A comment worth pulling out and highlighting (even if it is somewhat critical of me). Mickey Glantz, whose ideas about climate science and policy and politics I greatly respect, takes me to task for airing Jim Hansen’s draft ENSO paper:

You know that Hansen’s paper was just a draft…. Anyone could (and some did) jump on a bandwagon that was still parked in the barn.

So was it appropriate to air this thing in public, given that Hansen himself was clear that it was just a draft, subject to revision? Or is Mickey right? On balance, I think these discussions are most useful and interesting in full view. But it’s a close call. Comments and discussion encouraged.

Via Roger Pielke Jr., reference to a new draft from Jim Hansen that usefully clarifies the El Nino discussion of last week:

Present version should make clear that what we are saying is: Global warming has increased the east-west equatorial temperature gradient and that should increase the probability of a super El Nino. It is still a crap shoot, so it requires many rolls of the dice for empirical verification. Even in the last 30 years there have been only two super El Ninos.

Steve McIntyre approvingly cites a piece in the Daily Telegraph by Bob Carter arguing that the planet has not warmed in the last eight years. And if you pick the El Nino year of 1998 as your starting point, that’s true, making this a rhetorically powerful argument. But there’s a problem. Given the variability in the temperature record, in any given year, even with a continuing upward temperature trend, you are likely to be a bit cooler than the previous record year – until you set a new record. So by cherrypicking the previous record year as your starting point, you can argue most of the time that things are really cooling off, not warming. This is what Carter has done. But the really interesting thing here is the evidence offered by Tim Lambert that Carter himself has complained, in other contexts, that the extreme El Nino year of 1998 is an outlier that you shouldn’t use to establish a temperature trend.

update: Dave Dardinger accurately points out that Steve’s post was neutral, rather than approving, of Carter’s piece. I stand corrected.

On Hansen on El Niño (sub. req.)

I blogged earlier today at work about the latest ENSO assessment from the Climate Prediction Center, and said this:

We’re at the point in the year where forecasting is hard, so the computer models used to try to get a handle on where this is headed are all over the map.

It’s the conventional wisdom the ENSO forecast community has been giving me for years, a sort of “pivot point” around this time of year when their forecast skill is weakest.

Then comes this draft paper from Jim Hansen and others:

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.

(hat tip RP for the link to the Hansen piece)

Hansen goes on to “argue further that global warming has increased the likelihood of super El Ninos.'”

As it happens, I’ve been poking at this question as well because of the important relationship between ENSO and the climate here in the southwest. More/bigger (less/smaller) El Ninos means less (more) drought where I live. As Eric Guilyardi recently put it, “Predicting El Nino occurrence and amplitude… is a key societal need.”

The problem, as Hansen acknowledges, is that the global climate models are all over the map on this one.

Haiyan Teng and colleagues at NCAR just published a model intercomparison in GRL that looked, among other things, at the way a suite of global climate models handled ENSO under CO2 held constant at 2000 values.

In the present set of models, 9 out of 16 models show a committed El Nino-like response, while the rest have either a La Nina-like response or more uniform committed warming across the tropical Pacific. The reasons for these differing responses require further investigation.

That’s the paper that got me curious, which led me to Guilyardi’s paper in Climate Dynamics which also did a big model comparison. Again, the results tilted toward increased El Nino amplitude in a warmer climate, “though there is a considerable spread of El Nino behaviour among the models.”

I’m not a scientist, so I won’t try to evaluate Hansen’s claims, but were I a journalist trying to write about this and looking for the sweet spot where the bulk of the science lies, I’d have to explain to readers that Hansen is well to one side of it on both the question of what can be said now about what will happen in the coming year, and of what effect greenhouse warming might have on ENSO.

Climate model fidelity and projections of climate change:

Relative entropy, which is a measure of the difference between two probability distributions, has been calculated for the simulations of the climate of the 20th century from 13 climate models and the observed surface air temperature during the past 100 years. This quantity is used as a measure of model fidelity: a small value of relative entropy indicates that a given model’s distribution is close to the observed. It is found that there is an inverse relationship between relative entropy and the sensitivity of the model to doubling of the concentration of CO2. The models that have lower values of relative entropy, hence have higher fidelity in simulating the present climate, produce higher values of global warming for a doubling of CO2. This suggests that the projected global warming due to increasing CO2 is likely to be closer to the highest projected estimates among the current generation of climate models.

Shukla et al., Geophys. Res. Lett., 33, L07702, doi:10.1029/2005GL025579