When it comes to global warming from CO2, the toughest question is how are clouds going to respond? The IPCC has about 20 differant climate models, but only one includes a change in cloud cover. The reason is that there hasn't been much evidence (until now) about how clouds would actually respond to CO2 warming. However, what is being found is not encouraging...
http://www.sciencemag.org/cgi/content/short/325/5939/376The first reliable analysis of cloud behavior over past decades suggests—but falls short of proving—that clouds are strongly amplifying global warming. If that's true, then almost all climate models have got it wrong. On page 460, climate researchers consider the two best, long-term records of cloud behavior over a rectangle of ocean that nearly spans the subtropics between Hawaii and Mexico. In a warming episode that started around 1976, ship-based data showed that cloud cover—especially low-altitude cloud layers—decreased in the study area as ocean temperatures rose and atmospheric pressure fell. One interpretation, the researchers say, is that the warming ocean was transferring heat to the overlying atmosphere, thinning out the low-lying clouds to let in more sunlight that further warmed the ocean. That's a positive or amplifying feedback. During a cooling event in the late 1990s, both data sets recorded just the opposite changes—exactly what would happen if the same amplifying process were operating in reverse.
http://www.time.com/time/health/article/0,8599,1912448,00.htmlScientists create climate models to try to predict how the earth will respond to higher levels of greenhouse-gas emissions, but only one model — created by the Hadley Centre in Britain — includes the possible impact of changing cloud behavior. And the bad news is that the Hadley model contains particularly high temperature increases for the 21st century, in part because it sees dissipating cloud cover as a positive-feedback cycle — meaning the warmer it gets, the less cloud cover there will be, which will further warm the earth. Though it's just one data set over one part of the earth's surface, the Science study indicates that the pessimistic Hadley model may be right. "These low clouds are like the mirrors of the climate system," says Clement. "If they disappear, you might see that positive-feedback cycle."