Wikibooks: Historical Geology/Milankovitch cycles

• Format: Book
• Language: English
• Subjects: North Pole
• Online Access: https://en.wikibooks.org/wiki/Historical_Geology/Milankovitch_cycles

In this article we shall discuss what Milankovitch cycles are their effect on climate and how we know that this effect exists. =The cycles= The reader should remember from high school that the reason why the Earth has seasons is that its axis of rotation is at an angle to the plane of its orbit. When the northern hemisphere is tilted towards the sun then it is summer in the northern hemisphere and winter in the southern hemisphere when the southern hemisphere is tilted towards the sun it s the other way round. The magnitude of this effect depends on how tilted the Earth s axis is and this angle varies between 22.1° and 24.5° in a 41 000 year cycle. The reader should also recall that the Earth s orbit is not perfectly circular it is an ellipse with the Sun at one focus meaning that the Earth is closer to the Sun during some months of the year than others. The effect of this is less than you might suppose the Earth is five million kilometers closer to the Sun in January than in July but this doesn t stop the Northern Hemisphere from undergoing winter. The magnitude of this effect depends on how far the Earth s orbit deviates from being circular and a number of factors affecting this figure add up to a cycle of about 100 000 years. Finally there is the precession of the Earth s axis. At present as we have seen the North Pole is tilted away from the Sun at the Earth s point of closest approach to the Sun. However this too varies in this case in a 21 000 year cycle. These then are the three Milankovitch cycles as they are of different lengths their interaction will produce rather a complex pattern as they go in and out of phase with one another. Together they will affect both the total annual insolation (the amount of solar radiation that reaches the earth s surface) and also season variations in insolation. Although the idea of Milankovitch cycles as a factor in the Earth s climate was initially greeted with some suspicion by climate scientists it is now generally accepted that Milankovitch cycles account for ...