[SA]
This is interesting. How do you know this? I've never seen any
records that show that the Ice Age regresses and progresses with
exact amounts, but that doesn't mean it's true.
[Arlo]
Chapter One of Columbia's History of the World is a good overview of
the large-scale geologic forces that operate on the planet. The
geologic record indicates that during each "Ice Age" (defined as a
period when the north and south magnetic poles occur over land
masses, allowing for the formation of large polar ice caps) there are
recurring cycles of glaciation, where glaciers progress towards the
equator and then rescind to the caps. The end of the actual Ice Age
occurs when the magnetic poles move over water, allowing for the mass
de-icing of the polar caps. Earth's geologic record indicates that
the "norm" for the planet is the long periods of "tropic" conditions,
as during the Mesozoic Era, that occur when the magnetic poles occur
over water. "Ice Ages", times when the poles occur over land (as they
do now), are period aberrations to the norm.
From Wikipedia on the glaciation cycle:
"Within the ice ages (or at least within the last one), more
temperate and more severe periods occur. The colder periods are
called glacial periods, the warmer periods interglacials, such as the
Eemian interglacial era.
Glacials are characterized by cooler and drier climates over most of
the Earth and large land and sea ice masses extending outward from
the poles. Mountain glaciers in otherwise unglaciated areas extend to
lower elevations due to a lower snow line. Sea levels drop due to the
removal of large volumes of water above sea level in the icecaps.
There is evidence that ocean circulation patterns are disrupted by
glaciations. Since the Earth has significant continental glaciation
in the Arctic and Antarctic, we are currently in a glacial minimum of
a glaciation. Such a period between glacial maxima is known as an interglacial.
The Earth is now in an interglacial period known as the Holocene. It
was conventional wisdom that "the typical interglacial period lasts
about 12,000 years" but now appears to be incorrect from the evidence
of ice core records. Therefore, it has been widely contradicted
recently; for example, an article in Nature[4] argues that the
current interglacial might be most analogous to a previous
interglacial that lasted 28,000 years."
From Wikipedia on the causes of Ice Ages:
"The geological record appears to show that ice ages start when the
continents are in positions which block or reduce the flow of warm
water from the equator to the poles and thus allow ice sheets to
form. The ice sheets increase the Earth's reflectivity and thus
reduce the absorption of solar radiation. With less radiation
absorbed the atmosphere cools; the cooling allows the ice sheets to
grow, which further increases reflectivity in a positive feedback
loop. The ice age continues until the reduction in weathering causes
an increase in the greenhouse effect."
There are, to be fair, other theories as to the cause of periodic Ice
Ages, such as reduced solar activity, the build up of greenhouse
gases, etc, but the coinciding of magnetic poles over land masses
seems to me to be the higher quality explanation (keeping in mind
that the evidence of the Ice Age cycles, or the intra-Ice Age
glaciation cycles, is not in dispute).
I'm not saying, of course, that you will have to begin crafting
mammoth-hide snow boots, but someday one of your great-great....
grandchildren will. And if mankind weathers out the thousand-plus
years of Hoth (Star Wars geek reference), they have a future of
Dagobah ('nother one for ya) to look forward to.
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