First off the evidence of the global warming/cooling cycles has been linked to varitations in the Earths orbit (eccentricity, axial tilt, and precession). These variations affect the Earths climate they're called Milankovitch cycles. So there are perdictable climate changes. We are in a warming phase; however, increased pollution will most likely exagerate the this phase and lead to a spike in global temperatures. This is not law, but theory.

But just becasue somethimg is a theory does not mean it lacks weight or truth. In every day language a theory is thought to be the equivalent of a guess, but that just isn't so. A theory is based on evidence, and as time goes on evidence either supports the theory or does not. If it does not the theory could be thrown away or it could be modified to account for this new information. So in science when someone uses the word theory don't think it's them just throwing out an idea; it is the result of research and possibly years of evidence. Science is wonderfully dynamic, it's always shifting to accomodate new ideas and evidence. But this aspect of sciene doesn't gel well with humanity. Our whole existence is driven to maintaining a homeostasis not only internally but externally as well. That's natural, it's instinctual but the fact is that the physical world is constantly changeing in almost imeasurable ways when we examine it within the span of a life.

I think that's one of the reasons people struggle with evolution (religion aside) because evolution occured on a HUGE time scale, a scale that is almost unfathomable, a scale that's only had us existing for a few minutes! I think evolution is beautiful and marvolous to look at.

But I digress; people like all animals modify their environments but we've been doing it on a scale that is greater than animals. As far as volcanoes go, they do emit a lot of gases and affect global climate but those sorts of eruptions are not common, the Earth eventually rebalances but we are always adding we never give the physical world time to readjust.

What the goal should be is sustainability, a balance that sustains both the "human" and the "natural" world. But that means changes which people will naturally resist, ie a carbon tax on industry is a very effecient way to affect policy but extremely unpoular.