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View Full Version : The trouble with models: analogies and thought experiments



c1ue
12-14-10, 11:50 AM
http://wattsupwiththat.com/2010/12/13/the-details-are-in-the-devil/#more-29381

Excellent primer on yet again why GCM (climate models) are suspect.

Key points:

1) Emergent phenomena are not model-able from averages of underlying strata behavior
2) NASA's GCM model, with all forcings held constant, still shows a rise of 0.7 degrees C/century. Or in other words, according to this model global warming will continue even if a magic wand were waved to stop all future CO2 output. And this is roughly equal to the IPCC estimate of warming from the past century (0.6C/100 years)


I love thought experiments. They allow us to understand complex systems that don’t fit into the laboratory. They have been an invaluable tool in the scientific inventory for centuries.

Here’s my thought experiment for today. Imagine a room. In a room dirt collects, as you might imagine. In my household I’m responsible for keeping the dirt down, so I get out my vacuum cleaner to clean up.

But suppose I had a magic way to handle that dirt. Suppose there was a class of beings such that whenever there was a concentration of dirt in some small part of the room, one of these beings would pop into existence, clean up the dirt, and then disappear. We’ll say this being is a rare relative of the Tasmanian Devil, call it a “Tasmanian Dirt Devil” (TDD).

http://wattsupwiththat.files.wordpress.com/2010/12/tasmanian-dirt-devil-2.jpg?w=640&h=476 (http://wattsupwiththat.files.wordpress.com/2010/12/tasmanian-dirt-devil-2.jpg)

Figure 1. One of the few authenticated photos of the elusive Tasmanian Dirt Devil (TDD) cleaning a floor in its natural habitat. PHOTO SOURCE: Annals of Cryptozoology, Vol. 6, 1954

After observing the room for a while, we realize that the the TDD only appears when there is some small area of the floor with more than a certain concentration of dirt. However, we see that the TDD does not limit itself to that small concentration of dirt. It moves around and cleans out any other smaller concentrations of dirt around it as well. Once the area is cleaned to a certain level, the TDD vanishes, leaving the room somewhat cleaner. We also see that on days when traffic is heavy, often there are a number of Tasmanian Dirt Devils working on the room at once. No single TDD cleans the whole floor, but the floor is never dirty anywhere for long.

Now, here’s the question for our thought experiment: can we use a computer to model the effect of the TDDs, for example to calculate the rate at which dirt is being added to the floor, based on the average amount of dirt on the floor?

I say we cannot model it adequately if our only input to the model is the average level of dirt on the floor. Here’s two circumstances to explain part of why the problem is ugly.

1. Someone spills a very small bit of dirt on one corner of the floor. Because the dirt is concentrated in one area, a TDD materializes, cleans up the dirt and the surrounding area, and vanishes.

2. Four people simultaneously spill a very small bit of dirt in all four corners of the floor. Four TDDs materialize, clean up the dirt and the surrounding areas, and vanish.

If all we have is the average dirtiness of the floor, a few bits of dirt which are rapidly cleaned up will make little difference in a daily average of floor conditions. Despite those small fluctuations, in one case there is four times as much dirt being added to the system as in the other case.

So I think we can agree that in our thought experiment, the average dirt level of the floor is not linearly related to amount of dirt being added to the system. If we want to model what’s going on, it is very difficult to do it based on the average dirt level. We need much more detailed information in both time and space.

Here’s an illustration of a different problem. Again, two conditions.

1. Someone spills a very small bit of dirt on one corner of the floor. Because the dirt is concentrated in one area, a TDD materializes, cleans up the dirt and the surrounding area, and vanishes. Average dirt level on the floor ends up slightly below where it started.

2. Someone spills a very small bit of dirt evenly all over the floor. There is no concentration of dirt above the threshold level, so no TTD appear. Average dirt level on the floor ends up slightly above where it started.

Again, as you can see, average dirt levels and amount of dirt added show no correlation.

So what do Tasmanian Dirt Devils have to do with the climate? If we saw something like a TDD in our kitchens, we’d be amazed. However something just as amazing exists in the climate.

We’re not astounded by it all purely because are so familiar with it. However, let me take a small digression on the way to explaining the relationship between climate and Tasmanian Dirt Devils.

Emergent phenomena are a special class of things. They can be recognized by certain traits that they have in common. In general, emergent phenomena arise spontaneously at a certain time and place. Typically they exist for a definite duration, and eventually dissipate at another time and place. Their appearance is often associated with some natural variable exceeding a threshold. Often they can move about somewhat independently. If so, although they have general tendencies, their specific actions are usually very difficult to predict.

In general, the definition of emergent phenomena is that the properties of emergent phenomena are not apparent in the underlying stratum from which they arise.

Examples of natural emergent phenomena with which we are familiar include sand dunes, the behaviour of flocks of birds, vortexes of all kinds, termite mounds, consciousness, and indeed, life itself.

Regarding climate, there is one particularly important class of natural emergent phenomena. These are the natural “heat engines”. Heat engines are able to turn heat into work. Examples of these natural emergent heat engines include hurricanes, thunderstorms, tornadoes, and the Hadley Circulation itself.

