JB73, the Earth's distance from the sun at various times of the year has VERY little to do with the climactic changes. Although Earth's orbit is eliptical, there is very little difference in temperature at it's farthest vs. closest point to the sun. The seasons are caused soley by the tilt of the earth's axis.
As an amatuer astronomer, I love this kind of stuff. History suggests the Earth goes through repeated cycles of global warming, then ice ages. This is mainly caused by volcanic activity which releases carbon dioxide into the atmosphere which acts like a greenhouse. There are many theories as to why, including a group of very large asteroids that pass by the earth every 65 million years or so. The gravitational tidal effects of these asteroids passing by the earth causes increases in tectonic activity which in turn causes increases in volcanic activity. The more volcanoes, the more carbon dioxide, and the warmer it gets. The only drawback is that sometimes these asteroids actually hit the Earth causing mass extinctions (dinosaurs, etc...).
I think we're due for another pass in about a million years or so, so LOOK OUT!!!!!!!!!!! LOL