Friday, September 9, 2011

The Pleistocene Ice Ages [Original Post Date: 3/26/06]

Currently, as defined by geologists, we live in the Cenozoic Era, Quaternary Period, and Holocene Epoch (since the last 10,000 years or so).

The previous epoch was the Pleistocene, which lasted from approximately 2 million years ago to 10,000 years ago. Before the Pleistocene Epoch was the Pliocene Epoch, which lasted from approximately 5 million to 2 million years ago.

Most people have a vision of the Pleistocene climate as being a single large Ice Age, when in reality there were several periods of glacial advance, separated by shorter interglacial periods, some of which were as warm or warmer than today's climate. The primary, attributed reasons for the fluctuations were variations in Earth orbit, Earth axial tilt, and variations in solar output. Some of these fluctuations exaggerate each other, while others moderate each other. Random events such as large volcanic eruptions may play a role, also. Evidence suggests that the global cooling began in the latter part of the Pliocene.

Some authors suggest that the emergence of the Isthmus of Panama (above Sea Level) during the Late Pliocene played a role in the cooling of the climate. Prior to the emergence of the isthmus, there was a narrow seaway that allowed the movement of warm waters from the Caribbean into the east Pacific. The closure of this seaway altered oceanic currents, while opening a land bridge that allowed the migrations of mammals between North and South America.

This series of slides is from a lecture on glaciers. Proceed forward to cover the Pleistocene. There have been previous periods of global glaciation, but those were before humans. Oxygen-18 isotopic data is one type of proxy data used to reconstruct paleoclimate history, including the Pleistocene data listed here in Wikipedia.

From youngest to oldest, as identified in North America, the Pleistocene glacial stages were:

Wisconsinan Glacial Stage
Sangamonian Interglacial
Illinoian Glacial Stage
Yarmouthian Interglacial
Kansan Glacial Stage
Aftonian Interglacial
Nebraskan Glacial Stage

Slide 37 shows reconstructed temperature curves for the last billion years and the last 2 million in more detail. Slide 38 shows the maximum extent of the Pleistocene continental ice sheets. Slide 39 shows estimated sea level for the last 20,000 years. These particular slides are from the University of Portland.

After each glacial stage, the interglacial represented a period of natural global warming, a period of rebound. Since the end of the Wisconsinan Glacial Stage approximately 10,000 years ago, there have been several alternating shorter periods of warming and cooling.

Previous ice ages include:

A poorly-documented, possible ice age from 2.7 to 2.3 billion years ago, during the early Proterozoic Era.
The earliest, well-documented ice age was during the late Proterozoic Era, from approximately 800 million to 600 million years ago.
Late Ordovician Period 460 million to 430 million years ago.
Late Carboniferous Period to Early Permian Period 350 million years to 260 million years ago.

From this above-linked Wikipedia article:

..."The present ice age began 40 million years ago with the growth of an ice sheet in Antarctica, but intensified during the Pleistocene (starting around 3 million years ago) with the spread of ice sheets in the Northern Hemisphere. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000 and 100,000 year time scales. The last glacial period ended about 10,000 years ago."

This Wikipedia link shows temperature trends during the last 5 million years.

The Pleistocene Epoch is the recent geologic past. If there were repeated periods of natural global cooling and global warming then, why are people so convinced that any and all unusual changes and variations are triggered by humans?

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