What was done
Throughout the first half of the 1990s, earth's climate system exhibited a pattern variously described as a sequence of El Niño events or a continual El Niño episode.  As a result, there has been much debate concerning the uniqueness of this event and whether or not it is a manifestation of global warming.  In an attempt to provide some insight to this complex phenomenon, the authors of this paper examined the instrumental record and proxy indices over the last three centuries to learn if the 1990-1995 persistent El Niño episode was in fact unique.

What was learned
Analysis of the instrumental record showed four persistent El Niño sequences during the period 1876-1996, with the 1911-1915 sequence being "comparable [in] structure" to the 1990-1995 event.  Tree-ring proxy data over the period 1706-1977 revealed the presence of several ENSO events of prolonged duration similar to the 1990-1995 event, such that there occurred about "four or five 'persistent' El Niño sequences … in each of the eighteenth and nineteenth centuries."  The authors also reported finding no indication of a change in ENSO frequency and magnitude in their record, but they did note the presence of decadal and multidecadal signals.

What it means
In the end, the authors concluded, based upon their data, that there is "no evidence for an enhanced greenhouse influence on the frequency or duration of 'persistent' ENSO event sequences," thus weakening the merits of the notion that El Niño events will become both longer and of greater magnitude in a CO2-induced globally-warmed world.  In addition, the authors' documentation of decadal and multidecadal signals in the ENSO data suggest that variability in ENSO frequency and magnitude is a natural feature of climate.