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Emerging from the Last Ice AgeIce Retreating but Today's Temperatures not Historically Warmest
Sep 26, 2010 © Harry P. Schlanger
Glacial retreat from the last ice age is in progress with today's average global temperatures 2°C cooler compared to the Medieval Warm period of 1000 years ago.
A previous article entitled,
"Ice Ages Throughout Geologic
Time" examined the concept of a geologic time, how time periods were measured and classified, and
where each of five major ice ages fit into the historic timeline.
The focus here is on the most recent ice age, which spanned from part of the Tertiary period
through to the Quaternary period (Fig. 1).
Quaternary Period
Two epochs make up the Quaternary period: the Pleistocene, also known as
"Pleistocene Glaciation", which was a period of maximum cooling, and the Holocene epoch, the most recent
geologic time and period of warming - called an interglacial period.
Pleistocene Glaciation
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Figure 1. Historical global temperatures of the last ice age
(cartoon).
Note, mya = million years ago
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During the Pleistocene, or Ice Age, large glacial ice sheets covered much of North America, Europe,
Asia, and bordering Southern Australia for extended periods of time. The extent of glacier ice
oscillated between retreat because of warmer temperatures and advance due to colder
temperatures.
At the peak of the Ice Age, some 20,000 years ago (0.02 Mya), average global temperatures were in the
vicinity of a minimum of 12°C. The glacial advance locked up so much water in ice sheets that sea levels
were below today's levels by 120 m (Hoffman & Simmons, 2008).
Holocene Warming
The last glacial retreat began about 14,000 years ago (0.014 Mya).
Human civilization, in its most widely used definition, dates entirely within the Holocene, which began
about 11,700 ago (0.0117 Mya).
The graphic above is that of Ötzi the Iceman, discovered frozen in the ice in 1991 by two German tourists.
He lived some 5300 years ago, during a time when Europe's climate was comparable to today's.
Figure 2 is a graph of average global temperature during the Holocene epoch, compiled by Archibald (2007)
after published data from Dansgaard and Schonwiese. The main features are:
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Holocene Climatic Optimum: Between 9000 - 4000 years BP (before present) average
global temperatures reached their maximum level. Temperatures were warmer than they are today
and many of the Earth's great ancient civilizations began and flourished.
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Roman Climatic Optimum: During the Roman Empire (150 BC - 300 AD), a cooling began that lasted
until about 900 AD. At its peak, the cooling caused the Nile River and the Black Sea to freeze.
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Medieval Warm Period: The period 900 - 1200 AD represents the warmest climate since the Climatic
Optimum. The Vikings established settlements on Greenland and Iceland.
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Little Ice Age: From 1550 to 1850 AD global temperatures were coldest since the
beginning of the Holocene. The Thames River was frozen over. Cold weather in Iceland from 1753 and 1759
caused 25% of the population to die from famine due to crop failure.
Figure 2. Holocene global average temperatures
(compiled by Archibald, 2007).
Modern Warming
Since the Little Ice Age came to an end around 1860, the Earth has been experiencing a general warming
trend. However, there were periods of cooling in the 1940s to 1970s before warming resumed. Temperatures
peaked in 1998 and the trend has been down since. It should be noted that today's temperatures are about
2°C cooler compared to those recorded during the Medieval Warm period (Archibald, 2007).
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.
- Wikipedia. "Ice Age", 2010.
The copyright of the article Emerging from the Last Ice Age: Ice Retreating but Today's Temperatures not Historically Warmest 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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