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Gavin's proclamation that the earth's sensitivity lies around 3.3 degrees C, discussion about that figure is far from over.

Err, no ... what he actually wrote was (reply to comment #1 at link to RealClimate):

We can even go one step further - what climate sensitivity would have given a perfect forecast given the actual (as opposed to projected) forcings? The answer is 3.3 deg C for a doubling of CO2.

So the 3.3 deg C is a retrospective determination of the value that would have (does) give the best agreement between the 1988 model and the current observations. As such, discussion about that precise figure is over. It is a cold, hard mathematical fact (unless he made an error in running the calculation). Even future measurements do not change this value, because it is specifically an evaluation of an old model against the data available as of early 2011.

A more appropriate comment regarding current opinion on the value for climate sensitivity is in the body of the post:

... and the old GISS model had a climate sensitivity that was a little higher (4.2ºC for a doubling of CO2) than the best estimate (~3ºC).

"best estimate" and "~" acknowledge uncertainty and do not indicate a belief that discussion is over.

by det on Tue Feb 8th, 2011 at 03:42:16 AM EST
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What is visible in the graphs are trendlines, which are converted into CO2 sensitivity by a calculation that is not motivated.

The big hangup about climate sensitivity has never been much about the  sensitivity of purely CO2 - which is something that can be calculated and measured in lab conditions (it's about 1 degree for doubled CO2). The issue always has been about the effects of feedbacks, positive and negative, to increasing atmospheric CO2, in addition to the higher forcing of increased CO2.

Hansen, and Schmidt, assign certain weights to certain forcings based on certain estimates and certain model runs to explain the observed trendline, thereby finding a certain sensitivity.

There's mathematics involved, but it is far from being all hard.

by Nomad (Bjinse) on Tue Feb 8th, 2011 at 01:53:28 PM EST
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