There was a comment on one of my previous posts suggesting that I should stop being “sympathetic” towards climate models since, without criticism, climate scientists will never put any effort into correcting the problems with their models. The suggestion being that they could do better, but it suits them that their models predict higher global surface temperatures than is observed.
So, am I sympathetic to the climate models? I guess I should acknowledge that I probably am. One reason is simply that a lot of what I do involves complex modelling (although not as complex as climate modelling) and so I have some sense of how complicated these models can be. It seems highly unlikely that they’re getting it wrong simply because it suits them to do so. Another reason is related. If a large number of well-trained, experienced scientists think that these models have some merit (despite the known problems) then they’re probably right. The chance that some educated lay people know better than those who actually work with these models is probably small. This doesn’t mean that the scientists are defintely right, simply that it is unlikely that they’re wrong.
However, none of the reasons above is actually a particularly scientifically credible reason for being sympathetic towards climate models. I do, however, have a more justifiable reason for thinking these models haven’t actually failed, as many skeptics would have us believe. The main reason for believing these models have failed is that they’re predicting higher global surface temperatures than has been observed. The basic claim being that the prediction is wrong, therefore the model has failed.
These, however, are not simply models for predicting global surface temperatures, they’re climate models. They do much more than simply predict global surface temperatures. If I understand the basics (and I’m happy to be corrected if I don’t) what they’re actually doing is modelling the transfer of energy through the climate system. One of the fundamental reasons why the evidence for global warming is strong is that there is a measured energy imbalance at the top of the atmosphere. This imbalance is measured to be between 0.5 and 1 Wm-2. It’s my understanding that climate models produce top of the atmosphere energy imbalances that are consistent with these observations.
If so, then climate models are actually get the fundamentals correct. They are correctly modelling the increase of energy in the climate system. So, how significant is it that recently they’ve been overestimating global surface temperatures. Well, it’s clear that about 90% of the excess energy goes into the oceans, leaving 10% or so to heat the land and atmosphere. I don’t know precisely what fraction actually heats the surface, but lets assume a few percent. It’s clear, therefore, that a small error in the amount of energy going into the oceans can have a big impact on the estimated rise in the surface temperatures. To assume that a model has failed simply because something that only involves a few percent of the energy being modelled is maybe not quite consistent with observations, seems a little extreme; especially as the models are probably correctly modelling the evolution of the total energy.
There is, however, a little more to this. There are two parameters that are typically of interest. One is the Transient Climate Response (TCR) and the other is the Equilibrium Climate Sensitivity (ECS). The TCR is the change in surface temperature at the instant at which CO2 has doubled. The ECS is the change in surface temperature required to return the system to equilibrium after CO2 has doubled. It is a little more complicated than this, but this is essentially what these parameters are.
Since climate models are currently over-prediciting the rise in global surface temperature, this could indicate that the TCR is lower than previously expected. There is, in fact, some evidence that this may be the case, but it is still uncertain. However, if climate models are correctly modelling the change in total energy in the climate system, then their estimates of the ECS is likely to still be quite accurate. This could mean that we have more time (i.e., the TCR is lower than expected) but the ultimate rise in surface temperature will be as climate models have predicted. I don’t know about you, but this doesn’t really make me feel any less concerned.
So, that’s the real reason why I am sympathetic towards climate models. They may currently be over-predicting the rise in global surface temperatures, but there’s no evidence that they’re not correctly modelling the evolution of the total energy in the system (feel free to correct me if this is wrong). At best, this means we have more time than we previously thought, but – despite this – the end result will still be the same.