Week 7: IPCC Chap 2 Review of Feedback Mechanisms Climate Variations

Feedback Mechanisms A List of Feedback Mechanisms –Water Vapor, Lapse Rate –Snow/Ice Albedo –Cloud –Oceanic –Vegetative The Formalism of Feedbacks in Global Climate Change

Ice Cap Feedback Mechanism ice-freeice retreats ice Initial Increase Q planet is more absorbing of sunlight new equilibrium

Water Vapor Feedback initial increase Q more water vapor in boundary layer means IR must be emitted at a higher altitude, therefore it will be weaker T increases solar 0 solar 1 solar IR 0 IR<IR 0 IR=IR 0 +  IR IR is less so T must increase even more

Lapse Rate Feedback solar IR temp vs z solar IR temp vs z If the lapse rate changes, the change at the surface may be less (or more) than that at the emitting layer top

Feedback Formalism Now increase the solar constant (e.g.): Then First consider the case with no feedbacks For a 1% change:

In the most general case, I and a p will depend on water vapor, Surface ice, etc. We get:

The convention is to rewrite this as G o is called the gain and the feedback factors are the f’s G o is the coefficient if there are no feedbacks, and the f’s increase the response if they are positive and less than unity. If there were no feedbacks other than water vapor, f water turns out to be about 0.5; in other words, it doubles the response.

Feedback Summary Water vapor feedback is largest, f water ~ Ice feedback (positive), f ice, may be of the order of 0.1 The biggest uncertainty is with cloud feedback. The models differ widely in their depiction of cloud feedback