Background
Climate models have long suggested that global warming will foster stronger and more frequent El Niņos ((Timmermann et al., 1999; Collins 2000a,b; Cubasch et al., 2001). Palaeoclimate investigations, on the other hand, have generally indicated just the opposite, as evidenced by the materials we have archived in our Subject Index under the heading of El Niņo (Relationship to Global Warming).

What was done
In a model study designed to explore this inconsistency, Meehl et al. used two global coupled climate models: the Parallel Climate Model (PCM, described by Washington et al., 2000) and the Community Climate System Model version 3 (CCSM3, described by Collins et al., 2006), which they say "have been shown to simulate El Niņo reasonably well."

What was learned
The three National Center for Atmospheric Research scientists report that "El Nino frequency in these experiments does not change appreciably with increased positive radiative forcing," in contradiction of most earlier model studies; and in even stronger contradiction of earlier model studies, they found that the amplitude of Niņo3.4 SST standard deviations decreases somewhat with 2xCO2 in various PCM and CCSM3 scenarios, "and even more with 4xCO2." In addition, they say that "the decrease in El Niņo amplitude with larger positive radiative forcing for 4xCO2 ... is well outside the range of inherent low frequency El Niņo variability, and the reductions in amplitude for these experiments are statistically significant."

With respect to a closely related topic, Meehl et al. note that previous climate model studies showed "an intensification of El Nino precipitation anomalies in a future warmer climate." In their PCM and CCSM3 model studies, however, they found that even though El Niņo anomalies are superimposed upon warmer base state SSTs, "precipitation anomalies in the tropics are not [our italics] intensified in the future El Niņo events because El Niņo amplitudes decrease with greater positive radiative forcing."

What it means
In concluding their paper, Meehl et al. say that although they "identified physical processes that would lead to lower amplitude El Niņo events" in the two models they studied, Guilyardi (2005) and Merryfield (2006) analyzed multi-model datasets and found not only decreases in El Niņo amplitude in some models, but "increases or little change in others." Hence, their bottom-line conclusion is that "there is currently no definitive answer to the question of what will happen to El Niņo in the future," at least from the perspective of what climate models suggest. As noted in our Background statement, however, real-world palaeoclimate studies suggest that El Niņo amplitudes and frequency will likely both decline if the earth continues to warm, more in line with the findings of Meehl et al.

References
Collins, M. 2000a. Understanding uncertainties in the response of ENSO to greenhouse warming. Geophysical Research Letters 27: 3509-3513.

Collins, M. 2000b. The El Niņo Southern Oscillation in the second Hadley center coupled model and its response to greenhouse warming. Journal of Climate 13: 1299-1312.

Collins, W.D. et al. 2006. The community climate system model: CCSM3. Journal of Climate (in press).

Cubasch, U., Meehl, G.A., Boer, G.J., Stouffer, R.J., Dix, M., Noda, A., Senior, C.A., Raper, S. and Yap, K.S. 2001. Projections of future climate change. In: Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P., Dai, X., Maskell, K. and Johnson, C.I. (Eds.). Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the 3rd Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK, pp. 525-582.

Guilyardi, E. 2005. El Niņo-mean state-seasonal cycle interactions in a multi-model ensemble. Climate Dynamics 10.1007/s0038-005-0084-6.

Merryfield, W. 2006. Changes to ENSO under CO2 doubling in the IPCC AR4 coupled climate models. Journal of Climate (in press).

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.

Washington, W.M., Weatherly, J.W., Meehl, G.A., Semtner Jr., A.J., Bettge, T.W., Craig, A.P., Strand Jr., W.G., Arblaster, J.M., Wayland, V.B, James, R. and Zhang, Y. 2000. Parallel climate model (PCM) control and transient simulations. Climate Dynamics 16: 755-774.