Ice Age Theory
The past ten ice ages have been cycling at 100 thousand year intervals. Before that, they were more varied.
Morel mushrooms re-evolve during each ice age. High humidity and rainfall is required to start morel evolution, because a filamentous yeast must resist dehydration on tree bark during the early stages.
When considering these points, morel evolution creates the impression that ice ages start with high humidity and rainfall. This conclusion is interesting, because other theorists have been assuming that ice ages involve dry and cold conditions, even at their beginning.
Another possible explanation for the evolution of morels is that a suitable environment is created in front of the ice sheet which covers part of the northern continents during an ice age. Cold air sweeping off the ice may prevent summer heat from dehydrating the yeast as it adapts to the soil.
Geologists have been totally mystified about the cause of ice ages, and their theories are extremely varied. A recent theory involves the earth's orbit, and it has numerous flaws.
The cyclic nature of recent ice ages creates extremely strong evidence that the cause of ice ages is one or more hot spots in the earth's core which moves around and comes closer to the surface from time to time. When near the surface, it heats the oceans causing more rain to occur. The increased rainfall causes snow to accumulate faster than it can melt. The increased snow and cloud cover reflect sunlight causing a cool-down of the planet.
With this theory, the reversal can be attributed to the movement of the hot spots in the earth's core. The reason why ice ages increase 80% of the time and decrease 20% of the time could be that the earth's surface is slow at responding to warm up but fast at responding to cool down.
An explanation for the large variations in the cycle time of ice ages before the previous ten is that the movement of tectonic plates could cause oceans to heat differently, or the movement of hot spots in the earth could be variable.
Update The Garden of Eden
In 1994 a Stone Henge-like ritual site was found in western Turkey. It's called Gobekli Tepe. It was dated to 12,000 years ago. This is when the last ice age was ending. The environment was a paradise resembling the Biblical "Garden of Eden." Now the environment is rocky and barren. The assumption is that humans destroyed the environment by abusing it.
I would theorize that 12,000 years ago, that location was at the leading edge of a glacier. Cold and humid air coming off the glacier created a rapid transition between something like a rain forest and a desert. In between the extremes would have been all imaginable ecosystems. To transition between diverse ecosystems in such a short distance gave humans the ability to exploit the environment to a maximum. The soil looks like glacial till pointing to such a history.