YOGI BERRA -- or was it Winston Churchill?--once said, "It's tough to make predictions, especially about the future." Actually, it was neither; the proverb apparently is of Danish origin. Climate models make predictions about the future and are the basis for the Intergovernmental Panel on Climate Change public policy recommendations. However, the current state of ocean-atmosphere general-circulation models used for predicting future climate are not a sound basis for public policy decisions. Since a new president will be elected in November, this is an important topic for the presidential debate; billions of dollars are at stake.
There are many different climate models used to predict the possible temperature rise by the year 2100. The predictions range from about 1.2[degrees]C to 5.8[degrees] from the base year of 1990. For individual models, the uncertainty is from 1.2[degrees] to 2.2[degrees]. This is an enormous variation, as the rise from 1990 to 2016 is only about 0.3[degrees] and natural variations over the last 10,000 years are about 2[degrees].
The IPCC narrows the uncertainty in model predictions by using what is called "ensemble averaging." What this means is that one starts the various models with the same initial conditions and averages the output to get a single number. In doing this, the models implicitly are assumed to be statistically independent and unbiased; they are not, and therefore such an averaging is illegitimate. A single number may be useful for public relations, but the very large uncertainty does not go away by employing such shenanigans, and neither does the bias.
A second issue has to do with the second law of thermodynamics involving the quantity known as "entropy." As any physics student can tell you, the three laws of thermodynamics amusingly can be summarized as: you cannot win; you cannot break even; and you cannot get out of the game.
Sir Arthur Eddington, a physicist, mathematician, and astronomer of die early 20th century, put it this way: "The law that entropy always increases--the second law of thermodynamics--holds, I think, the supreme position among the laws of nature.... If your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation."
However, according to a colleague who has done extensive work on the issue--none of the major climate models explicitly incorporate the second law of thermodynamics...