[NOTE to “GHG” vocab list entry]

Re: comparing 2 GHGs – H2O vapour > CO2

Q: Why target CO2 if water vapour causes the largest climate feedback, and is natural? (In Hansen’s experience, a recurring contrarian pique)
A: As to Hansen’s immediately quoted standard rebuttal to the pique above – “… but the amount of water vapour in the air is determined by t” (p 43), I inadvertantly collided with what looks like an implied “by contrast, the amount of CO2 in the air is NOT determined by t”.

quasi-syllogistic practice
Premise I: Liquids and solids + heat = gas (+ x)
Premise II: CO2 and H2O vapour are gases
=> CO2 needs (an increase in) t to become a GHG, as does H2O vapour.

Premise I’: CO2 needs t to become a GHG, as does H2O vapour.
Premise I”: H2O vapour’s turning into snow or rain naturally is a function of t (Hansen, pp 42-43: “…[water] vapor… condense[s] out as water or ice”)
=> The difference that matters is not between H2O vapour’s and CO2’s (natural) becoming a GHG but in their (natural) un-becoming.

working hypothesis
For CO2’s departure (the part that is in excess, compared to the range of what the global ecosystem’s business-as-usual can handle) from the atmosphere to necessitate the active – and prompt – participation of humans – unlike H2O vapour:
The precondition would have to be:
1) either there is no natural mechanism to extract CO2 from the air,
2) or, if such (a) mechanism exist(s), it is/they are not sufficiently active on its/their own to serve as the much needed negative feedback(s). [to help de-throne CO2 from its elevated GHG position, so that an unprecedented (in human history) atmospheric CO2 over-saturation (however “small” the quantity compared to H2O vapour) does not tip the global ecosystem toward reshuffling its energy balance (global warming), chemical balance (poisoning the seas/oceans), bio balance (who/what stays and who/what goes?) etc.]

Q: Whose business-as-usual: humans’ OR the Earth ecosystem’s?
If the above is correct, then it is a toss between us humans continuing business as usual, or allowing the Earth’s ecosystem to continue its business as usual.

A: …no-brainer?!
“Thinking about it” would seem to be necessary if we were not part of said ecosystem. Since the likelihood of what upsets it will upset us, moreover in ways that we not only may not like but may not be able to change either…, shouldn’t the choice – while we still have that chance – be a no-brainer?!

May 13 addition
Oh, a Hansen quote that’s right on target (pp 279-280):
LINDZEN: “Even if all the other greenhouse gases (such as carbon dioxide and methane) were to disappear, we would still be left with over 98 per cent of the current greenhouse effect.” Cato Review, Spring 1992, 87-98. “If all CO2 were removed from the atmosphere, water vapour and clouds would still provide almost all of the present greenhouse effect.” Research and Exploration 9, 1993, 191-200.
HANSEN AND ANDY LACIS: Removing CO2, with water vapour kept fixed, would cool Earth 5-10 [degrees] C; removing CO2 and trace gases with water vapour allowed to respond would remove most of the natural greenhouse effect.

Well, “no GHG effect” would effectively propel the planet’s ecosystem outside of the habitability zone (cf. “habitability” vocab list entry), H2O would “respond” by “condensing out” as… not even water but straightforward ice (?)

Assuming that Prof Hansen and Lacis’s assessment is backed up by computer-modelling, even back-of-the-envelope squiggles, could it be that MIT’s Prof. Richard Lindzen was… “eyeballing it” somewhat, rather than bothering with the math???