Yes, obviously the one factor with the greatest effect on the earth's climate is the sun.
By a huge margin.

If the sun flared up, it could burn the earth to a crisp, or if it burned out, the earth would freeze. Now that's climate variation.

But the question is, what other factors might cause the climate to go outside the boundary of what we humans have become comfortable with over the last 12,000 years?

One factor is Co2, of course. The correlation between excess Co2 and the end of the last ice age does not necessarily mean that it was what brought about the warming. It may be that the warming initially had other causes, but since one result of the melting was the freeing of trapped Co2, a feedback mechanism may have been triggered, leading to still further warming.
In fact, I've read speculation that the current high output of carbon is helping to ward off the next ice age.

One thing that I don't think there's much argument about, though, is that atmospheric Co2 is reaching levels that the earth has not seen in something like 30 million years, long before the current series of ice ages.

That was an entirely different world, one in which there were no humans. We really don't know how our species would do in a world with that sort of climate, but it's fairly certain that there will be major disruption.

The whole system is far too complicated to say "aha, so this is the reason!"

Personally, I'm not all that concerned about much of the changes (though - as I mentioned - there will be major disruptions, tens of millions no doubt dead... damn, I'm a bastard,) except for the ocean acidification, which could lead to some real problems. Like Permian Extinction level problems. Crossing my fingers on that.