Comments on Lindzen & Choi (2009)
Posted by Ari Jokimäki on December 5, 2009
Lindzen & Choi (2009) (“L&C” from hereafter) are studying how outgoing longwave radiation (OLR) and outgoing shortwave radiation (SWR) are responding to the changes in sea surface temperature (SST). L&C already have been criticized by others, but I’ll add some comments. Some of the comments I present here I have already made in some online-discussions. First, I’ll take a look at what others have said on this so far.
Additionally, AMIP runs are known to have very counter-intuitive behaviour when it comes to surface energy fluxes (for instance, an oceanic warm anomaly caused by atmospheric anomalies in the real world is associated with an anomalous downward flux of heat – however in an AMIP run, the flux anomaly is upwards (completely opposite)). AMIP runs are useful, but it may be that Lindzen’s analysis is one of those things that is particularly sensitive to that.
So, L&C used model runs that are known to sometimes give exactly the opposite behaviour than in the real world. Also Roy Spencer (of all people) noted the AMIP model selection, and he said that he had got different results (indicating positive feedback) using CMIP model runs.
Rob Dekker had noted that L&C had offset of 4 W/m2 in the shortwave data in their Figure 3. He said:
Of course, after correcting this error, the conclusions of his paper would need to be adjusted as well. Not only is the ERBE data essentially is in line with the model predictions, but also the ERBE data shows that there is NO feedback at all (feedback factor 0) for short-term sea surface temperature changes.
Roy Spencer also noted the smoothing problems I’ll discuss next.
Knowingly working with faulty data
The anomalies include a semiannual signal due to the temporal aliasing effect that needs to be eliminated [Trenberth, 2002]. The relevant sampling error of the tropical monthly ERBE data is about 1.7 W m–2 for SWR and 0.4 W m–2 for OLR [Wielicki et al., 2002a, b]. This spurious signal, particularly in the SWR, can be removed in a 36-day average, reducing the SWR error to the order of 0.3 W m–2.
So, they know that there is a false signal in the data, and they know how to eliminate it. Nevertheless, their action is not to correct the data:
However, in this study, the 36-day average was not applied because we wish to relate monthly SSTs to monthly ERBE TOA fluxes.
It seems to me that there’s not much point to this study after this. Whatever you find, you cannot be sure if it’s real or an artifact due to the false signal in the data.
Above we saw L&C say that they are not correcting the data because they want to compare monthly values, but then they say anyway:
Instead, the moving average with a 7-month smoother was used for the SWR anomalies alone;
They left data uncorrected because they don’t want to mess monthly values, but then they go ahead and mess them for one dataset? And they do it by using strange value of 7 months, which they don’t explain. They do say that they are later going to show that this smoothing doesn’t affect their results. But later they show it by comparing it to equally strange smoothing values of 3 and 5 months. Let us also emphasize this: they only smoothed one dataset out of the three they are using.
Temporal and spatial coverage
The study of L&C is limited to tropics. They try to generalize this by suggesting that higher latitudes are neutral and therefore the negative feedback factor would reduce by a factor of two. I would rather have it measured.
The study is limited to short time variations (from few months to less than 2 years), of which they use only 9. They don’t study long time response at all. They say:
Simple calculations as well as GCM results suggest response times on the order of decades for positive feedbacks and years or less for negative feedbacks [Lindzen and Giannitsis, 1998, and references therein].
Assuming that this claim is correct, by limiting their study to timescales of years or less, they are focusing their study only to the feedbacks they are claiming to be negative. In other words, they are not even trying to study positive feedbacks by their own words.
Keep also in mind that they are not trying to measure the feedbacks relating to the theory of AGW where the changes in the forcing are slow, measured on interdecadal scales. Here in this blog we already have seen that water vapor feedback has been measured to be positive. Cloud feedback has been more uncertain, but recent paper by Clement et al. (2009) measured a positive feedback for low level clouds which has been the primary cause for the general feedback uncertainty. For clouds, situation has been uncertain because we haven’t had observations that are stable enough in decadal timescales, but that situation has improved in recent years (Loeb et al. (2007)).
