One of my favorite topics, glad to know some familiar USMB personalities are still fighting the good fight. I'll offer my take.
Anthroprogenic Global Warming theory is just that, a theory. Scientific
facts, on the other hand, come from testing hypotheses, not by convincing enough people to accept your theory.
Kirk, you are right that mankind has been pumping carbon dioxide into the atmosphere - no one will dispute that. But it is by no means a pollutant. It's plant food. It's what we and other mammals expel from our lungs when we exhale. It is not carbon monoxide, sulfur dioxide, or CFCs.
Can CO2 act as a greenhouse gas? Absolutely. But Brian is correct - it is a relatively weak GHG. In fact, there is a diminishing return relationship between increased atmospheric CO2 and temperature increase, that levels off logarithmically around 1.2 degrees C. That is the maximum warming possible from CO2 alone, and you can find that equation in the IPCC reports.
In An Inconvenient Truth, Al Gore's "really big charts" actually demonstrate an opposite relationship - temperature increases precede CO2 increases by about 800 years. This is because increased temperatures heat the oceans and cause CO2 to evaporate and enter the atmosphere. Don't believe me? Open 2 cans of soda, put one in the fridge, and leave one outside. See which one goes flat quicker.
So where do the doomsday scenarios come from?
Assumptions that the Earth's climate is dominated by positive feedbacks (reduced ice albedo, more water evaporation, which is the key greenhouse gas, etc) that amplify the initial warming cause by CO2. AGW alarmists build climate models with assumptions that strong positive feedbacks dominate any negative feedbacks, or they ignore the negative feedbacks altogether (due to a lack of understanding... shocker). That's rather sloppy for supposedly "settled science."
Almost any natural process you can think of is kept in balance by negative feedbacks, and I believe the Earth's climate is as well. The Earth has warmed and cooled countless times, alternating between Ice Ages and Warm Periods like the Medieval Warm Period and the Roman Warming. Climate is constantly changing. Greenland has been farmed before. The Thames has frozen over in England. Thousands migrated to North America over the Bering Strait Land Bridge. If you can point to anecdotal evidence about the North Pole melting as proof of AGW theory, I would assume that I can point to similar anecdotal evidence to refute the claim. Climate is changing, absolutely, but there's no empirical evidence that CO2 is driving the change, or that it will lead to runaway warming.
As a last note, below is a chart that James Hansen presented to Congress 20 years ago, with his predictions for future temperatures based on his climate models. The red, orange, and yellow lines are his predictions based on differing CO2 production forecasts (our actual production would put his forecast somewhere between the red and orange lines).
And though actual temperatures are far below his projections, he sticks to his original theory instead of rethinking his hypothesis. Who's the real denier?
