Background
The authors write that "the El Niño Southern Oscillation (ENSO), centered in the tropical Pacific Ocean, is the leading mode of interannual climate variability in the global climate system," and they state that climate models forced by increased greenhouse gas concentrations can be used to "simulate changes in ENSO variability." However, they note that these simulations do not agree on even the sign of change they predict, nor on the resultant mean state of the tropical Pacific Ocean. Thus, they rightly state that paleoclimate records that document past ENSO variability, over a long enough time period to include a significant range of mean global air temperature, "provide a useful reference against which to test models that attempt to simulate the natural behavior of ENSO," as well as what that phenomenon would be like in a warmer world of the future.

What was done
In order to provide such a climatic sounding board, Khider et al. developed a history of ENSO variability over a period of time that included both the Medieval Climate Anomaly (MCA, AD 800-1300) and the Little Ice Age (LIA, AD 1500-1850). This they did "by comparing the spread and symmetry of δ¹⁸O values of individual specimens of the thermocline-dwelling planktonic foraminifer Pulleniatina obliquiloculata extracted from discrete time horizons of a sediment core collected in the Sulawesi Sea, at the edge of the western tropical Pacific warm pool," and by interpreting the spread of individual δ¹⁸O values "to be a measure of the strength of both phases of ENSO," while the symmetry of the δ¹⁸O distributions was used by them "to evaluate the relative strength/frequency of El Niño and La Niña events."

What was learned
The five researchers state that their results indicate that "the strength/frequency of ENSO, as inferred from the spread of the δ¹⁸O distributions, during the MCA and during the LIA was not statistically distinguishable and was comparable to that of the 20th century," but they write that their results suggest that "ENSO during the MCA was skewed toward stronger/more frequent La Niña than El Niño," an observation that they note is "consistent with the medieval megadroughts documented from sites in western North America." On the other hand, they note that a coral record from the central Pacific (Cobb et al., 2003) suggests that the LIA was characterized by an increase in the strength/frequency of ENSO events compared to the MCA and the 20th century. And they acknowledge that whereas the MCA was skewed toward "stronger/more frequent La Niña than El Niño" in their reconstruction, they indicate that the studies of Moy et al. (2002) and Conroy et al. (2008) "show an increase in the frequency of El Niño events during this time period."

What it means
With such discrepancies as these existing among real-world reconstructions of the effects of mean global temperature on the ratio of El Niños to La Niñas, it would appear that we don't even have the means for determining which of the similarly-divergent scenarios of current state-of-the-art climate model simulations is correct, or how close or how far from reality they each may be, which surely does not make for a solid foundation for divining what a future warmer world might be like with respect to "the leading mode of interannual climate variability in the global climate system," which sure sounds like something one would want to get right.

References
Cobb, K.M., Charles, C.D., Cheng, H. and Edwards, R.L. 2003. El Niño/Southern Oscillation and tropical Pacific climate during the last millennium. Nature 424: 271-276.

Conroy, J.L., Overpeck, J.T., Cole, J.E., Shanahan, T.M. and Steinitz-Kannan, M. 2008. Holocene changes in eastern tropical Pacific climate inferred from a Galapagos lake sediment record. Quaternary Science Reviews 27: 1166-1180.

Moy, C.M., Seltzer, G.O., Rodbell, D.T. and Anderson, D.M. 2002. Variability of El Niño/Southern Oscillation activity at millennial timescales during the Holocene epoch. Nature 420: 162-165.