Some minor issues
In the first paragraph of their introduction L&C say:
This is important since most current estimates of climate sensitivity are based on global climate model (GCM) results, and these obviously need observational testing.
One would need to quantify the “most” and “current” here, but it seems to me that this statement is belittling the situation, as there are many empirical estimates made on climate sensitivity in recent years.
Let’s also remember that the Iris hypothesis of Lindzen has been tested by others with bad results, no closing iris is seen.
Here’s some earlier discussion where Lindzen apparently didn’t use most recent data that was available, and when most recent data was used, the results changed remarkably, and not in Lindzen’s favor.
Forcing the feedback
As mentioned above, there is a problem in L&C handling of direct response of SST change. When temperature of SST increases, the sea surface starts to radiate more thermal energy, which causes OLR to increase according to Stefan-Boltzmann law. L&C correctly noted that when trying to determine the feedback component from OLR, one has to subtract the direct response from the OLR, leaving only the possible component caused by feedbacks. L&C say:
In the observed ΔOLR/ΔT, the nonfeedback change of 4 W m–2 K–1 is included. Also ΔSWR/ΔT needs to be balanced with ΔOLR/ΔT.
First part is correct, the nonfeedback change is the direct response I described above. But it is the second part here that is wrong. ΔSWR/ΔT is basically the change in the albedo of the Earth, it is the amount of change in the reflected sunlight. ΔOLR/ΔT is the change in OLR. Now, when the SST changes, it directly affects the amount of OLR, but L&C are suggesting here that it has a direct opposite effect of equal size to the reflected sunlight. Why would Earth’s reflectance change directly in accordance to changes in SST? There is a known feedback effect that affects the reflectance; the amount of high level clouds changes and causes a negative feedback, but remember that here we are not dealing with feedbacks yet, we are dealing with direct response. What direct response a warming event in SST could cause Earth to reflect more sunlight? That is what L&C are claiming here. Perhaps the reflecting properties of sea surface changes when it warms? Perhaps the reflecting properties of clouds change when the warmer thermal radiation from sea surface hits them? There might be some minor effects like that but L&C claim that they are of equal size to the change in OLR.
There is no reason why the Earth’s reflectance should be balanced with direct response shown in OLR. With this action, they are adding an extra 4 W/m2 to the SWR which works in the negative direction, so basically L&C are forcing (perhaps not intentionally) the feedback to be negative.
For argument’s sake, despite what we said above, let us assume that it is reasonable to balance the OLR and outgoing SWR like L&C suggested. So, how do we do balancing? We have a situation where there is an extra component of 4 W/m2 in OLR side. In that situation we can balance the situation by either A) taking the 4 W/m2 out of OLR or B) adding the 4 W/m2 to SWR. How L&C decided to do it? Here’s what they say about it:
From the consideration, FLW = –ΔOLR/ΔT + 4 and FSW = –ΔSWR/ΔΤ – 4.
As you can see, they did both. They took the 4 W/m2 out of OLR and added 4 W/m2 to the SWR. That means that they didn’t balance the situation, they just moved the unbalance to the other side.
I thank Kaj Luukko and AJ for enlightening discussions on this paper.
UPDATE (January 9, 2010): There has been some studies published dealing with L&C. They are still “in press” so there’s not much to link to yet, but see these RealClimate articles: First published response to Lindzen and Choi and Lindzen and Choi Unraveled. The comments presented are largely different than the ones presented here, so the articles are “must read” for anyone interested in this particular issue. The comment sections there also contain interesting stuff.
Clement, Amy C., Robert Burgman, and Joel R. Norris (2009), Observational and Model Evidence for Positive Low-Level Cloud Feedback, Science, Vol. 325. no. 5939, pp. 460 – 464, DOI: 10.1126/science.1171255 [ABSTRACT] [FULL TEXT]
Loeb, N. G., B. A. Wielicki, F. G. Rose, and D. R. Doelling (2007), Variability in global top-of-atmosphere shortwave radiation between 2000 and 2005, Geophys. Res. Lett., 34, L03704, doi:10.1029/2006GL028196 [ABSTRACT]