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Climate Model Predictions of CO2-Induced ENSO Changes
Volume 10, Number 13: 28 March 2007

Shortly after the powerful 1997-98 El Niņo - which some have described as "the strongest in recorded history" (Jimenez and Cortes, 2003) - climate models began to suggest that future CO2-induced global warming would lead to stronger and more frequent El Niņos (Timmermann et al., 1999; Collins 2000a,b; Cubasch et al., 2001). These predictions gave the world's climate alarmists yet another club with which to pound home their claims of impending climate catastrophe if anthropogenic CO2 emissions are not drastically reduced as soon as possible. Subsequently, however, a number of paleoclimate studies have cast great doubt upon those claims, revealing that ENSO frequency and intensity often tend to decline during periods of relative warmth (see the materials archived under ENSO (Relationship to Global Warming) in our Subject Index).

So what do the best and most up-to-date climate models now suggest?

In an arduous effort to definitively answer this question, Merryfield (2006) examined the sensitivity of ENSO-like variability to increased atmospheric CO2 concentration via detailed analyses of the output of 15 coupled climate models being evaluated for the IPCC's Fourth Assessment Report, where "differences in ENSO amplitude, period, and pattern between a low-CO2 (pre-industrial) control climate and a climate in which CO2 has stabilized at twice its pre-industrial concentration are considered."

According to Merryfield, this work revealed that "under CO2 doubling, 8 of the 15 models exhibit ENSO amplitude changes that significantly exceed centennial time scale variability within the respective control runs," which also means that 7 of the 15 models don't exhibit such behavior. In addition, in five of the eight models where significant change occurred, he reports that the amplitude decreased, whereas in the remaining three models it increased. Hence, with respect to predicted CO2-induced warming effects on ENSO amplitude, he correctly concludes "there is no consensus as to the sign of change."

With respect to ENSO period, Merryfield reports that the model intercomparison "yields a more consistent picture." However, that picture is one of very little impact of predicted CO2-induced global warming on ENSO periodicity. In fact, only five of the 15 models showed significant changes, with four exhibiting decreases and one an increase; and when all 15 models were considered together, the mean change in ENSO period was a decrease of only about 5%. As for ENSO patterns, the climate models "show a range of behaviors," in Merryfield's words, "with more than half of the models exhibiting a pronounced intensification in the central Pacific and two exhibiting an intensification in the far eastern Pacific."

In concluding, Merryfield says that "because the models vary widely in their ability to simulate present-day ENSO, their associated predictions of warming-induced changes cannot be viewed as equally reliable." In fact, the ENSO responses produced by the best and most up-to-date climate models of today are so widely scattered that the most that can be said about their predictions is that they tell us nothing of any substance about what we might realistically expect in a future warmer world in the way of ENSO behavior.

Is this progress? We guess that it is. After all, it is better that the models tell us nothing at all than that they tell us that which is contrary to the way the real world works. The only shame of the matter is that it has cost us so much in time, talent and funds to get from a state of not knowing anything about the future of ENSO to a state of not knowing anything more about the future of ENSO.

Sherwood, Keith and Craig Idso

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.

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.

Jimenez, C.E. and Cortes, J. 2003. Coral cover change associated to El Niņo, eastern Pacific, Costa Rica, 1992-2001. Marine Ecology 24: 179-192.

Merryfield, W.J. 2006. Changes to ENSO under CO2 doubling in a multimodel ensemble. Journal of Climate 19: 4009-4027.

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.