Calculating the greenhouse effect

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

….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).

The quote comes from a lecture by an Australian climate ‘contrarian’ and frequent contributor to the southern hemisphere op-ed pages. Where did he get this from? One might assume that reference ‘8’ was a scientific text, but one would assume wrong. It was in fact our old friend at Fox News, who may in turn have picked up his (junk)science from here. It is not clear whether this is the original source, but it’s close enough.

So, starting at the top:

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