There is no scientifically valid mechanism for CO2 causing global warming.

Carbon dioxide absorbs all radiation available to it in about ten meters. More CO2 only shortens the distance, which is not an increase in temperature. In other words, the first 20% of the CO2 in the air does most of what CO2 does, and it doesn't do much.
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•The • The carbon dioxide greenhouse effect provides a scientifically valid mechanism. CO2 produces a warming effect not by soaking up the radiation in its immediate vicinity, like a sponge, but by raising the effective radiating level of the atmosphere to a colder layer.

•The • The atmosphere doesn't act as a single unit, but is made up of hundred of different layers. Some of the energy radiated from Earth is stopped in each of these layers. The energy is then re-radiated in a random direction, but on average the energy is moving either "up" or "down." The atmosphere gets thinner as altitude increases, so eventually the radiation will reach a layer high enough and thin enough to escape to space.

•If you • If increase the concentrations of a greenhouse gas in the atmosphere, the thin upper layers will become more opaque and thus absorb more of the IR; therefore the place where the majority of the energy finally escapes moves to a higher level. These higher levels are much colder (until you reach the stratosphere, but as most infrared photons escaping to space are emitted by the troposphere, this can be ignored), and so they do not radiate heat very well. Thus the rate that radiation escapes to space is lower, and the planet will take in more than it radiates. As the higher levels emit some of the excess downwards, the lower levels will warm all the way down to the surface.

•The imbalance will • The remain until the higher levels get hot enough to radiate as much energy back out as the planet is receiving.

•19th century physicist John • 19th Tyndall described the effect neatly like so:

"As a dam built across a river causes a local deepening of the stream, so our atmosphere, thrown as a barrier across the terrestrial rays, produces a local heightening of the temperature at Earth's surface."