What was done
The authors retrieved two 8-m cores and two 0.5-m cores from the center of lake Laguna Pallcacocha in the southern Ecuadorian Andes; and from careful analyses of the cores' sediments, they derived a continuous history of El Niņo/Southern Oscillation (ENSO) events over the past 12,000 years.

What was learned
The authors report that for the moderate and strong ENSO events detected by their analytical techniques (weaker events are not registered), "the overall trend exhibited in the Pallcacocha record includes a low concentration of events in the early Holocene, followed by increasing occurrence after 7,000 cal. yr BP, with peak event frequency occurring at ~1,200 cal. yr BP," after which the frequency of events declines dramatically to the present.  Superimposed upon this long-term trend, in their words, "is a millennial-scale oscillation that is coherent throughout the Holocene, but displays less significant variance in the early Holocene."

With respect to the last 1,200 years of the record, the decline in the frequency of ENSO events is anything but smooth.  In coming out of the Dark Ages Cold Period, which was one of the coldest intervals of the Holocene (McDermott et al., 2001), the number of ENSO events experienced by the earth drops by an order of magnitude, from a high of approximately 33 events per 100 yr to a low of about 3 events per 100 yr, centered approximately on the year AD 1000, which is right in the middle of the Medieval Warm Period, as delineated by the work of Esper et al. (2002).  Then, at approximately AD 1250, the frequency of ENSO events exhibits a new peak of approximately 27 events per 100 yr in the midst of the longest sustained cold period of the Little Ice Age, again as delineated by the work of Esper et al.  Finally, ENSO event frequency declines in zigzag fashion to a low on the order of 4 to 5 events per 100 yr at the start of the Modern Warm Period, which according to the temperature history of Esper et al. begins at about 1940.

Going back in time from 1,200 years ago, the declining long-term trend in ENSO event frequency reduces the frequency differences that exist between millennial-scale warm and cold periods.  Nevertheless, at 2000 cal. yr BP, the Roman Warm Period, as delineated by McDermott et al., is near its peak warmth and ENSO event frequency is again at a very low level.  By 3000 cal. yr BP, however, ENSO event frequency is yet again significantly elevated in response to another millennial-scale cold period that appears in the climatic reconstruction of McDermott et al.

What it means
The authors say "the Laguna Pallcacocha record provides evidence for millennial-scale oscillation of ENSO activity during the late Holocene" and that "two processes known to operate at this timescale are the deposition of ice-rafted detritus in the North Atlantic (Bond events)" and "changes in the carbon cycle represented by the residual ¹⁴C record."  They also note that the modeling study of Clement et al. (2000) "produced results consistent with observations from the Laguna Pallcacocha record ... forced with orbitally induced changes in insolation," in that it produced "reduced ENSO amplitude and frequency during the early Holocene, with a gradual increase in both parameters towards the late Holocene."  In addition, they note that this model (Clement and Cane, 1999) "has demonstrated that ENSO can display millennial, as well as modern-day, variability."  Hence, they conclude that "although the links with solar and North Atlantic climate warrant further study, internal ENSO dynamics operating independently are a sufficient explanation for the millennial variability that we observe."

In light of the fact that our review of the data indicates moderate and strong ENSO events to have been much more prevalent in the colder portions of the late Holocene than they were in its warmer portions, we tend to lean toward the solar-induced millennial-scale climate oscillation linkage.  Also, our observations strongly suggest that any future warming of the globe should lead to a decline in the frequency of moderate to strong ENSO events, which should allay irrational fears created by climate alarmists intent on scaring the nations of the world into believing that more frequent super-ENSO events will accompany a warming of the globe.

References
Clement, A.C. and Cane, M.A.  1999.  Mechanisms of global climate change at millennial time scales.  Geophysical Monograph Series 112 (Clark, P.U., Webb, R.S. and Keigwin, L.D., Eds.) American Geophysical Union, Washington, DC, pp. 363-371.

Clement, A.C., Seager, R. and Cane, M.A.  2000.  Suppression of El Niņo during the mid-Holocene by changes in the Earth's orbit.  Paleoceanography 15: 731-737.

Esper, J., Cook, E.R. and Schweingruber, F.H.  2002.  Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability.  Science 295: 2250-2253.

McDermott, F., Mattey, D.P. and Hawkesworth, C.  2001.  Centennial-scale Holocene climate variability revealed by a high-resolution speleothem delta¹⁸O record from SW Ireland.  Science 294: 1328-1331.