Background
The author notes there has been a "long-running debate as to how the El Niņo-Southern Oscillation (ENSO) might react to global warming," and that "the focus in most model studies on ENSO and climate change has been on whether the Pacific will tend to a more permanent El Niņo state as the world warms due to an enhanced greenhouse effect."

What was done
In an attempt to resolve the riddle, Nicholls examined "trends in the seasonal and temporal behaviour of ENSO, specifically its phase-locking to the annual cycle over the past 50 years," where phase-locking, in his words, "means that El Niņo and La Niņa events tend to start about April-May and reach a maximum amplitude about December-February," which is why he examined trends in ENSO indices for each month of the year.

What was learned
The Australian researcher determined "there has been no substantial modulation of the temporal/seasonal behaviour of the El Niņo-Southern Oscillation" -- as measured by (1) the sea surface temperature averaged across the region 5°S-5°N by 120°W-170°W, and (2) the Southern Oscillation Index (the non-standardized difference between sea level pressures at Tahiti and Darwin) -- over the past 50 years, during what he describes as "a period of substantial growth in the atmospheric concentrations of greenhouse gases and of global warming."

What it means
The fact that Nicholls found that "the temporal/seasonal nature of the El Niņo-Southern Oscillation has been remarkably consistent through a period of strong global warming" clearly repudiates the early climate-model-derived inferences of Timmermann et al. (1999), Collins (2000a,b), and Cubasch et al. (2001) that global warming will increase both the frequency and intensity of ENSO events -- which projections (not surprisingly) followed fast on the heels of the powerful 1997-98 El Niņo that was subsequently described by some as "the strongest in recorded history" (Jimenez and Cortes, 2003).

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.

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.

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.