Hansen's El Nino forecast reprised

I have no idea why Roger Pielke chose to bring up James Hansen's old 2006 El Nino forecast again just recently, but since he did, and it generated some hot air in the blogosphere, I might as well add my view.

1. Hansen originally said in a draft paper:

We suggest that an El Nino is likely to originate in 2006 and that there is a good chance it will be a “super El Nino”, rivaling the 1983 and 1997-1998 El Ninos, which were successively labeled the “El Nino of the century” as they were of unprecedented strength in the previous 100 years.

[quote taken from Prometheus, I don't have Hansen's draft myself]. The meaning of "likely" is usually interpreted in the range of 66-90% probability, so clearly the "good chance" of a super El Nino, which is conditional on there being an El Nino at all, is lower than this. Given the historical context of what a "super El Nino" is (there have been 2 in the past 30 years by Hansen's definition) a "good chance" cannot reasonably be interpreted at above 50% probability, and may have been meant as something substantially lower (albeit higher than climatology). Incidentally, I am not inclined, as some are, to give Hansen a free pass on that "super El Nino" prediction on the basis that it did not make it to further versions of the paper - it was clearly his original opinion, despite the fact that he withdrew it in the face of opposing views. I have also been known to have reviewers and editors prevent me from presenting my views fully in the peer reviewed literature, this does not always mean they have convinced me I am wrong! But anyway, it was hardly a confident prediction in the first place.

2. At the time, more established seasonal forecasters were not predicting El Nino. From Roger's previous post, attributed to NOAA/NCEP/CPC:

Most of the statistical and coupled model forecasts indicate ENSO-neutral conditions in the tropical Pacific through the end of 2006 (Fig. 6). The spread of the most recent statistical and coupled model forecasts (weak La Niña to weak El Niño) indicates uncertainty in the outlooks for the last half of the year. However, current conditions (stronger-than-average easterly winds over the central equatorial Pacific and below-average upper-ocean heat content) support those forecasts indicating that La Nina conditions will continue for the next 1-3 months.

3. An El Nino actually occurred, but it was a weak one.

So, what can we conclude from this?

Well, Hansen clearly went out a little on a limb, at a time when established forecasters were more circumspect. He got it right, with events turning out absolutely in line with his prediction. I'm not sure how Roger justifies his "something for everyone" which is clearly intended as a jibe at Hansen for over-egging things, as the prediction of a super El Nino was only ever presented as a reasonable possibility, not a high probability event. As I've said before, I do believe that scientists bear responsibility when their words and research are predictably misunderstood as a result of them presenting their results in a misleading manner. I don't think this is one of those occasions. His words were admirably clear (admittedly not numerically quantitative).

As Roger himself said at the time of the forecast:

If he is proven right with this forecast, contrary to all of the models and statistics, then his credibility will rise far beyond its already stratospheric levels.

(To avoid the appearance of piling onto Roger for the sake of it, I will mention that although I don't claim great expertise in the area, I actually agree with him here, a point which I may blog about in more detail later.)