This is a tricky question. (Hell, it could well be a trick question.) El Niño takes place 2-3 times per generation. A stronger than usual phenomenon is a singl

This is a tricky question. (Hell, it could well be a trick question.) El Niño takes place 2-3 times per generation. A stronger than usual phenomenon is a single data point, and we all know how much validity one should give to an outlier. In order to quantify the strength of consecutive events, and especially to rule out freak outliers in both directions you will need a few distinct data points. Let's say 6-8 data points at a minimum. So the timescales for measurements will be in the range of 40-50 years. By the time you can reliably use El Niño as a yardstick to say that it's getting late to act, it will have been too late for quite some time.