An ice age is
a long period
when Earth's
surface and
atmosphere
are much colder
than average.
That results in
continental and
polar ice sheets,
and alpine glaciers.
Earth's
climate
alternates
between
ice ages and
greenhouse
periods.
During greenhouse
periods, there are
no glaciers at all.
Earth is currently
in the Quaternary
glaciation.
( aka "Ice Age" )
Within an ice age
are very cold
"glacial periods"
and warm periods,
called "interglacials".
"Ice age"
implies
extensive
ice sheets
in both the
northern and
southern
hemispheres.
We are living
in a "warm"
interglacial
period called
the Holocene.
Three types
of evidence
for ice ages:
Geological Evidence:
Rock scouring
and scratching,
glacial moraines,
drumlins, valley
cutting, the
deposition of
till or tillites and
glacial erratics.
Sediment and
ice cores studies
reveal that glacials
last a long time,
but interglacials
are short periods.
Chemical Evidence:
Variations in the
ratios of isotopes
in fossils present
in sediments,
sedimentary rocks
and ocean sediment
cores.
Paleontological Evidence:
Changes in the
geographical
distribution
of fossils.
During a cold
glacial period
living creatures
will move to lower,
warmer latitudes,
or become extinct.
There have been
at least five major
ice ages:
(1) Huronian,
2.4 to 2.1 billion years ago,
during the early
Proterozoic Eon.
(2) Cryogenian,
720 to 630 million years ago,
(3) Andean-Saharan,
460 to 420 million years ago.
(4) late Paleozoic,
360 to 260 million years ago.
(5) Quaternary,
Started about 2.58 million
years ago.
2.58 million years ago
is based on formation
of the Arctic ice cap.
The Antarctic ice sheet
began to form about
34 million years ago.
There have been
cycles of glaciation,
with ice sheets
advancing
and retreating,
on 40,000-year
and 100,000-year
time scales.
The Earth
is now
in a warm
interglacial
warm period.
The last cold
glacial period
ended about
10,000 years
ago.
All that remains
of the continental
ice sheets are
the Greenland
and Antarctic
ice sheets, and
some smaller
glaciers.
Outside of ice ages,
the Earth seems
to have been ice free,
even in high latitudes,
in periods known as
greenhouse periods.