http://SaturnianCosmology.Org/ mirrored file For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== Page Last updated: Wed, 29 Jan 2003 08:02:37 GMT Croll-Milankovich Cycles and Climate Change According to the geologic record, long cooling and warming trends throughout the last million years seem to adhere to an approximate 100,000-year cycle. Why, though, should the Earth move into and out of ice ages cyclically? We now believe that these cycles correspond to changes in the Earth's relation to the Sun. In the mid-1800s, this idea received its earliest compelling formulation by the self-taught Scottish scientist James Croll. However, it was discovered that the geologic record do not match Croll's theory well enough to verify it. In the 1930s, the idea that astronomical factors explained ice ages was refined by a Serbian mathematician named Milutin Milankovich. It is thought that the onset of glaciation and the subsequent interglacial periods have been brought on by changes in the Earth's orbit around the Sun, also known as the Milankovich cycles. Further glaciation has been blamed for at least one (the late Ordovician) of the five mass extinctions on Earth and it is believed that the glacial period responsible for the extinction was brought about by eccentricity. Other effects relating to climate change, sea level changes and sedimentation (due to temperature change, and surface-runoff, for instance) can also be related to these cycles. There are three cycles, which combine to give a complicated pattern. * eccentricity <#eccent> * precession <#prec>, and * axial tilt <#tilt> Eccentricity *Eccentricity* is the shape of the Earth's orbit around the Sun. It varies between more or less elliptical on a cycle of about 110,000 years. (This 110 000 year cycle is actually a combination of a 100 000 year cycle, and a much weaker one of 413 000 years. It is sometimes referred to as the 100 000 year cycle). These variations are important because they change the distance that the Sun's energy waves must travel to reach the Earth, resulting in the Earth receiving more or less radiation. (This is the only one of the cycles that affects the actual /amount/ of radiation reaching the Earth.) Today the Earth experiences about a 6% difference in the amount of solar radiation received in January compared to July. When the Earth's orbit is more elliptical, the amount of energy received would be vary much more between seasons, in the range of 20-30%. Precession Earth does not have a perfect spin about its axis. It wobbles, and this wobble is defined as *precession*. precession02 (2K) 22 000 year precession cycle This Earth 'wobble' is with respect to the direction its rotational axis points. Within the framework of the stars, it represents a change of direction, from having the North pole pointing at the North Star, to having the North pole pointing at the star Vega. Precession oscillates between the two positions in a period of about 22,000 years. (The 22 000 year cycle is in fact a combination of a 19 000, and a 23 000, year cycle). Precession is climatically significant when the North pole is pointing towards Vega. There will be a large seasonal variation in temperature (harsh winters and hot summers). In this situation ice sheets would retreat in the Northern hemisphere. In contrast, if precession resulted in summer at the perihelion, then milder summers would allow more of the winter ice sheets to survive to the next winter. This could increase the size of these ice sheets, and possibly usher in a period of glaciation. According to the precession cycles, we should presently be entering into an extended period of ice sheet growth - were we not adding vast quantities of greenhouse gases to the atmosphere and upsetting the natural climatic variations. Axial tilt Axial tilt refers to the inclination of the Earth's axis in relation to its plane of orbit with the Sun. tilt02 (1K) 40 000 year tilt cycle This inclination oscillates in a range of 21.8^o and 24.4^o . It takes the Earth about 41,000 years to move through this range. The difference in tilt affects /where/ on the Earth receives the most and least solar radiation, but has global climatic consequences. Again, like precession above, axial tilt could cause warmer winters and cooler summers. This would in turn lead to more moisture being available to precipitate, leading to more snowfall, despite the warmer winters. This snow and ice could then last through the cooler summer months. At present, axial tilt is in the middle of its range. For any comments, suggestions or contributions, please e-mail me at: portsdown at bbm.me.uk Site written by hilma