But the 5 C figure is a magical one to global warming theories. That is the size of the difference between ice ages and the present. It was the size of change predicted by the first theorists of CO2 induced warming 100 years ago, when they knew precious little about the subject. It was the size of change predicted by the initial mid 1970s computer models, with large known errors since found in their assumptions. The real source of ice ages, long-scale variations in earth's orbital eccentricity, was rediscovered and became widely understood only in the 1970s, soon after the first global warming computer models.
The 5 C figure remains the key prediction of the UN climate modeling group, no matter how many extra mystery power terms they have to add, when CO2 changes are shown to be unable to produce effects that large. One wag called it "Planck's constant" - a law of nature. The theory can change, but the prediction never. Because the whole thing originated in an attempt to explain ice ages, and so is tied to predicting a change of the same scale as ice ages.
Ever since we found out what really causes ice ages, all such theories have "overexplained" them. Meaning, if such theories were right ice ages should have been twice as intense as they actually were, because they are independently explained twice over. This is one of two basic problems with all the existing CO2 warming theories.
The other basic problem is they predict the wrong scale of temperature response from the power terms they find for greenhouse effect of increased CO2 concentration. The power expected from the scale of changes seen in the past is on the order of 1 watt per square meter, while that for projected future changes, up to doubling of atmospheric CO2, are around 2-2.5 watts per square meter.
But they only arrive at large scale predicted temperature changes from those by the mistake of a linear predicted response between power and temperature. When the well known physical law in the matter is that the power needed to maintain a higher equilibrium temperature goes as the fourth power of the temperature (in degrees kelvin). So even on the highest projections of CO2 effects, they are missing 3/4ths of the power they need.
The real scale of expected effect from observed CO2 variations is in 10ths of 1 degree C. Which fits the data better than their own models. Since they must save the 5 C prediction to maintain the scare mongering interest and policy implication aspects of the whole affair, they just wave their hands and pretend there must be positive feedbacks somewhere that quadruple the power effect, for some reason only of CO2 variations, without amplifying every other variation in the system. Which is just an epicycle hunt. They have no reason to expect such things physically, and can't even name the power source.
Whenever they are asked for one, they wave their hands and invent some possible mechanism without any evidence. When it is checked by serious physicists, the sign is usually in the wrong direction or the wrong order of magnitude or both. For instance, for a long time they hope to find such amplifiers in cloud effects. But clouds are net coolers, and cloud cover has shown a small net increase since the CO2 spike, making it a damping force not an amplifying one.
The global warming emperor has no clothes. They engage in this sort of popular-opinion astrology instead of explaining where the missing power is supposed to come from because they can't do the latter, while the former is as easy as prophesying doom due to unfavorable omens. Which is all it amounts to, until they produce a non-mystical power budget explaining what is supposed to keep the "lights" on in their predicted future.
It takes an enourmous amount of power operating continually to keep a big object glowing even slighter hotter indefinitely. Just like an electric stove, unless the power is still on, any warmer body just cools off again rapidly by radiating away its heat energy in the infrared. Any allegation of a higher equilibrium temperature therefore requires a power budget to explain where the sustaining power is coming from.
They don't have one. Yes, atmospheric greenhouse from CO2 can contribute a continually operating power source for this, but it is an order of magnitude too small for the scale of effects they are predicting, and can only account for 10ths of a degree C temperature changes. It is easy to see why.
CO2 is a trace element in the atmosphere, far below 1% of its composition. Changes on the order of double or half in its concentration are therefore changes on the same order - less than 1% of the atmosphere. And CO2 acts only slightly differently than other gases, due to different sensitivities to this or that color of light. Second order changes in 1000s of parts of the atmosphere, therefore. And the atmosphere as a whole is only the third factor in global temperatures, behind sunlight heating and rotation cooling. Neither of which changes with atmospheric concentrations of anything.
Small changes in marginal effects of trace elements in the third cause of earth mean temperature do not produce enourmous changes in earth mean temperature.
The Global Warming Debate (PDF)
Comment on Slide 7, page 12.
According to review article in CO2science.com on Ice Ages: From studies of climate over 2,000,000 years, Interglacials consistently last 10,000 years. But our current Interglacial began 11,500 years ago. We're overdue for another Ice Age.
We're worrying about the wrong thing.