Background
The authors say there is an "expectation that ENSO [El Niño-Southern Oscillation] statistics would change under global warming, although the details remain uncertain because of the large spread of model projections for the 21st century (Guilyardi et al., 2009)." In addition, they indicate there is evidence of "increasing intensity as well as occurrence frequency of the so-called Central Pacific (CP) El Niño events since the 1990s." And the question thus arises: Is the latter a consequence of global warming?

What was done
In exploring this highly unsettled (Collins et al., 2010) situation, Yeh et al. ran a multi-millennial CGCM (coupled general circulation model) simulation "to assess whether the natural changes in the frequency of CP El Niño occurrence simulated by the model are comparable to the observed changes over the last few decades," suggesting that "if the changes are similar then we cannot rule out the possibility that the recent changes are simply natural variability."

What was learned
The five researchers report that their control simulation -- run for 4200 years with the present values of greenhouse gases -- "exhibits large variations of the occurrence frequency of the CP El Niño versus the eastern Pacific (EP) El Niño," and that it also "simulates to some extent changes in the occurrence ratio of CP and EP El Niño in comparison with the observations." Therefore, as they continue, they conclude that ...

What it means
... "we cannot exclude the possibility that an increasing of occurrence frequency of CP El Niño during recent decades in the observation could be a part of natural variability in the tropical climate system," providing one more piece of evidence for the likely benign nature of recent global warming.

References
Collins, M., An, S.-I., Cai, W., Ganachaud, A., Guilyardi, E., Jin, F.-F., Jochum, M., Lengaigne, M., Power, S., Timmermann, A., Vecchi, G. and Wittenberg, A. 2010. The impact of global warming on the tropical Pacific Ocean and El Niño. Nature Geoscience 3: 391-397.

Guilyardi, E., Wittenberg, A., Fedorov, A., Collins, M., Wang, C., Capotondi, A., Jan, G., Oldenborgh, V. and Stockdale, T. 2009. Understanding El Niño in ocean-atmosphere general circulation models: Progress and challenges. Bulletin of the American Meteorological Society 90: 325-340.