Background
Climate-alarmists often claim that global warming will increase both the frequency and intensity of ENSO events; and they use this claim in an attempt to convince the general public that continued burning of fossil fuels, which they claim is the major cause of the past century's rise in temperature, will lead to disastrous climatic consequences.

What was done
The authors of this important study generated "multi-century, monthly resolved records of tropical Pacific climate variability over the last millennium by splicing together overlapping fossil-coral records from the central tropical Pacific."  This exercise allowed them "to characterize the range of natural variability in the tropical Pacific climate system with unprecedented fidelity and detail."  The record they developed was not complete, but covered the periods AD 928-961, 1149-1220, 1317-1464, 1635-1703 and 1886-1998.

What was learned
In the words of the authors, "ENSO activity in the seventeenth-century sequence [was] not only stronger, but more frequent than ENSO activity in the late twentieth century."  They also note "there [were] 30-yr intervals during both the twelfth and fourteenth centuries when ENSO activity [was] greatly reduced relative to twentieth-century observations."

What it means
As noted by Tudhope and Collins (2003) in their perspective on this study, the Little Ice Age is generally acknowledged to have occurred over the seventeenth to nineteenth centuries, while the Medieval Warm Period is typically described as occurring from the eleventh to fourteenth centuries.  Hence, although both they and Cobb et al. describe the ENSO phenomenon as not being related to the mean thermal state of the climate, it should be obvious to even the casual observer that the two most extreme examples of ENSO strength and frequency detected in this study are just the opposite of what is typically predicted by climate alarmists: the strongest and most frequent ENSO activity occurred during the Little Ice Age, while the weakest expression of the phenomenon occurred during the Medieval Warm Period.

Reference
Tudhope, S. and Collins, M.  2003.  The past and future of El Niņo.  Nature 424: 261-262.