The PDO is a real measurement, but it's not likely a real effect.  In the literature that I've read, the PDO appears to be just an echo of whatever multiple ENSO episodes recently did, in which case ENSO is the effect.  A PDO can last from 10 to 40 years.  However, there are frequent short sets of 1 to 5 warm years embedded within a generally cool phase and vice-versa, so stating a long PDO phase length is misleading.  Unlike ENSO, where heat is buried deep and rises to the surface at times, PDO is just a sloshing back and forth and has little measurable global temperature effect, it just shifts the jet stream around a bit.  What is called a cool phase, for instance, is actually associated with extremely high sea surface temperatures on the other side of the Pacific. A PDO is not capable of causing a long-term warming trend, just short-term mostly local temperature variations, because it is not adding heat to the planet.  Predictions aren't useful for something that has no real effect.