First, it's been fun posting here (and I appreciate the healthy discussion with Canuck and Imran) but it seems the insulting will and blog rading will begin now, and there is a deal of information already here. I'll just try to make a few final points through clarification and rounding out the discussion, without usurping the “last word.” I look forward to continuing this discussion in other areas.
First, I start with a Raymond Pierrehumbert quote:
The forcings by the greenhouse gases have little uncertainty because their concentrations are accurately measured in the atmosphere, and their infrared absorption properties are very accurately measured in the laboratory. The two are put together using highly accurate numerical methods that have little error. The small uncertainty is primarily because the radiative forcing depends on temperature profiles (well observed), water vapor profiles (somewhat less well observed) and cloud profiles (with additional uncertainties). The water vapor and clouds come in because you get little additional radiative forcing from greenhouse gas increases below a thick cloud. Similarly, water vapor competes somewhat with absorption due to the other greenhouse gases. There is no plausible ways these uncertainties could be stretched to accomodate the scenario you are proposing.
(for context, the stretch in the original comment was forcing being off by 20%)
There is still a good deal of uncertainty in many specifics of the climate change- El nino's, aerosols, feedbacks and tipping points, clouds, biological responses, etc. In a way, evolution has the same uncertanties- exactly when and how and where did we first get humans, what was the perfect transiton between this time and that time? But I can say with near certainty you will never find a gap so good as to show me a poodle in the Permian, or a chimpanzee in Cambrian time. We still have uncertanties into the mechanics behind plate tectonics, but we know they move and what happens at various boundaries. Similarily, anthropogenic climate change, like all these things has an especially strong solid base and has grown for decades with contributions to various aspects of it (ie the proposal of a greenhouse effect) from the time of Fourier. I am extremely confident that this foundation will turn out to be fallacious, and so are the experts that matter. For a translation of Fourier's Mémoire sur les Températures du Globe Terrestre et des Espaces Planétaires see http://geosci.uchicago.edu/~rtp1/papers/Fourier1827Trans.pdf. fourier doesn't actually mention a greenhouse, or write down complex equations, or how the outgoing thermal flux increased with temperature (which today is the T^4 stefan-boltzmann law), the dynamics of convection and water vapor and CO2 or clouds, but it was a start. Fast forward a couple hundred years and we arrive at a time that is stil asking questions, and still expanding on our knowledge, but to say we know nothing is hopelessly illogical. To say we don't know enough is wrong; we know very well what happens with a given amount of CO2, how things will respond, and we most certainly have clouds in models. We know the direction we are going in, and we know the things at stake. I can't "prove" anything for future scenarios (which really isn't how science works), but give you a "highly confident" warning.
Just as a response to some points above- on the feedbacks, I think I already went over this, but the inseparable water and clouds is a falsehood. First, the cooling due to evaporation is how the surface comes back to equilibrium after being perturbed by the increased radiative heating; that is automatically accounted for in models, and is necessary for them to reach equilibrium, which is a prerequisite for an estimate of the "equilibrium climate sensitivity" to a doubling of CO2 that is always cited from the IPCC reports (i.e., 1.5-4.5 deg. C until the most recent report, now 2-4.5 is more like it). Then, it is not true that increased evaporation forms clouds and is an example of a negative feedback. Increased evaporation increases water vapor; the competition between increased water vapor and increased temperature cause relative humidity to change little as the climate warms (you need saturation to get a cloud), and it is relative humidity that determines the formation of clouds. As proof: In midlatitudes it's a lot warmer in summer than winter, and there's lots more evaporation in summer, yet if anything there are fewer clouds because relative humidity is on average lower. But Climate models now predict that cloud feedback will be either close to neutral or positive in a warmer climate.
The conclusions from the U.S. Climate Science Program are clear: "The observed patterns of change over the past 50 years cannot be explained by natural processes alone , nor by the effects of short-lived atmospheric constituents (such as aerosols and tropospheric ozone) alone." Measurements and models are still being worked on, but the IPCC and others make it clear the anthropogenic signal is certainly noticeable. '98 has been tirelesly addressed and will not be repeated- there is clearly a trend and a one-year anomaly which is irrelevant for showing how the century will warm.
I did not say we expect antarctica as a whole to be warming, and in fact in my post above I made it clear we expect quite the contrary over many areas. It is not at all established that the Arctic warming is due to natural variability alone. Even if the AO is part of the Arctic climate change, one has to face the possibility that changes in GHG's are affecting the natural change( Palmer and Molteni). the changes are large and fast, and consistent with warming. Natural variability is large in the Arctic however. Probably the best explanation from Dr. Richard Alley is that "we changed the atmosphere in ways that made warming and sea-ice loss more likely, and they happened".
The temperature trends in the atmosphere have nothing to do with Smith et al. (2007) on how ocean currents may offset some warming or Camp and Tung (2007) on the solar cycle enhancing warming for 5 years (which I am a bit skeptical of for reasons beyond this post).
Paleoclimate- while Shaviv and Veizer have been skeptics, there is still a great amount of literature on CO2 as a primary climate influence over geologic time, and I think the AR4 paleoclimate chapter will do justice to the enormous body of information, or really, just a geology or paleoclimatic textbook.
These are two that came out after the AR4
http://www.nature.com/nature/journal/v449/n7159/abs/nature06085.html (this one has co-author Veizer so apparently he thinks there is some influence) and
http://www.nature.com/nature/journal/v446/n7135/abs/nature05699.html .
On realclimate- the posts there are from climate scientists generally speaking from primary literature they recently put out, or guest posts from top scientists in their areas (like Soden in the feedbacks reference or Ray Pierrehumbert on radiative transfer). Ad hominems aside, the "both sides" is becoming a bit toppled over to one side, as the scientific literature has become a bit lacking of anti-GW data, but I guess I'll go to the flat earth society to make sure we get "both sides" in the Earth shape "controversy"
I hope some people continue to go over what I've referenced, but it looks like I've worn out my welcome. I strongly suggest that people go over the IPCC report at their main page, and other primary sources, rather than the blogosphere and other wingnut articles. NASA and NOAA and Geophysical Union and NAS and the science, nature, etc journals are there for good reason-- Chris