Ice Ages are periods in the earth's history when a significant,
extended cooling of the atmosphere and ocean took place. The
earth last entered such an ice age about 1.6 million years ago,
at the beginning of the Quaternary period. Although continental
ice sheets withdrew from North America and Europe about 10,000
years ago—at the end of the Pleistocene epoch—many scientists
believe that the Quaternary Ice Age is not over yet. Evidence of
earlier ice ages also exists. the time of the earliest recorded
life on earth (about 3.6 billion years ago), the planet's
average surface temperature has been about 20°C (70°F), with a
range of uncertainty of about 5°C (9°C). For more than 90 per
cent of that time the earth has been free of ice ages, and no
large glaciers have existed except in high mountains. Ice ages
occur about every 150 million years, and last a few million
years. the cause of ice ages is still a subject of controversy,
an argument based on astronomical observations of the galaxy has
gained credibility i