Greenhouse earth is the climatic state where no ice caps or
continental glaciers exist. Icehouce earth is the state where polar ice caps do
exist, and is our present state. An extreme form of this is "snowball
earth" when frozen conditions reach the equator. There is geological
evidence for this, in till-like deposits in tropical latitudes.
The evidence for these periods is incomplete enough that there is no strong consensus on their having occurred. But two main periods of global glaciation seem to have occurred. The nearer in time is the Cryogenian period, from roughly 726 mya to 650 mya. The peculiar type of sediment deposited by glacial ice--a mix of sediment grain sizes from fine clay, to sand, to gravel and cobbles up to boulders--is called diamict, or more commonly, till. Deposits of diamict dating to the Cryogenian have been found in paleo-tropical latitudes.
The main objection to findings of these possible tropical
glaciers is the question of whether different deposits coexisted. But
radiometric chronologies can't establish this with enough precision, and the
most common biological markers (fossils) are absent from those deposits.
Another, earlier, and longer, snowball earth episode is
thought to have occurred during the Huronian ice age (which was very cold,
whether the globe was fully froze or not), which lasted 300 million years, from
2.4 to 2.1 bya. This massive ice age is thought to have coincided with the
oxidation of that atmosphere, which removed most of the carbon dioxide and
nearly all the methane from it. These being two powerful greenhouse gases, the
early gradually descended into a long, profound ice age.
Whether the planet was frozen pole-to-pole or only mostly
so, periods of profound coldness have occurred, partly in response to
atmospheric changes in composition due to the products of living organisms. The
warming currently happening on the planet, though unprecedented in quickness,
is very deeply precedented in scale.
Tomorrow: Antarctica.
Be well!
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