An Ice age may be defined as an interval of time when planet wide climate undergoes a definite drop in average global temperature. When this drop in temperature continues on a geologic time scale it leads in turn to frozen precipitation being deposited in a volume so vast during the cold seasons that it cannot all melt away during the warmer ones. This leads to ice fields growing larger and locking up more of the planets water supply, with a proportional decline in sea levels; these may decline by 400 feet or more which dramatically alters continental coastlines and marine habitats.
The mounting pressure of the deepening ice shields that can sometimes be two and a half miles in depth causes rivers of ice called glaciers to spread into the earth’s temperate zones, blanketing immense blocks of land now well populated, and burying rich agricultural districts under a mile or more of ice. The impact of this process on flora, fauna and topography are profound. The Great Lakes of Canada and the United States were revealed during the last glacial period as were the present system of rivers and inland waterways. When the next cycle of glaciations advances and retreats, these structures will be erased; and a new and generally unrecognizable terrain will emerge.
The entire Quaternary period is referred to as an Ice age, broken down into a myriad of cycles of glaciation during which ice shields grow and expand and inter glacial periods where glaciers have retreated and temperate lands emerge once more. Earth is presently nearing the end of one such inter glacial cycle, being about 18,000 years into a 20,000 year period.
While the Quaternary period is recognized as the “Ice Age” enormous glacial episodes have occurred throughout geologic time and are often associated with major extinction events, as is the case with the end Ordovician, end Cretaceous and end Permian events. It is also asserted that the Quaternary Ice age is but one of perhaps 5 enormous ice ages occurring at intervals of hundreds of millions of years and related closely to plate tectonics and the forming, location and break up of the super continents but that exceeds the scope of this article.
A pattern emerges:
Milotan Milankovitch was a Serbian civil engineer, astronomer and mathematician. During the early part of the 20th century and particularly during the Great War when he was a perpetual prisoner of Austria Hungary Milankovitch developed a theory to explain the progress of the more recent – Quaternary – glaciation episodes. His theory is based on the variability of interactions between the sun and earth which are cyclic in nature. Although it is simplifying the Milankovitch theory somewhat these interactions reflect three basic astronomical processes.
The first process is the eccentricity of earths orbit around the sun. Generally thought of as circular, this is true only at intervals of celestial time, and cycles over a period roughly 100,000 years in duration. Interaction with other celestial bodies primarily Jupiter and Saturn cause earths orbit to be either more or less circular, or elliptical (eccentric). Presently the status of earths orbit varies by about 3% from a true circle which causes an approximate 7% variation in received seasonal solar energy. At maximum eccentricity which is about 9 % deviation from circular orbit, the difference between solar energy received differs by 20%.
Low orbital eccentricity correlates to an increase in glaciation.
The second process is known as Obliquity, or the angle of the tilt of earth’s axis relative to the sun, which is not a constant. At present the axis inclines about 23.5 degrees from true, and varies between 22 degrees and 24.5 degrees over a 41,000 year period. It appears that at 22 degrees, i.e. lesser tilt, there is less seasonal variation in temperature at high and mid latitudes. This status translates to milder winters and cooler summers, a condition which favors glaciation.
The final process, known as precession, relates to the way the earth wobbles, much like a top, around the point where the axis passes thru the South Pole. The cycle occurs over a 23,000 year period and affects once again, proximity to the sun and therefore how much solar energy is received in northern latitudes. Currently the earth is closest to the sun in January, farthest in July. In 11,000 years when that situation is reversed northern latitudes will be subjected to very savage winter conditions. Additional factors must also be considered some of which include the amount of atmospheric dust and/or greenhouse gasses in the air, the reflectivity of the ice sheets themselves, the nature of the prevalent cloud cover and even the rebound effect of the earths crust as it rises once relieved of glacial masses.
Milankovitch theories are eloquent and persuasive and predicted the most severe glaciations at times when the three cycles coincided. Indeed this seems to be the case but empirical truth was not available until the 1970’s; years after the death of the original theorist.
Advancing technology aids research:
Proof of the cyclical nature of recent periods of glaciation and glacial recession lay buried beneath the ancient ice fields of the Antarctic and of Greenland, and come in the form of Isotopic Geochemistry. Using these techniques researchers can take core samples of ancient ice shields and by analyzing the air pockets trapped in the ice calculate the relative frequency of two oxygen isotopes, O16 and O18 respectively. O16 is about 500 times more common than O18 but in periods of intense cold the percentages shift, and a far greaten number of O18 atoms appear. Correlating this information with the predictions of the Milankovitch theory bears that theory out very well.
But ice cores older than 400,000 or so are not available and that is barely a blink of an eye in geologic time. Fortunately it seems that the calcium carbonate skeletal remains of very ancient sea creatures contain the same tell tale oxygen isotope markers that are trapped in ice core samplings, and my be obtained in much the same way, by taking core samples from fossil rich former seabed samples whose age we know. When these samples are analyzed it appears that Milotan Milankovitch was right on the money with his system and with his calculations; at least as far as the glaciation events of the Quaternary period are concerned.
A look at the predictive factor of Milankovitch theory indicates that earth is now emerging from a 20,000 year long interglacial cycle and can expect another period of deep ice cover for North America, Europe and Northern Asia in 2000 years, give or take a reasonable margin of error. This is of course, not even a second on the vast geologic clock.
A world of research beckons:
What Milankovitch theory cannot help science do is to identify the entire mechanism by which the Quaternary period of glacial advance and retreat began in the first place, after many hundreds of millions of years without such cycles, at least none that have been detected. As has been stated earlier there appear to have been at least 4 other major such events over earths 4.6 billion year time line, and perhaps many more. These also seem to be cyclic in nature.
Discovering the prime movers in that vast cycle will provide a fascinating field of investigation for many years to come.
