One thing to consider is that for all we know, we're still in the later stages of an ice age, possibly still in an interglacial period that'll precede a new glacial period sometime in the future. A combination of the
Malkovich Milankovitch cycles and changes in the circulation of heat in the oceans (something that changes most significantly as the continents move around) is more or less what determines these glacial and interglacial periods and ice ages in general.
If there are natural forces at work, those are the likely causes, not biological or due to volcanoes or something. What we do know with essentially absolute certainty is that humans are accelerating the warming process immensely through our industrial activities, as well as our destruction of forests and breeding of livestock. It would be ridiculous to claim that this natural shift's sudden acceleration followed the industrial revolution by coincidence when you consider the staggering vastness of geologic time in comparison with the last 200 years of human activity.
Climate change would happen regardless of us, but it would happen at a rate at which the world's ecosystems could adapt along with it, whereas what we're going to see in the near future (if nothing is done) are waves of mass extinctions from the bottom up, particularly in the oceans. A lot is said about rising tides, and while the damage that'll do to coastal cities in the developing world is very serious, that would be relatively benign compared to ocean and atmospheric acidification and a global collapse of irreplaceable ecosystems. We're finding more and more from our existing experience with ecological destruction and extinction that the Earth's systems are more connected than is immediately obvious, and the upshot is that a global catastrophe like what we're heading towards wouldn't just be a conservation tragedy, but something that could make human life increasingly difficult to support.
Anyway, to end this ramble, that's why people are generally pretty miffed about global warming skeptics.
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nawp, I'm out