that may be present.  In the current study, the authors improve our understanding of the natural component of climate variability by examining an extraordinarily high number of pollen grains in a lake sediment core from the southwest German alpine foreland (48°06'00"N, 9°43'44"E) and thereby producing a climate proxy for this region for the penultimate interglacial (126,000 - 110,000 years before present).

What was learned
The results of the authors' analysis revealed the presence of 11 major cold events having an average recurrence time of approximately 1450 years over the course of the last interglacial, which periodicity is essentially identical to the millennial-scale oscillation of climate throughout the current interglacial (Bond et al., 1997, 2001; deMenocal et al., 2000; McDermott et al., 2001; Gupta et al., 2003; Hu et al., 2003), as well as the similar oscillation that has been documented throughout interglacial and glacial periods alike over the past half-million years (McManus et al., 1999) and the equivalent oscillation that persisted even before the development of the large 100,000-year ice sheets characteristic of the late Pleistocene (Raymo et al., 1998).  As for the cause of the 1450-yr cycle reported by Müller et al., the authors have no definitive answer, but suggest that it may ultimately be the product of variations in solar activity.

What it means
This study adds to the growing body of evidence that earth's climate oscillates in a well-defined manner on a timescale of approximately 1500 years.  This knowledge is very important, for it suggests that something other than the historic buildup of carbon dioxide in the atmosphere may be responsible for 20th-century warming, the timing of which fits nicely with the prior history of this oscillation.  Indeed, there is absolutely no evidence for any concomitant oscillation in the air's CO2 content accompanying the 1500-year oscillation of climate that was responsible for the warmings that produced the prior Medieval Warm Period and the still earlier Roman Warm Period, nor is there any evidence for such a CO2 oscillation accompanying any of the similar warmings of the penultimate interglacial or the other interglacial and glacial periods through which this persistent oscillation of climate has been documented to occur as far back in time as scientists have been able to peer.

References
Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G.  2001.  Persistent solar influence on North Atlantic climate during the Holocene.  Science 294: 2130-2136.

Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., de Menocal, P., Priore, P., Cullen, H., Hajdas, I. and Bonani, G.  1997.  A pervasive millennial-scale cycle in North Atlantic Holocene and glacial climates.  Science 278: 1257-1266.

Gupta, A.K., Anderson, D.M. and Overpeck, J.T.  2003.  Abrupt changes in the Asian southwest monsson during the Holocene and their links to the North Atlantic Ocean.  Nature 421: 354-356.

Hu, F.S., Kaufman, D., Yoneji, S., Nelson, D., Shemesh, A., Huang, Y., Tian, J., Bond, G., Clegg, B. and Brown, T.  2003.  Cyclic variation and solar forcing of Holocene climate in the Alaskan subarctic.  Science 301: 1890-1893.

McDermott, F., Mattey, D.P. and Hawkesworth, C.  2001.  Centennial-scale Holocene climate variability revealed by a high-resolution speleothem δ¹⁸O record from SW Ireland.  Science 294: 1328-1331.

McManus, J.F., Oppo, D.W. and Cullen, J.L.  1999.  A 0.5-million-year record of millennial-scale climate variability in the North Atlantic.  Science 283: 971-974.

Raymo, M.E., Ganley, K., Carter, S., Oppo, D.W. and McManus, J.  1998.  Millennial-scale climate instability during the early Pleistocene epoch.  Nature 392: 699-702.