Background
The authors write that over the period 1871-2008, "it is widely agreed that the earth's average temperature has warmed," but they say that "what is far less clear is how this warming trend has altered forms of climate variability such as ENSO," noting, for example, that "Trenberth and Hoar (1996, 1997) argue that there has been an increase in the occurrence of El Niņos since 1976" and that they suggest that this change "is due to global warming."

What was done
Noting that DelSole and Tippett (2009) have recently demonstrated that meteorological records that are relatively short, on the order of 50 years or less, "are not sufficient to detect trends in a mode of variability such as ENSO," Ray and Giese decided to explore the postulated global warming-ENSO connection in greater detail, by first comparing sea surface temperatures (SSTs) derived from an ocean reanalysis with three widely-used SST reconstructions, after which they used the reanalysis data to evaluate potential changes in ENSO characteristics over the period 1871-2008.

What was learned
"Overall," in the words of the two researchers, "there is no evidence that there are changes in the [1] strength, [2] frequency, [3]duration, [4] location or [5] direction of propagation of El Niņo and La Niņa anomalies caused by global warming during the period from 1871 to 2008."

What it means
Quoting Ray and Giese one final time, they write that "climate change is an ongoing process that is expected to continue in the future, but how El Niņo and La Niņa variability will react to this change is yet unresolved." And especially is this so, in light of the fact that global temperatures appear to have neither risen nor fallen by any significant amount over the past decade and a half, suggesting that even the direction of future global temperature change remains a mystery.

References
DelSole, T. and Tippett, M.K. 2009. Average predictability time. Part I: Theory. Journal of the Atmospheric Sciences 66: 1172-1187.

Trenberth, K.E. and Hoar, T.J. 1996. The 1990-1995 El Niņo-Southern Oscillation event: Longest on record. Geophysical Research Letters 23: 57-60.

Trenberth, K.E. and Hoar, T.J. 1997. El Niņo and climate change. Geophysical Research Letters 24: 3057-3060.