Wednesday, 30 November 2011

Milankovich cycles

The cycles are caused by changes in the amount of radiation reaching the earth from the sun over time. This is not because the Sun changes its output of energy, as it has remained relatively constant, but because the earths orbit around the sun varies in three predictable cycles.

  • Eccentricity: the earths orbit changes shape to become more elliptical over a period of 100,000 years. At present the eccentricity is almost at a minimum with a difference of around 6% in received radiation between January and July. At maximum eccentricity this difference increases to between 20% and 30%, which has a massive effect on climate. 
  • Obliquity: the tilt of the earths axis, which is responsible for our changing seasons, changes up to 3 degrees with a cycle of 41,000 years. A smaller tilt promotes the growth of ice sheets as warmer winters result in more moisture ans snow.  
  • Precession: eccentricity and obliquity together cause this further cycle where the inclination of the Earths axis changes in relation to where it is on the orbit. The cycle operates in periods of 19,000 and 23,000 years. At the moment we are closest to the sun so northern winters are slightly warmer than 10,000 years ago when the planet was furthest from the sun. Slow changes in the direction of the axis of the earth as it orbits results in greater seasonal contrasts.

Evidence for Milankovitch cycles

The blue Lias and Kimmeridge clay

For many years geologists studying the Blue Lias rocks to the lower Jurassic in Lyme Regis noticed the way the layers of rock changed from clay to limestone and back again. The pattern of beds created seemed so regular that it must have an explanation.

Analysis of these rocks has shown that the change in environment from a clay-rich sea to a limestone-producing sea happened on a roughly 41,000 year cycle. This correlates with the obliquity of orbit predicted by Milankovitch cycles.

Attempts to identify a 100,000- year cycle have been met with scepticism from some experts, but work carried out on clay found at Kimmeridge bay in Dorset on Upper Jurassic rocks has identified regular Milankovitch cycles.

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