It's nutjobs like this guy who make it difficult for actual scientists to get thier point across to the masses without being laughed at. Fact is, we don't know how much humans have affected the natural climate change for the reasons Dan pointed out: we haven't been collecting the data on it long enough to really know. What cannot be disputed is that CO2 does absorb infrared raditation and will cause a warming affect. Yes, volcanic eruptions do in fact put large amounts of particulates and greenhouse gases into the atmosphere, the thing, is, they don't happen on a constant basis, as is the case with the byproducts of combustion.
The other fact that has not been mentioned here, that concerns me much more than rising sea levels in and of themselves is the effect that all that freshwater will have on global climate and thermohaline circulation in the worlds oceans. All that water doesn't just slosh around, there are actual currents and patterns in effect, which directly drive weather on Earth.
Bottom line is that we just don't know how much we are affecting climate change. We haven't been doing it long enough to say for sure. Either way, we have to find other ways to supply energy to our masses eventually. Peak Oil will take care of the problem eventually...