The climate models
"There is considerable confidence that Atmosphere-Ocean General Circulation Models (AOGCMs) provide credible quantitative estimates of future climate change, particularly at continental and larger scales."
―IPCC Fourth Assessment Report, The Physical Science Basis, p. 591 
"The IPCC report ... is very clear in stating that the detection and attribution of climate changes is only clearly possible at continental scales and above."
―Gavin Schmidt, prominent climatologist 
"The increase in rainfall in East Africa, extending into the Horn of Africa, is also robust across the ensemble of models, with 18 of 21 models projecting an increase in the core of this region, east of the Great Lakes."
―IPCC Fourth Assessment Report, The Physical Science Basis, p. 869 
The melting ice, the extreme weather events, and the rising temperature of the Earth the last few decades, are pointed to, sometimes by media and environmentalists, and sometimes by scientists, as the devastating result of man-made global warming, but they are usually not used in scientific circles as evidence of such warming. Scientists who support the man-made global warming hypothesis will usually admit that, while the climate is changing, this is no news; this has happened before. But this time, they claim, it's different, and the reason they know it's different is that this is what the models say. All humanity's fears about the future of the climate are because of what the models say. Let's take a look at these models.
What is a model?
A computer model is a program that simulates a real process. For example, car manufacturers perform computer simulations of crashes. The car structure is divided in little elements, and the computer calculates how each each one of these elements will behave, based on the forces that act on it from its neighbouring elements; and this calculation is repeated for many successive time instants.
The reason such a simulation system is called a model is that it is a representation of reality, in the same way that a model aircraft is a representation of a real aircraft. Similarly, a map models the landscape, i.e. represents it.
Computer models accept input and produce output. The input to a crash model could be the speed of the car, the point of impact, etc; the output could be the forces that are applied to the bodies of the passengers. Making the required calculations is frequently called "running the model".
You must always bear in mind that a model is a model and reality is reality. Good models approximate reality well, but they are still models; they are not reality.
Climate models, also known as General Circulation Models or GCMs, are very complicated models that accept a number of inputs such as carbon dioxide, sun radiation, and many more (the input variables of climate models are often called forcings). The models are run in huge supercomputers, simulating the evolution of climate around the globe, and they produce the temperature and rainfall around the earth as a function of the input. The argument goes that when the models are run without the input that is allegedly caused by humans (i.e. when they are run with the "normal" or "pre-industrial" concentration of carbon dioxide in the atmosphere), then there is no global warming; but when they are run with the alleged "anthropogenic forcings", i.e. with increased carbon dioxide concentrations, then there is global warming at the end of the 20th century, which continues throughout the 21st. This, goes on the argument, proves that the recent warming of the Earth, unlike previous climate changes, is caused by human activity; and the models, goes on the argument, can tell us how the climate will be several decades from now.
I can claim that if you drive your car at 30 km/h and you hit a wall flat on, you will only get a bruise on your left shoulder; and if you ask me why I think that, I can tell you that this is what the computer crash model says. This can be a convincing answer only if we have reasons to believe that the model works well. Therefore, the crucial question is: "How reliable are the climate models?" The Intergovernmental Panel of Climate Change (IPCC) attempts to answer that question in pages 600–601 of "The Physical Science Basis" , which is part of the IPCC Fourth Assessment Report. They mention three reasons: (1) that the models are based on established physical laws; (2) that the models can reproduce features of the current climate; (3) that the models can reproduce features of past climate. We need to examine them one by one.
(1) The models are based on established physical laws. If we know the speed and direction of a die, its dimensions, and the hardness of the materials of which the die and the surface on which it falls are constructed, can we predict the outcome? The physical laws of movement are very well known and understood in that case, but any model would fail miserably. The reason is that the problem is extremely complicated. If a model that attempts to simulate the physical movement of a die fails to predict the outcome, when the physical laws are fully known and really well understood, the fact that climate models are based on physical laws constitutes absolutely no evidence whatsoever that the models can work well. Climate is incredibly more complex than the throwing of a die. What's more, by the IPCC's own words, in climate there are "limitations in scientific understanding" (, p. 601).
(2) The models can reproduce features of current climate. It's not difficult to make a die throwing model that can reproduce features of die throwing. A model could be made that reproduces the fact that a die accelerates downwards; that when it hits the surface it rebounds; that it maintains its angular momentum; that it loses energy through friction; and so on. If a model can reproduce these features, this does not create any confidence whatsoever that it is able to correctly reproduce the final outcome of a real throw. Similarly, the fact that climate models can reproduce winters and summers, or dry areas and humid areas, has nothing to do with their ability to tell us how the climate will be in the future.
(3) The models can reproduce features of past climate. This argument is summarized in Figure 1. Climate models, it is claimed, are successful in reproducing the climate of the 20th century well, and therefore it is expected that they will be able to also reproduce the 21st century. In the Figure, the black line represents the (estimated) reality; and the yellow lines are 58 different outputs from several different climate models. The red line is the average of these outputs. Your first impression would be "wow, the models are good!" But let's take a more careful look at the Figure.
