the peak of the last glacial cycle, giant ice
sheets a mile thick covered half of North America.
is an ice age?
ice age is a period of timeusually millions or
tens of millions of yearswhen vast glaciers, called
ice sheets, cover much of the Earths land surface.
Several ice ages have occurred throughout our planet's
history. The latest ice age began about 2.5 million
years ago. During this time, giant ice sheets have advanced
and retreated many times in North America and Europe.
cycles of advancing and retreating ice sheets have occurred
every 100,000 years. Each cycle consists of a long,
generally cold period during which the ice sheets slowly
reach their maximum extent over tens of thousands of
years, and a relatively short warm period during which
the ice sheets rapidly retreat.
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 still holds
true, we should be about due for the beginning of the
next cold phase. Indeed, several hundred years ago,
global temperatures began decreasing during a period
known as the Little Ice Age.
the last century, however, average global temperatures
have instead started to rise. Scientists have recently
concluded that much of the recent warming is due to
the release of greenhouse gases from human activities
such as the burning of fossil fuels. A sobering possibility
is that continued human-caused global warming could
disrupt or override the natural climate cycle of the
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