Vecchi, G.A. and Wittenberg, A.T. 2010. El Nino and our future climate: where do we stand? WIREs Climate Change 1: 10.1002/wcc.33.
In doing so, the two researchers at the US National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory report that "the amplitude and character of ENSO have been observed to exhibit substantial variations on timescales of decades to centuries," and they say that "many of these changes over the past millennium resemble those that arise from internally generated climate variations in an unforced climate model." In addition, they write that "ENSO activity and characteristics have been found to depend on the state of the tropical Pacific climate system, which is expected to change in the 21st century in response to changes in radiative forcing and internal climate variability." However, they indicate that "the extent and character of the response of ENSO to increases in greenhouse gases are still a topic of considerable research," and they state that "given the results published to date, we cannot yet rule out possibilities of an increase, decrease, or no change in ENSO activity arising from increases in CO2."
Vecchi and Witenberg conclude their review of the subject by stating "we expect the climate system to keep exhibiting large-scale internal variations," but adding that "the ENSO variations we see in decades to come may be different than those seen in recent decades," and admitting that "we are not currently at a state to confidently project what those changes will be."