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MAJOR ENVIRONMENTAL FACTOR: 118,000 BC to 8,000 BC: the last Ice Age; a third of Earth's surface is sheathed in ice for much of this period-- and world geography undergoes drastic changes

For the 15,000 years preceding this time, world sea levels never rose more than two meters above that of the sea level circa 1999 AD. But now sea levels worldwide drop to 1999 AD levels and significantly below between 118,000 and 8000 BC. This means there are greater chances for landbridges connecting islands and continents, and considerably more land exposed along the coastlines of continents and islands, among other things. All this extra dry land offers places where human civilizations might spend centuries or even millennia building cities and nations, and migrating over to explore new territories-- only to have the sea take them away again in the centuries to follow (and possibily leaving some folks permanently stranded on places like islands afterwards). In 1998 AD archaeologists will be discovering many human works submerged this way during prehistoric times.

Another implication of the great ice sheets are 'ice bridges' essentially offering yet another means of connection between many land masses during this time that would otherwise be inaccessible due to surrounding seas. Note that the Earth's north pole will have little or no exposed land area by 1998 AD; and yet it will be covered by an ice sheet sufficient to support lengthy migrations of human and animal species across the region (assuming the cold and hunger don't kill them along the way). Thus, technically there exist paths allowing exploration of the americas and other regions for ancient hominids as far back as two million years or more as of 1998 AD. Such access ways may be as forbidding as deserts, or as temporary as seasonal sea ice, but they are there none-the-less. Indeed, early humans could theoretically explore almost every continent on earth now requiring little more than their feet for locomotion, due to ice sheets and lowered sea levels exposing various land bridges. No boats or rafts are required for most excursions. And yet, any humans intelligent enough to use floating constructions may trod the last few percentage points of the Earth's surface remaining out of reach of their land locked peers as well.

Unfortunately, towards the end of this period, when the ice sheets are retreating and the ocean levels rising again, volcanic action tends to increase as the weight of the ice sheets themselves seemed to have restrained them earlier. And the huge cataracts of flood waters released from melting glaciers and overflowing of sea-sized inland lakes during this time also wreak havoc on many regions.

Sudden large climate changes worldwide occurred during the last Ice Age.

In the northern hemisphere at least (and likely true of the rest of the world as well), substantial climate change for vast regions can take place within ten years or less-- well within a single human life time. The so-called Little Ice Age was documented to end in a mere ten years during the 1840s.

Climate changes can happen with little or no warning. Around 8000 BC in Michigan there was around a 145 year window of opportunity for a certain small forest to spring up in the wake of shrinking Ice Age glaciers. A sudden climate change opened the window, while further escalations in that climate change later closed it, when the glaciers rapidly collapsed and flooded the area with silt carrying water, burying and drowning the forest where it stood. The flood was gentle enough not to topple or strip the trees.

From studies of the tree growth rings, it appears that there was no indication beforehand that the climate was going to warm up when it did, in either instance.

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