Ice ages are times when the entire
Earth experiences notably colder climatic conditions where the addition of snow consistantly exceeds
melting. During an ice age, continental-size
glaciers can cover considerable large regions of the earth. The
polar regions become much colder than they are presently and there are noticeably large differences in temperature from the
equator to the pole.
There have actually been a number of ice ages in the history of the Earth. Large glaciations occurred during the late
Proterozoic (between about 800 and 600 million years ago), during the
Pennsylvanian and
Permian (between about 350 and 250 million years ago), and the late Neogene to
Quaternary (the last 4 million years and includes the Pleistocene epoch).
There have been at least 4 major ice advances since the
Pleistocene epoch (which began about 1.6 million years ago). Called the
Wisconsinan,
Illinoisan,
Kansan, and
Nebraskan, these glaciations refer to the southernmost limit of glacial movement in
North America.
When scientists refer to the ice age, they are usually talking about the most recent glaciation, the Wisconsinan.
The Pleistocene ice age is important in human history because it is theorized that this was the time when
Homo sapiens evolved and spread through most of the world.
Water was stored in massive sheets of ice resulting in the lowering of sea level. This allowed our species to migrate from continent to continent by
landbridges.
There are many discussions among scientists to whether or not we are entering
global warming or just experiencing a warming trend before the next ice age. So, it is possible that our modern
climate is simply a very short, warm period between glacial advances.