Wednesday, 1 August 2012

Stability, Feedback and Tipping Points - (Part 2)


In positive feedback, a change in one parameter causes other changes which then amplify the magnitude of the original change.

Example:  Arctic Ice Loss.  Global temperatures have risen but temperatures in the Arctic have increased much more rapidly over the past century.  This has resulted in melting of the ice cover in the Arctic circle.

Change (increase) in the one of the climate parameters (temperature) has caused change of a second parameter ( area covered by ice in the Arctic).

Ice is highly reflective.  Fresh snow sends back about 80% of Sun's energy falling on it, while ice is ~50% reflective. 
Melting converts the ice to darker water that reflects less than 10 percent of solar energy and absorbs the rest.
Greater absorption of solar energy increases the temperature which then causes more ice to melt...
A positive feedback cycle operates...

Another example of positive feedback is thawing of the permafrost - frozen earth in Siberia, Alaska and Canada. 
http://sitemaker.umich.edu/section2_group1/arctic_issues__permafrost
secondfeedback.jpg

Positive feedback causes a drift away from the original stable situation. 
The drift may continue and become uncontrollable. 
Alernately, due to changes happening, other internal mechanisms of the system may come in and pull it back towards stability.
If the drift due to positive feedback continues then we can reach a tipping point - we shall discuss this in Part 3
   
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