The Cimate Change Poll
Out of context: Reply #41
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- georgesIII0
let see if you get it,
http://en.wikipedia.org/wiki/Nor…North African Climate Cycles have a unique history that can be traced back millions of years. The cyclic climate pattern of the Sahara is characterized by significant shifts in the strength of the North African Monsoon. When the North African Monsoon is at its strongest annual precipitation and subsequent vegetation in the Sahara region increase, resulting in conditions commonly referred to as the "green Sahara". For a relatively weak North African Monsoon the opposite is true, with decreased annual precipitation and less vegetation resulting in a phase of the Sahara climate cycle known as the "desert Sahara".[1]
Variations in the climate of the Sahara region can at the simplest level be attributed to the changes in insolation due to slow shifts in Earth’s orbital parameters. These parameters include the precession of the equinoxes, obliquity, and eccentricity as put forth by Milankovitch theory.[2] The precession of the equinoxes is regarded as the most important orbital parameter in the formation of the "green Sahara" and "desert Sahara" cycle.
... However, shifts in the insolation cycle are too gradual to cause abrupt climate transitions like those seen at the onset and termination of the African Humid Period all on their own. So to account for these rapid shifts in the climate of the Sahara, several nonlinear feedback mechanisms have been proposed. One of the most common sets of nonlinear feedback mechanisms considered, are vegetation-atmosphere interactions.[17] Computer models looking at vegetation-atmosphere interactions and insolation across North Africa have shown the ability to simulate the rapid transitions between "green Sahara" and "desert Sahara" regimes.[1][18] Thus the results from these models suggest the possible existence of a vegetation-insolation threshold, which if reached, allows the Sahara region to rapidly transition from "green Sahara" to "desert Sahara" and vice versa. ...