Hansen et al 1988 noted very sensibly that there were radically different approaches to some physical problems in GCMs and looked forward to the “real world laboratory in the 1990s” providing empirical information on these conundrums. One such conundrum were temperatures offshore Antarctica. They noted that their model showed a strong warming trend in sea ice regions bordering Antarctica while Manabe’s model showed cooling after CO2 doubling, a difference hypothesized to arise from differing ocean heat transport assumptions.
This was a fairly posed observation and I thought it would be interesting to see how things turned out in the next 20 years, not, as criticism of the 1988 model, but merely to see whether subsequent information shed any light on these questions. In an earlier post, we inquired about Waldo in Antarctica . Today his cousin Hansen.
First here is an excerpt from Plate 2 from Hansen et al 1988 (lowball Scenario C not being shown in this excerpt) showing the decadal mean temperature increase relative to the control run mean for Scenarios A and B in the 1990s for DCF and JJA. The forcing history has been closer to Scenario B than to Scenario A; but, in the absence of such a graphic in Hansen et al 1998 showing the 2000s, perhaps Scenario A in the 1990s can be interpreted as an rough approximation to Scenario B in the 2000s. (I’m not placing any weight on this interpretation here, merely noting it to guide your eye.) I want you to look today at the offshore Antarctica area, where Hansen expressed uncertainty about his physical parameterizations. We can talk about other areas on other occasions.
For comparison, here is an IPCC AR4 figure showing 1979-2005 tropospheric trends. This is an annual basis, while the Hansen figure shows two seasons. So you’ll have to “add up” the colors in the Hansen figure to compare. But, for a qualitatitive impression, it’s easy enough to do. John Christy pointed out yesterday that you need to multiply troposphere trends by 1.2 to match surface trends so keep that in mind – though the discussion here is only of patterns. There’s not much texture in the DJF offshore Antarctica in the 1990s, so most of the texture comes from the JJA period shown in the left bottom Hansen figure above; these changes would be attenuated when an annual average is taken.
I’d like you to look at the pattern of increase and decrease around Antarctica in each figure. In the real world, there is a a cooling area off Antarctica to the south of Africa, a warming area to the southwest of Australia and another cooling area on the bottom right of the graphic reaching up towards New Zealand.
Now compare this to Scenario B in the 1990s. As noted above, most Scenario B texture offshore Antarctica occurs in the SH winter (JJA), where we see almost the reverse pattern to the one predicted in Hansen et al 1988. Off to the bottom right of the Hansen figure (bottom left panel) we see strong warming predicted in the area SW of New Zealand where cooling occurred in the real world; we see cooling predicted in the area to the SW of Australia where warming occurred in the real world and we see strong warming predicted south of Africa, where cooling occurred. At the Antarctic Peninsula, Hansen predicted negligible change in summer (DJF) temperatures. In the area south of the Atlantic just to the east of South America, both model and result were warm.
What struck me here was the “remarkable similarity” in the geometric pattern between the model and the outcome, with lobes southwest of New Zealand, southwest of Australia and south of Africa nicely matching. The only defect from a modeling point of view was that these predictions offshore Antarctica had the wrong sign. So to the extent that Hansen was wondering about his Antarctic sea ice model in 1988, it looks like some wrong choices were made for this particular aspect of his model. It would be an interesting inquiry to track how the GISS sea-ice module was modified in subsequent versions as to the sea-ice module.
I note that there is also a problem in sea-ice formation in the more recent ECHO-G model which I noticed in their data, but haven’t discussed previously.
I am not suggesting that a mis-step in modeling Antarctic sea ice in Hansen et al 1988 (especially where uncertainty was noted at the time) invalidates other aspects of his predictions. However as to Hansen’s predictions for offshore Antarctica, I would certainly not regard them as “eerily” prescient and indicate the need for a little caution in other parts of the package to understand better what aspects have stood the test of time and what haven’t.
One of the retorts to observing any inaccuracy in a model always seems to be – well, the error doesn’t “matter”, we get the same answer anyway. My instinct from mathematics is then: well, if you get the same answer for doubled CO2 both ways, surely there is some sort of relevant simplification that would assist in laying bare the mechanics of the GCM in question. My instinct in all of this is that the GCMs are an interesting intellectual exercise, but a needless complication of the relevant physics for developing policy on doubled CO2. I’m not asserting this ex cathedra; it’s merely my instinct at this time.