The vertical axis in Figure 1 represents the departure from the mean temperature. If, for example, the temperature of the planet is 15°C on the average, and in 1962 the temperature of the planet was 15.3°C, then the black line will be at +0.3 in 1962. The problem is that the comparison is between the real departure and the departure of each model. If a model says that the temperature raised from 10 to 10.4°C, and in reality the temperature raised from 15 to 15.4°C, then the model would appear to coincide with reality in Figure 1. This means that the 60 lines of Figure 1 are artificially made in order to approximately fall on each other; if plotted without subtracting the mean, they would be 60 distinct lines, often very far from each other; if the chart were still only 2°C high, as it is now, many lines would be entirely outside the chart.
You may argue that if a model could get the departure from the mean, this would be a remarkable achievement, even if it could not get the actual value. Indeed. But it's different if you are shown Figure 1 and you think "wow, the models are good!", and it's different if you were shown a Figure of 60 entirely distinct lines, far from each other, in which case you'd think "hmm, the models are crap; however, they do seem to get the slope right."
Do they get the slope right? From what we can see, the (estimated) reality indicates relatively constant temperature from 1930 to 1970; in contrast, the models indicate a continuous temperature rise from 1930 to 1970, with an abrupt drop in 1962, when the Agung volcano errupted. We don't see, in that Figure, that the erruption has particularly influenced reality. If you look carefully into Figure 1, you'll notice other problems as well.
A third problem is that it is completely unscientific to draw the red line that shows the average of the models. There are absolutely no scientific grounds to believe that the average of the models is something special, or that it is better from the models. If the models get it wrong, there is absolutely no reason why the average of many models would get it less wrong. In fact, the mere existence of that red line makes us wonder: how were the 58 model outputs chosen? Why were those 58 outputs chosen, and not some other 58 outputs? Could that be cherry-picking?
Finally, even if the models match reality to some extent, how do we know that they were not made to fit reality? Normally scientists are extremely careful to adjust their models to match a specific set of data, and then they check how well their models match an entirely different set of data. In this case, for example, one could adjust the models so that they'd match the period 1900―1950, without looking at 1950―2000, and then checking how well the models can match reality for the latter period. This is not as easy as it seems in the case of climate models, and it may be impossible because of the huge complexity of the problem and the fact that there is data for only about one century; but we've never been given evidence that the match, to the doubtful extent that it exists, is anything better than data fitting.
Extraordinary claims require extraordinary evidence. The claim that models can predict future climate is extraordinary; but instead of clear evidence, we are provided with confusing arguments and strange linguistic formations: rather than being told that "there is evidence that the models can predict future climate", we are being told that "there is considerable confidence that [the models] provide credible quantitative estimates of future climate, particularly at continental scales and above." What is "considerable confidence" and how far is it from certainty? Who is it that is considerably confident? What is "credible"? Does it merely mean plausible? What does "particularly at continental scales and above" mean? Does it mean "only at continental scales and above", as the quotation by Gavin Schmidt implies? Or does it mean "which is good at regional scales and even better at larger ones", as is implied by the IPCC's 94-page-long Chapter 11 , that is crammed with regional details?
A number of studies explain why the models are not credible (e.g. , ). But we have chosen, in this chapter, to only look at what IPCC says. The IPCC should have provided evidence that models do a decent job, and then we'd be able to start discussing on the merit of that evidence. But the IPCC, as you can see, has not provided any evidence. Rather than clear answers to clear questions, they have provided confusion and lawyerish language.
Next: The scientific consensus
|||(1, 2, 3, 4) David A. Randall, Richard A. Wood, Sandrine Bony, Robert Colman, Thierry Fichefet, John Fyfe, Vladimir Kattsov, et al., "Climate models and their evaluation", in IPCC Fourth Assessment Report: Working Group I Report: "The Physical Science Basis", 2007; http://www.ipcc.ch/ipccreports/ar4-wg1.htm, accessed on 2009-04-22.|
|||G. Schmidt, “Hypothesis testing and long range memory,” RealClimate, 10 August 2008; http://www.realclimate.org/index.php/archives/2008/08/hypothesis-testing-and-long-term-memory/, accessed on 2009-04-24.|
|||(1, 2) Jens Hesselbjerg Christensen, Bruce Hewitson, Aristita Busuioc, Anthony Chen, Xuejie Gao, Isaac Held, Richard Jones, et al., "Regional Climate Projections", in IPCC Fourth Assessment Report: Working Group I Report: "The Physical Science Basis", 2007; http://www.ipcc.ch/ipccreports/ar4-wg1.htm, accessed on 2009-04-24.|
|||Patrick Frank, "A climate of belief", Skeptic 14(1), pp. 22―30, 2008; available at http://www.skeptic.com/the_magazine/featured_articles/v14n01_climate_of_belief.html, accessed on 2009-04-06.|
|||D. Koutsoyiannis, A. Efstratiadis, N. Mamassis, and A. Christofides, "On the credibility of climate predictions," Hydrological Sciences Journal 53(4), pp. 671―684, 2008; available at http://dx.doi.org/10.1623/hysj.53.4.671, accessed on 2009-11-10.|