There were several ice ages as well.. Most of those differences man wasn't even around. Guess we'd have to blame dinosaur farts or whatever.
Again, these are just my thoughts, but it there were all those climate changes and man not even in the picture, what might be the common denominator? I think it may be the sun. maybe over time the sun waxes and wanes in its intensity with the hot times being global warming and the cool spots down enough to create an ice age.
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The change in the tilt of the Earth's axis also has an effect on climate:
