Explaining how the "greenhouse effect" is calculated. repost Gavin Schmidt Jan, 2006 article

I'm taking the liberty of reposting this detailed description of how real world scientists calculate the greenhouse effect.  It's a follow up to my previous post and my conversation with another character who thinks climate scientists have their calculations all wrong.

Calculating the greenhouse effect

By Gavin Schmidt  (Gavin who? link here) @ 21 January 2006  @  RealClimate.org 

Filed under:  Climate Science  Greenhouse gases 

In another forum (on a planet far, far away), the following quote recently came up:

{And get's regurgitated ad nauseam to this day, thus the need to repost this authoritative and most informative article.}

….the combined effect of these greenhouse gases is to warm Earth’s atmosphere by about 33 ºC, from a chilly -18 ºC in their absence to a pleasant +15 ºC in their presence. 95% (31.35 ºC) of this warming is produced by water vapour, which is far and away the most important greenhouse gas. The other trace gases contribute 5% (1.65 ºC) of the greenhouse warming, amongst which carbon dioxide corresponds to 3.65% (1.19 ºC). The human-caused contribution corresponds to about 3% of the total carbon dioxide in the present atmosphere, the great majority of which is derived from natural sources. Therefore, the probable effect of human-injected carbon dioxide is a miniscule 0.12% of the greenhouse warming, that is a temperature rise of 0.036 ºC. Put another way, 99.88% of the greenhouse effect has nothing to do with carbon dioxide emissions from human activity⁸.

We’ve discussed the magnitude of the greenhouse effect before, but it might be helpful to step through this ‘back-of-the-agenda’ calculation and see what the numbers really give. (Deltoid has also had a go at some of these mis-statements).