The idea behind the theory of global warming is simple. It is that the earth receives a certain amount of energy from the sun. When a beam of sunlight hits the earth, several things can happen. It can be absorbed by clouds, the air, or the ground and sea, or it can be reflected by any of those three and return to space. If it is absorbed, then the energy is converted to heat. In order for the earth's surface to stay constant (on the average) the same amount of energy absorbed and converted to heat must be radiated back into space. The amount of absorbed heat energy radiated into space depends on the average temperature of the surface of the earth. The higher the temperature, the more energy radiated. If anything causes the fraction of energy absorbed from the sun to increase, then the earth's temperature must increase until absorbed energy re-radiated into space equals energy gained from the sun.
Global warming advocates believe that an increase in gasses in the atmosphere that can absorb light or infrared radiation and convert them to heat will increase the total amount of energy absorbed by the earth. Less energy will be reflected and more will be absorbed, and this will result in an increase in the earth's surface temperature.
Most of the information published by global warming advocates lists the gasses responsible for energy absorption by the atmosphere, but they always leave water vapor off of the list. This is confusing because water vapor contributes a huge amount to the overall absorption of energy by the atmosphere from the sun. Moreover, the water vapor content of the air varies tremendously depending on the temperature, the proximity of bodies of water, the turbulence and mixing of the air, the altitude, and a number of other factors. If graphs listing the greenhouse gas effect of the various greenhouse gasses included the contribution of water vapor, and information concerning the variability of water vapor content was included in these graphs, then it would be immediately intuitively obvious to anyone that carbon dioxide and other greenhouse gasses are a drop in the bucket compared to water vapor. It would be obvious that efforts to predict global temperatures by looking only at carbon dioxide concentrations while ignoring water vapor are ridiculous.
Even more problematic is the contribution of clouds. Clouds receive a large percentage of the radiation from the sun. Some of it is sent back to space, some of it is absorbed, some of it is sent to the ground. Moreover, some of the energy radiated from the ground to the air is caught by clouds and then sent back to the earth, a tremendously important effect that can determine whether the temperature on a single night is ten degrees higher or lower in any give location on the earth, an effect that is two orders of magnitude larger than anything that carbon dioxide could produce. Any model that attempts to predict global temperatures into the future will have to be able to first predict how prevalent clouds will be, what altitude they will form, what conditions will accompany their formation, how much air mixing there will be, what kind of clouds they will be, how much precipitation will be associated with them. There is no model proposed that even comes close to doing any of this.
Predictions about global warming might be possible if one could assume that water vapor and clouds remained more or less constant, on the average, while, say, carbon dioxide increased. But nothing could be further from the truth. Any of the proposed effects of global warming will affect water vapor and clouds tremendously and in ways that cannot be predicted.
Therefore, predictions of future climate change are not yet possible.