A very good point, and one that could get so politally twisted as to deny the Eath ever, in all its billions of years, changed temperature at all.

Then there's that South Polar volcano that I think has been causing the ozone-hole effect over the past million years or so.

I like the albedo approach -- the whiter the planet the more reflective, and therefore the colder it gets. Of course there's a limit to this in the Habitable zone -- not a very comfortable limit, but a limit none the less. If we end up with a new Snowball-Earth, then we'll need to polute the hell out of it to lower temps. Can do, as keeping warm would require a lot of dark exhaust from all that fossile fuel.

Then again, we could just darken the snow -- somehow.