I attended a talk by the distinguished climate modeler Warren Washington on Thursday: "Climate Modeling of the 20th and 21st Centuries." He spoke on the state of the art in climate modeling, the evidence for warming, and the likely impacts of future warming. The scientific consensus is that we the planet has warmed 0.7C since the beginning of the 20th Century. January 2007 was the warmest January in recorded history. It's easy to see why people are so worried.
The talk also featured a long Q&A. One Q: how well can models predict catastrophic change? A: Not very well, as relevant physics (e.g., Antarctic ice sheet collapse, methane hydrate emissions) are not well understood. From a climate change skeptic: might observed warming not be due to some other factor? (As: Physics of greenhouse greenhouse forcing well understood, no other mechanism known, rate of change unprecedented in record.)
Warren Washington co-authored An Introduction to Three-Dimensional Climate Modeling, from which I learned much of what I know of geophysical dynamics. He was also chair of the National Science Board for a while. A great scientist.