With Speed and Violence Fred Pearce - Global Commons Institute

It turns out that the eccentricity of Earth's orbit around the sun drives the
100,000-year cycles into and out of ice ages. Meanwhile, the other two
effects, especially the precession, seem to trigger the short warm episodes
that punctuate each ice age.
Croll realized that, averaged over a year, these changes made little
difference to the amount of solar radiation reaching Earth. The overall effect
was probably less than 0.2 watts per 10.8 square feet. But the changes did
alter where and when the sun hit. Croll calculated in great detail how these
influences waxed and waned over tens of thousands of years. And he
established, at any rate to his own satisfaction, that they coincided with what
geologists were then discovering about the timing of Earth's progress into
and out of ice ages.
Taken together, the changing orbital shape, planetary tilt, and rotational
wobble alter the strength of seasonality, making summers and winters more
or less intense. And it was this that triggered the growth of ice sheets on land
in the Northern Hemisphere, he said. Ice sheets would grow when northern
winters were coldest. That would be when Earth was farthest from the sun,
and when changing tilt ensured that it received the least sunlight. Once ice
sheets started to grow, they would reflect ever more sunlight back into space,
intensifying the cooling. Croll realized, too, that there was much less room
for ice sheets to spread in the Southern Hemisphere, because they were
confined to the continent of Antarctica. So the Northern Hemisphere would
dominate events, driving the overall heat budget of the planet. But, he
suggested, other feedbacks, such as changes to winds and ocean currents,
could help drive the world further into an ice age.