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Ice Ages Throughout Geologic TimeHistorical Timeline, Measurement and Theory of Cool Climate
Sep 20, 2010 © Harry P. Schlanger
The history of the Earth has been divided by a time scale called 'geologic time'. There is evidence for five previous Ice ages, the last one coming to completion.
This article focuses on past Ice ages but first examines methods of determining historical time periods
and global temperature measurement techniques. The question of uncertainty is considered throughout.
Geologic Time
Geologists developed a concept of historical time to divide up the Earth's history into appropriate
periods. This was achieved by examining layers of sedimentary rock and using radiometric-dating
techniques developed early in the 20th century.
The established
geologic timeline
has major time divisions are called "eons", which are subdivided into "eras" and
further into "periods" and "epochs". Some of these epochs may be grouped together to show less detail,
for example "Quaternary" or "Tertiary" periods in Figure 1.
Measurement of Global Temperatures
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Figure 1. Historical Global Temperatures
Modified from
Scotese (2010). Note, mya = million years ago
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Pidwirny (2006), Wikipedia (2010) and Scotese (2010)
list a wide range of indirect evidence for temperature that allows
climatologists to reconstruct the Earth's past climate. Physical, chemical and biological data can provide
fossil evidence of past climate fluctuations effects - called "proxy data", for example:
- Tree ring width and density measurements
- Fossilized plant remains
- Insect and pollen frequencies in sediments
- Moraines and other glacial deposits
- Marine organism fossils
Using proxy data inherently assumes uniformity in the data record. That is, today's measurement is assumed
to reflect the same physical or biological character of the past.
Five Major Ice Ages
Over geologic time, there have been five major ice ages as described by
Plimer (2009) in a detailed monograph of climate on Earth, and other authors (Archibald 2007, Scotese, 2010).
Figure 1 shows where in time, relative to the finest geologic time divisions, do the five major
ice ages "fit":
- The curve running down the centre is the historical average global temperature
- The temperature oscillates between "warm" times of up to 22° and "cool"
times of 12°
- Cool times define the Ice ages
Although the historical average global temperature curve is nice and smooth, the actual raw data coming from
temperature proxies from around the world is so highly variable, they are calibrated and processed in what
is called
"Spaghetti graphs".
This variability makes one wonder about the true meaning and significance of an averaged global temperature.
Fig 2. Sample of highly variable global temperatures, averaged in a
so-called "Spaghetti graph" (Frank Lansner).
The Last Ice Age
The period from 1.8 Mya to about 14,000 years ago (Fig. 1) is known
as the Pleistocene, or the last Ice Age. Large glacial ice sheets covered much of North America, Europe,
and Asia for extended periods of time, but were not static. Glaciers retreated and advanced because of
successive warmer and colder temperatures. The most recent glacial retreat is still going on today
(Pidwirny, 2006, Hoffman & Simmons, 2008).
Explanation for the Ice Ages
The causes the ice ages are not fully understood and remain controversial. There are basically two types of
ice age theories, terrestrial or extraterrestrial. A few of these are (Wikipedia, 2010):
- Solar theory
- Variations in Earth's orbit
- Continental and crustal displacement theories
- Changes in Earth's atmosphere
- Astronomical or Milankovitch theory
The solar theory explains variations of intensity and timing of heat from the sun, causing the
glacial and interglacial cycles. For example, the cool
period called "Little Ice Age" correlates with an extended period of low solar activity
called the "Maunder Mininum" (Archibald, 2007). However, some scientists believe that climate may be
influenced by a combination of mechanisms, not explained by a single theory.
The reader might be interested in a related article, "
Emerging from the Last Ice Age".
References:
- Archibald David. "
The Past and Future of Climate" (pdf). A presentation to The Lavoisier Group’s 2007
Workshop, "Rehabilitating Carbon Dioxide" held in Melbourne, Australia, on 29-30 June 2007.
- Hoffman, Doug L. & Simmons Allen.
"The Resilient
Earth: Science, Global Warming, and the Future of Humanity". 2008 (eBook).
- Pidwirny, M. "Earth's Climatic History",
Fundamentals of Physical Geography, 2nd Edition, 2006.
- Plimer, Ian. "Heaven and Earth. Global Warming: The Missing Science". Conner Court Publishing. Ballan,
Victoria, Australia, 2009.
- Scotese C. R. "Climate History",
PALEOMAP Project, 2010.
- Wikipedia. "Ice Age", 2010.
The copyright of the article Ice Ages Throughout Geologic Time: Historical Timeline, Measurement and Theory of Cool Climate is owned by Harry P. Schlanger. Permission to republish in print or online must be granted by the author in writing.
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