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A Closer Look: Ice Ages
What are ice ages?
Ice ages are recurring periods in the Earth's history,
usually thousands or tens of thousands of years in length, when
the entire Earth
experiences colder climatic conditions. During these periods,
enormous continental glaciers called ice sheets cover large areas
of the Earth’s
surface. Ice ages are separated by warmer periods called interglacial
periods. Several ice ages have occurred throughout our planet's
history. The last ice age peaked about 18,000 years ago, after
which the Earth
again began to warm.
Ice sheet advance in most recent ice
age.
What causes an ice age?
Ice age intervals seem to correspond with long
periods of worldwide climatic cooling. It should be noted that
there are numerous
factors that affect the Earth's climate. There are two main
natural factors, however,
that are thought to influence long-term changes in the Earth's
climate.
Plate Tectonics
- Changes in the positions of the continents are linked
to periods of multiple glaciations. The presence of
large landmasses at the poles seems to trigger the development of
extensive
ice
sheets. For example,
throughout the Ice age that occurred during the Pennsylvanian
and Permian
geologic eras (250 –350 million years ago), the southern
portion of the super-continent Pangaea was at the South Pole.
Scientists
believe this may have caused extensive glaciation
over what are now Africa, South
America, India, Antarctica, and Australia.
- Major uplift at
continental plate boundaries can cause profound changes
in oceanic and atmospheric circulation patterns. Changing
circulation patterns cause climate change. Some scientists hypothesize
that climatic changes caused by uplift are critical to the development
of
ice ages.
- Large or numerous volcanic eruptions, also the result of plate
tectonics, release huge amounts of gases (carbon
dioxide and water vapor) that act to trap heat within the atmosphere,
causing
global warming
(sometimes
referred to as the greenhouse effect). In contrast,
particles of ash can impact the global climate by reducing the amount
of solar radiation
reaching
Earth's surface, causing global cooling.
Milankovitch Cycles
Named after the Serbian mathematician Milutin Milankovich,
the Milankovitch cycles describe variations in Earth’s position
as it orbits the Sun that affect the amount of sunlight it
receives. One cycle
involves the shape of Earth’s orbit, which varies from being more
elliptical to more circular. Another cycle involves Earth’s “tilt” on
its axis, which is not perpendicular to the plane of its orbit
around the Sun. Presently, Earth tilts 23.5¾ from the perpendicular,
but there
is evidence that in the past this angle varied from 22¾ to
24¾. A third variation involves Earth’s “wobble.” Like
a top wobbling on its axis, Earth wobbles as it orbits. The
results are that sometimes
the North Pole points to the North Star, and sometimes it
points to the star Vega. These cycles vary from tens of thousands
to hundreds of thousands
of years and can be traced back through the Earth's geologic
record.
When did the ice ages occur?
Several ice ages have occurred throughout
our planet's history. Major periods of glaciations occurred
during the late Proterozoic
Era (between 600 and 800 million years ago), during the
Pennsylvanian and
Permian Eras (between about 250 and 350 million years ago),
and the late Neocene to Quaternary Eras (the last 4 million
years). Somewhat less extensive
glaciations occurred during parts of the Ordovician and
Silurian Eras (between about 430 and 460 million years ago).
The most recent ice age
began about 1.8 million years ago during the Pleistocene
Era. During this time, giant ice sheets advanced and retreated
many times in North America
and Europe.
Recent cycles of advancing and retreating ice
sheets have occurred approximately every 100,000 years. Each
cycle consists of a
long, generally cold period during which the ice sheets
slowly reach their maximum size,
and a relatively short, warm period during which the
ice sheets rapidly retreat.
We are now in a warm period that
has lasted more than 10,000 years, which is longer than many
of the previous warm intervals.
If the pattern of glacial cycles holds true, scientists
believe the Earth is
soon due for another cold period. In the 1800s, global
temperatures began decreasing during a period known
as the Little
Ice
Age. Currently, patterns
indicate that the Earth is nearing the end of an interglacial
period, meaning that another ice age is predicted in
a few thousand years.
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