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Isaac Held's Blog

24. Arbitrariness in feedback analyses

This post is concerned with arbitrariness in the terminology we use when discussing climate feedbacks.  The choice of terminology has no affect on the underlying physics, but it can, I think, affect the picture we keep in our minds as to what is going on, and can potentially affect the confidence we have in this picture.

In feedback analyses of a climate response to some radiative forcing, we start with a reference response, the response “in the absence of feedbacks”, and then we look at how this reference response is modified by feedbacks.   An electrical circuit analogy often comes to mind, with the reference response analogous to the unambiguous input into a circuit.  But the choice of reference response in our problem is ultimately arbitrary.  The following is closely based on the introductory section of Held and Shell 2012.

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23. Cumulative emissions

Schematic of three different idealized global warming scenarios.  The time period is roughly 1,000 years and each scenario starts with the CO2 increase and warming from the anthropogenic pulse of emission in the 20th and 21st centuries.  On the left, emissions are slowed so that CO2 is maintained at the level reached at the end of this pulse.  In the center, emissions are eliminated at the end of the pulse, resulting in slow decay of CO2.  On the right, CO2 levels are abruptly returned to pre-industrial levels –perfect geoengineering –  a scenario useful for isolating the recalcitrant component of warming discussed  in post #8.


If we stop emitting CO2 at some future time T how would surface temperature evolve over the ensuing decades and centuries — ignoring all other forcing agents?  This question (or closely related questions) has been looked at using a number of models of different kinds,  including Allen et al, 2009, Matthews et al, 2009, Solomon et al, 2009, and Frolicher and Joos, 2010.   These models agree on a simple qualitative result: global mean surface temperatures stay roughly level for as long as a millennium, at the value achieved at the time T at which emissions are discontinued, as illustrated schematically in the middle panels above.

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