My opinions, for what their worth:
Are you convinced that humans are the primary factor in this buildup of carbon dioxide, directly or indirectly?
Yes, I believe that human activity is directly responsible for the buildup of CO2 in our atmosphere since the Industrial Revolution. By burning fossil fuels for energy we are releasing huge amounts of carbon to the atmosphere that otherwise would remain locked beneath the Earth's surface.
If so, that the buildup of carbon dioxide is the primary factor in the current warming trend and not another, naturally occurring factor, such as increased solar activity?
This is a little trickier because it is true that the planet's climate has undergone significant changes due to natural rhythms such as the Milankovitch cycles. Over the last 400,000 years, the intensity of the Sun's radiation hitting the Earth has fluctuated because of slight changes in the Earth's orbit and angle of tilt. During that span, periods of high solar intensity correspond with warmer periods (interglacial periods) and periods of low intensity occurred during glacial times. But, and this is a big but, the concentration of atmospheric CO2 never rose higher than about 280 ppm during that 400,000 year span. Because of our reliance on fossil fuels we have raised the concentration of atmospheric CO2 to 380 ppm, increasing by about 2 ppm per year (a rate that will increase with China and India's continued industrialization).
And finally, that this trend will continue unless we humans intervene?
Without a doubt, the release of CO2 will continue and accelerate without the intervention of developed nations. As I said before, there are a host of reasons to ween ourselves off of fossil fuels that have absolutely nothing to do with climatology.
In my opinion, there is no question about the link between anthropogenic CO2 and climate change. Where it gets a little fuzzier is what the impacts are going to be. I tend to distance myself from folks who like to talk about 20 foot sea level rise and shutting down the Gulf Stream because things like that are hard to predict and smack of scare tactics, hence my negative attitude towards Gore.
Cliff Stryker Buck, Ph.D.
Department of Oceanography
Florida State University