Of course it's a natural event, as is La Nina. (Very) broadly speaking, La Nina can be thought of as a storage phase, when ocean currents and winds conspire to push warm surface water into the depths and hence push surface temperatures down. El Nino is what happens when that warm water pops back up. Over the long term, El Nino and La Nina should average to zero - La Nina pushes temperatures down, El Nino pushes them up, but they dance around a steady average.
That's not what we see any more, though. Instead, La Nina years tend to hold things roughly steady, while El Ninos give large sudden upward jumps - exactly as you'd expect if they were now overlaid on a constant warming trend. The escalator of SkepticalScience fame, in other words.
