Yes, the Big Chill is coming, but you won't need your industrial-strength
thermal underwear for another 3,000 to 20,000 years. Over the past billion
years, the earth has experienced three long periods during which ice built up
at its poles, each period made up of several 100,000-year "ice ages,"
when glaciers advanced to cover much of the world. These ice ages were
punctuated by 10,000-year "interglacials," warm spells marked by the
melting of the vast ice sheets. We live at the end of such a temperate
time-out; the last great ice age wound down about 7,000 years ago. At its peak,
20,000 years ago, glaciers encased much of North America, Europe, and Asia.
Days were about eleven degrees colder than they are now, forcing humans and
animals southward.
It's not hard to see how an ice age is caused by a
temperature drop, creating summers cool enough that the previous winter's snow
never melts. Several seasons' snows accumulate and compact to form glaciers.
But what turns down the thermostat? The cold facts have been hotly debated, but
the theory most widely accepted - the "astronomical" theory - states
that three periodic changes in the earth's position relative to the sun seem to
have launched ice ages by influencing the amount of solar radiation the earth
receives.
Because of the gravitational pull of the sun and moon on the
equator, the earth wobbles on its axis like a toy top slowing down. Every
22,000 years or so, it describes a circle in space. The axis also tilts,
causing the seasons. When the North Pole tips away from the sun, it's winter in
the Northern Hemisphere. Today, the angle of tilt is 23 ½ degrees but every
41,000 years it moves from 22 to 24 degrees and back again. Perhaps the most
important cycle is a change in the shape of the earth's orbit - from nearly
circular to highly elliptical and back to circular - every 100,000 years due to
the gravitational tug of fellow planets. The combined effect of these three
cycles is to place the earth farther away from the sun at certain times, cooling
the planet into an ice age.
