State-space models were applied to several climate indices associated with the El Ni¿o--Southern Oscillation (ENSO), including the Southern Oscillation Index (SOI) and its component sea level pressure series; the NINO3 sea surface temperature index; and the Northern Oscillation Index (NOI). The best models for each series include a significant long-term nonparametric trend combined with a stochastic stationary cyclic term that clearly delineates the El Ni¿o and La Ni¿a events. There is no evidence that the frequency of ENSO events has changed over the 20th century. The long-term trend, however, has contributed to an apparent increase in the magnitude of recent El Ni¿o events. This trend, potentially related to global warming, has increased the level of each series by an amount equal to 30--50% of the amplitude of their corresponding annual cycle or cyclic ENSO term. Thus, the background sea surface temperature in the eastern equatorial Pacific is more than 0.5¿C warmer now than prior to 1950, implying a greater overall impact of El Ni¿o events.