A 35-year record of satellite altimetry allows scientists to place this year's El Niño alongside its strongest predecessors. The comparison rests on continuous measurement of sea surface height, a direct indicator of heat accumulation in the equatorial Pacific that characterizes the phenomenon.

Satellite altimetry measures sea level to centimetre precision from orbit, recording the thermal anomalies that show up as bulges in the tropical Pacific Ocean. That continuous record, begun more than three decades ago, is now the benchmark for assessing the magnitude of each El Niño episode against the major historical events.

The value of such a long series lies in the ability to contrast current behaviour with that of the most intense episodes of the instrumental period, rather than describing it in isolation. The comparison helps to place the intensity of the ongoing event within the natural variability of the climate system.