|El nino: Image NASA|
Khider, D., Stott, L.D., Emile-Geay, J., Thunell, R. and Hammond, D.E. 2011. Assessing El Niño Southern Oscillation variability during the past millennium. Paleoceanography 26: 10.1029/2011PA002139.
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.
For non scientists like me, MCA is the Medieval Climate Anomoly refered to on this blog as the MWP or Medieval warming period.
For more details, refer to CO2 Science here.