Ice Age..... When the Earth was covered in ice...
If you are a movie fan, you must have heard or seen the popular
animated movie 'Ice Age'. It's a story about a mammoth, a sabre toothed
tiger and a sloth trying to survive the advent of the ice age.
Sure, most of us enjoyed the movie very much, but did anyone of you
query what an ice age is?
An ice age is a period of long-term downturn in the temperature of
the Earth's climate. This results in an expansion of the continental ice
sheets, polar ice sheets and mountain glaciers.
Ice age is often used to mean a period when ice sheets exist in the
northern and southern hemispheres; by this definition, we are still in
an ice age (because the Greenland and Antarctic ice sheets still
exist).More colloquially (used in informal speech), when speaking of the
last few million years, ice age is used to refer to colder periods with
extensive ice sheets over the North American and Eurasian continents. In
this sense, the last ice age ended about 10,000 years ago.
This article will use the term ice age in the former, glaciological,
sense; and use the term glacial periods for colder periods during ice
ages and interglacial for the warmer periods.
Many glacial periods (colder periods) have occurred during the last
few million years, initially at a 40,000-year frequency, but more
recently at 100,000-year frequencies. These are the best studied. There
have been four major ice ages in the distant past.
Origin of ice age theory
According to the Wikipedia encyclopaedia, the idea that, in the past,
glaciers had been far more extensive was folk knowledge in some Alpine
regions of Europe. No single person invented the idea.
At this early stage of knowledge, what was being studied were the
glacial periods within the past few hundred thousand years, during the
current ice age. The existence of ancient ice ages was as yet
There have been at least four major ice ages in the Earth's past. The
earliest hypothesised (assumed) ice age is believed to have occurred
around 2.7 to 2.3 billion years ago, during the early Proterozoic Age.
The earliest well-documented ice age, and probably the most severe of
the last one billion years, occurred from 800 to 600 million years ago
(the Cryogenian period) and it has been suggested that it produced a
Snowball Earth in which permanent sea ice extended to or very near the
equator. There were extensive polar ice caps at intervals from 350 to
260 million years ago, during the Carboniferous and early Permian
Periods, associated with the Karoo Ice Age.
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 three 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
The timing of ice ages throughout geologic history is in part
controlled by the position of the continental plates on the surface of
the Earth. When landmasses are concentrated near the polar regions,
there is an increased chance for snow and ice to accumulate.
Small changes in solar energy can tip the balance between summers in
which the winter snow mass completely melts, and summers in which the
winter snow persists until the following winter. Due to the positions of
Greenland, Antarctica, and the northern portions of Europe, Asia, and
North America in the polar regions, the Earth today is considered prone
to ice age glaciations.
Evidence for ice ages comes in various forms, including rock
scratchings, glacial moraines (mass of stones carried and deposited by a
glacier), drumlins (usually oval ridges formed under the ice sheet of a
glacial period), valley cutting, and the deposition of till or tillites
(a type of clay) and glacial erratic (unevenness). Successive
glaciations tend to distort and erase the geological evidence, making it
difficult to interpret. It took some time for the current theory to be
In between ice ages, there are multi-million year periods of more
temperate, almost tropical, climate, but also within the ice ages (or at
least within the last one), temperate and severe periods occur.
The colder periods are called 'glacial periods', the warmer periods 'interglacials',
such as the Eemian interglacial era. We are in an interglacial period
now, the last retreat ending about 10,000 years ago.
But, what causes ice ages? The cause of ice ages remains
controversial for both the large-scale ice age periods and the smaller
ebb and flow of glacial/interglacial periods within an ice age. The
general consensus (widespread agreement) is that it is a combination of
upto three different factors: atmospheric composition (particularly the
fraction of CO2-Carbondioxide and methane), changes in the Earth's orbit
around the Sun known as Milankovitch cycles (and possibly the Sun's
orbit around the galaxy), and the arrangement of the continents.
The first of these three factors is probably responsible for much of
the change, especially for the first ice age. The "Snowball Earth"
hypothesis(suggested explanation of something) maintains that the severe
freezing in the late Proterozoic was both caused and ended by changes in
CO2 levels in the atmosphere. However, the other two factors do matter.
An abundance of land within the Arctic and Antarctic Circles appears
to be a necessity for an ice age, probably because the landmasses
provide space on which snow and ice can accumulate during cooler times
and thus trigger positive feedback processes like albedo (a measure of
the reflecting power of an object, expressed as the proportion of
incident light it reflects) changes.
The Earth's orbit does not have a great effect on the long-term
causation of ice ages, but does seem to dictate the pattern of multiple
freezings and thawings that take place within the current ice age.