Ice Ages:
When people today think of major climate changes in Earth’s history, one of the first things to come to mind is always times of intense cold. This may be because people feel most uncomfortable in the few feet of snow that falls for a few weeks out of the year, and shudder at the thought of extended periods of complete ice. Humans today can hardly imagine surviving even a mild winter without their heated homes and Tommy Hilfiger sweaters, which is why many assume ice ages to be devastating periods that no one could have survived. To understand the implications of periods of glaciation, click here.
Immense global cooling, continual snowfall, and huge sheets of ice forming and consuming large portions of continents characterize ice ages. But what causes the Earth to get so cold? Milankovitch cycles are the leading theory behind the ice ages – the variations that contributed to the cooling of the planet. The eccentricity, or orbit, of the earth is an ellipse that "ranges from circular to more elliptical over 100,000 years" (Watson, 1997). When the earth’s orbit is more circular, there will be less sunlight hitting it because it is consistently further away from the sun than during more elliptical orbits. The earth orbits at an angle, or tilt, that is approximately 23.5 degrees from the perfectly flat plane. Tilt can fluctuate from 22.5 to 24.5 degrees every 41,000 years (Anderson, 1997). The earth "wobbles" on it’s axis "like a spinning top," effecting the amount of sunlight that will reach the earth (Illinois State Museum What…, 2001). Each of these factors contributes to cooler temperatures by limiting the amount of solar radiation (the earth’s source of heat) that strikes the planet. Tilt and precession (wobble) combined can "cause radiation changes of up to 15% at high latitude" (Illinois State Museum What… 2001) while the fluctuating orbit can limit radiation throughout the planet (Watson, 1997). Earth will undergo a cooling trend as the orbit, tilt, and wobble realign themselves, causing increased snowfall and glaciation. Many researchers agree that there are other factors that contribute to the onset of an ice age, but that Milankovitch cycles must occur along with such other factors for there to be significant glaciation. |
