CO2 causes warming! CO2 causes cooling! What the hell is going on? The fact is that CO2 can do both. Thinking purely of the radiative impact, initially, CO2 causes a little cooling if a lot is added at once, then the warming impact catches up and over takes the cooling impact. It is kinda weird, but there is a pretty logical though a little complicated explanation.
The big thing is where is the energy coming from? Early when there is a rapid increase in CO2, the energy is coming from the sun until the surface temperature catches up. In the upper atmosphere, it is easier for CO2 to radiate to space than it is to the surface. Since a good portion of the incoming solar energy is absorbed by the atmosphere, a higher percentage of that atmospheric warming will be lost to space with a jump in CO2. The warming effect is happening just initially, the cooling wins out a little. Since there is a disruption of the outgoing energy flux, the surface temperature responds by warming until it reaches a new average temperature that is a little warmer and the atmospheric cooling and surface warming come into a new balance that is warmer if you only consider the radiative impact. Unfortunately, the world's climate system tends to be a little chaotic, so if you only consider radiation you miss the big picture.
In the first of the Coming Ice Age series, I emphasized the important role of water. Water vapor, only a part of the puzzle, is emphasized in the radiation impacts of CO2. In a warmer world the air can hold more water vapor, CO2 causes warming, more water vapor adds to the warming, so OH My God, we are in trouble!
If you only consider water vapor you are absolutely right. Clouds though are more than water vapor, they are condensed water droplets. Water reacts different than water vapor. So the cloud issue is one of the largest unknowns in the climate change debate.
The recent global not warming is likely due to cloud cover increase that causes more reflection of incoming energy than it does retention of out going energy. These changes in average cloud cover are likely associated with the longer term internal climate oscillations. The impact of these oscillations are considered trivial to long term climate because they should tend to average out. On very long time scales that is probably true. With more CO2 though, that can change in several ways.
The first is more radical internal climate variability. More water vapor in the atmosphere means more intense rain and snow events. So visions of snow in winter so rare that future children will be amazed, is totally bogus. Certain areas will have a lot more snow and a lot less snow with climate oscillations. Rain events will be a lot stronger and droughts a lot deeper. But stronger and deeper than what?
The rain and drought events so far are not exceptionally different than past recorded events and do not seem to be significant at all compared to thousand year events as best as we can tell. So while climate events will be more extreme, it is not easy at all to confidently predict how much more.
During the past 30 years of climate science, most of the period indicated warming that agreed very well with warming predicted by climate models geared to estimate the climate's sensitivity to increased CO2. For the past ten or so years, the climate appears to have shifted due to the change in one of the internal oscillations, the Pacific Decadal Oscillation (PDO) an now the sun is going into a quite mode, which the satellite era of science has never experienced. So there are a lot of questions that will get better answers.
Second, increased climate extremes during a general cooling internal oscillation is likely to cause more cooling, IF water in its solid state, snow and ice, increases albedo to a point where it amplifies the impact of the cooling. That possibility increases with a cooling PDO and quieter sun. Should another internal oscillation synchronize with the PDO and quiet sun, the possibility increases greatly.
Third, with less outgoing radiation from the surface due to increased albedo, the cooling effect of CO2 on incoming solar energy will be enhanced. That will result in more cooling or less warming if you like, due to atmospheric CO2.
So there is the potential of a lot of stuff happening with more CO2. The scientific community in my opinion is screwing up royally by stressing their certainty in one scenario, when the uncertainty of the various scenarios in more important.
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