What was done
In the words of the authors, "an annually dated ice core recovered from [the] South Pole in 1995, that covers the period 1487-1992, was analyzed for the marine biogenic sulfur species methanesulfonate (MS)," after which "orthogonal function analysis [was] used to calibrate the high-resolution MS series with associated environmental series for the period of overlap (1973-92)."  This procedure allowed Meyerson et al. to derive a five-century history of (1) ENSO activity and (2) southeastern Pacific sea-ice extent, the latter of which parameters they say "is indicative of regional temperatures within the Little Ice Age period in the southeastern Pacific sea-ice sector."

What was learned
Among other things, the authors noted a shift at about 1800 towards generally cooler conditions.  This shift was concurrent with an increase in the frequency of El Niņo events in the ice core proxy record, which is contrary to what is generally predicted by climate models, where cooling generally leads to less El Niņo activity and warming leads to more (Timmermann et al., 1999).  On the other hand, the authors' findings were harmonious with the historical El Niņo chronology of both South America (Quinn and Neal, 1992) and the Nile region (Quinn, 1992), which depict "increased El Niņo activity during the period of the Little Ice Age (nominally 1400-1900) and decreased El Niņo activity during the Medieval Warm Period (nominally 950-1250)," as per Anderson (1992) and de Putter et al., 1998).

What it means
Once again, we have a situation where real-world data demonstrate model-based climate-alarmist claims to be 180 degrees out of phase with reality.

References
Anderson, R.Y.  1992.  Long-term changes in the frequency of occurrence of El Niņo events.  In: Diaz, H.F. and Markgraf, V. (Eds.), El Niņo.  Historical and Paleoclimatic Aspects of the Southern Oscillation.  Cambridge University Press, Cambridge, UK, pp. 193-200.

de Putter, T., Loutre, M.-F. and Wansard, G.  1998.  Decadal periodicities of Nile River historical discharge (A.D. 622-1470) and climatic implications.  Geophysical Research Letters 25: 3195-3197.

Quinn, W.H.  1992.  A study of Southern Oscillation-related climatic activity for A.D. 622-1990 incorporating Nile River flood data.  In: Diaz, H.F. and Markgraf, V. (Eds.), El Niņo.  Historical and Paleoclimatic Aspects of the Southern Oscillation.  Cambridge University Press, Cambridge, UK, pp. 119-149.

Quinn, W.H. and Neal, V.T.  1992.  The historical record of El Niņo events.  In: Bradley, R.S. and Jones, P.D. (Eds.), Climate Since A.D. 1500.  Routledge, London, UK, pp. 623-648.

Timmermann, A., Oberhuber, J., Bacher, A., Esch, M., Latif, M. and Roeckner, E.  1999.  Increased El Niņo frequency in a climate model forced by future greenhouse warming.  Nature 398: 694-696.