The most common and most important of these heat engines are thunderstorms. Thunderstorms do two kinds of mechanical work. First, they power the deep tropical convection that is the driving force for the circulation of the entire ocean and atmosphere.

Second, thunderstorms drive what can be thought of as a sophisticated air conditioner, using a variation of the standard refrigeration method. This method, used in your home air conditioner, uses ambient heat to evaporate a liquid. This removes the heat from the area where the evaporation is taking place.

Then you move the evaporated liquid (and the latent heat it contains) to another location. In the new location, you condense the liquid, releasing the latent heat of condensation. The heat is then transferred to the surroundings, and the condensed liquid is returned to start the cycle over.

In the natural Hadley air conditioner we call a thunderstorm, the same process takes place. Water is evaporated at the surface, cooling the surface. The water vapor rises to the clouds. There it is condensed. The latent heat it contained is released, rises, and is radiated out to space.

Meanwhile, in addition to losing latent heat through evaporation, the surface is further cooled by the fall of cold rain from the thunderstorm. This is accompanied by an entrained cold wind, which assists in the cooling.

In both cases (Hadley circulation and refrigeration) the net effect of a thunderstorm is to remove energy from the surface and move it up into the troposphere.

Having digressed, I return to what climate has to do with Tasmanian Dirt Devils.

Consider our thought experiment. If you replace TDDs with thunderstorms, and replace the room with a climate model gridcell of the tropical ocean, and replace dirt with energy, you have an excellent description of the action of the climate system at the hot end of the climate heat engine, the Tropics.

Whenever there is a “hot spot” on the tropical ocean or land, if it is hot enough, a thunderstorm springs up and starts pushing huge amounts of energy vertically. As the thunderstorm moves across the surface, it moves towards the warmest area in its path. This preferentially cools the warmest areas. In addition, it continues to do so until the local surface temperature is a few degrees below the initiation temperature.

There are some conclusions that we can draw from this thought experiment:

1. Increasing the rate at which dirt is added does not commensurately increase the average dirtiness of the floor. Similarly, increasing the rate at which energy is added to the Tropics does not commensurately increase the surface temperature.

2. Attempting to model our thought experiment using room-wide averages won’t work because Tasmanian Dirt Devils are driven by local conditions, not average conditions. Similarly, attempting to model our climate using gridcell-based averages won’t work because thunderstorms are driven by local conditions, not average conditions.

3. Modeling a system which contains simple linear feedback is not too difficult. In that case, average changes in the response variable are linearly related to changes in the forcings. Modeling a system with an active governor, like TDDs or thunderstorms, requires a much different type of model. As I showed above, in that case the response variable is not linearly related to the forcing.

4. Thunderstorms preferentially cool the warmest areas. Although the average temperatures might be the same, this has a different effect than a gridcell-wide uniform cooling. Again, this makes the modelling of the system more complex.

Let me be clear about what I am saying about models. I’m not saying that we can’t model the climate. I think we can, although it won’t be easy. But we have to model it the way it really is.
It is not a system with a linear relationship between forcing and temperature as conventional theory claims. It is a dynamic governed system with a complex, nuanced, non-linear response to forcing. Yes, we can model that. But as I show above, we can’t do it under the assumptions made by the climate models.

Could we model it parametrically, without having to model individual thunderstorms? Probably … but the model has to be designed to do that. And the current climate models either are not designed to do it, or are not doing it successfully.

How do I know that they are not doing it successfully? Drift. Consider the room with the Tasmanian Dirt Devils. If there is no change in the amount of dirt being added per day, the system will rapidly take up a steady-state condition.

The models are subjected to a very similar test. In this test, called a “control run”, every one of the forcings of the model is held exactly steady. Then the models are run for a number of model years. Figure 2 shows the results from the Coupled Model Intercomparison Project (CMIP) control runs. We would expect the models to rapidly take up a steady-state condition.

http://wattsupwiththat.files.wordpress.com/2010/12/cmip-latest-control-runs.jpg?w=640&h=297 (http://wattsupwiththat.files.wordpress.com/2010/12/cmip-latest-control-runs.jpg)

Figure 2. Results of control runs for 16 coupled atmosphere-ocean climate models. SOURCE (http://www-pcmdi.llnl.gov/projects/cmip/overview_ms/control_tseries.pdf)

Notice the drift in the surface air temperature in a number of the runs over the 80-year simulation. The CERFACS model is the worst, but even a mainstream model like the NASA GISS model of James Hansen and Gavin Schmidt shows drift over the 80 years.

How much drift? Well, the trend in the NASA GISS model control run is a warming of about 0.7C per century. This is about the same as the IPCC estimate of the warming over the last century, which is 0.6C.

Now, you could look on that GISS model 0.7C per century inherent warming drift with no forcing change as a bug. I prefer to think of it as a feature. After all, it lets Hansen and Schmidt simulate the warming of the 20th century without the slightest change in the forcings at all, and how many models can do that?

However, that drift does strongly suggest that they are not modelling the climate correctly …

As always, the quest for understanding continues. My best regards to all,

w.