I've seen Dr. Roy Spencer, a climatologist from the University of Alabama, on TV and read some of his comments, but until today I had not visited his website.
Spencer has a brief article, Global Warming 101, which is the most clear and concise explanation of the global warming conundrum I have ever seen. Well worth the time, even if it needs to be read twice. I will try to put this into memory, because it really does distill the whole question down into the most basic parts.
In short, energy coming in from the sun equals energy out, radiating from the earth. However, various factors can change the equilibrium for a time. That's what all the debate is about.
Greenhouse gases (water vapor is the biggest factor along with a related factor, clouds) retard the flow of energy back into space. CO2 is a factor, but by itself this is minor. The slight warming caused directly by CO2 can be amplified by water vapor or diminished by clouds. The degree of warming will be highly dependent on the latter two factors, which are in dispute.
Now we come to the issue of mathematical computer climate models. The warming predicted for our future, which global warming alarmists to inflame the media and the public, will depend greatly on how the H2O amplification factor and the cloud formation factor are represented in the mathematical model.
Dr. Spencer does a Climate Model Reality Check in another web page that packs the essential information into a short essay. A few quotes:
"...virtually everyone now agrees that the direct warming effect from extra CO2 is relatively small – too small to be of much practical concern."
"...the main reason the models produce so much warming depends upon uncertain assumptions regarding how clouds will respond to warming. Low and middle-level clouds provide a ‘sun shade’ for the Earth, and the climate models predict that those clouds will dissipate with warming, thereby letting more sunlight in and making the warming worse. [High-altitude (cirrus) clouds have the opposite effect, and so a dissipation of those clouds would instead counteract the CO2 warming with cooling..."
The key here appears to be the response of clouds to higher temperature. The earth has been warming slightly over the past century. What has happened to cloud cover and what will happen to cloud cover as a result is simply unknown. Therefore at the heart of the climate models predicting global warming is a factor for a decrease in clouds (amplifying CO2 warming or other causes of warming) which is a huge question mark. A giant "guess factor." That factor may in fact be at odds with observations of clouds via satellite.
And forget about cloud response to global warming. What caused the slight warming in the first place?
Guesswork. It's all guesswork. A savvy and honest scientist will say, "We don't know." Period.