Activating water vapour

I'm a bit bored.  WUWT is struggling at the moment so I'll comment on a comment on WUWT.

JPeden says, referring to the silly video that science deniers paid far too much for (archived here):

September 2, 2013 at 7:45 am

Great presentation so far – I’m about half way through. Money well spent!

But on feedbacks, according to the mechanism presumed by “mainstream” Climate Science, I still have no idea why water vapor didn’t already do completely by itself what the “mainstream” Climate Scientists say it will do to assist CO2. It makes absolutely no sense to me that water vapor would need any CO2 at all to do the same thing Climate Scientists say it will do when ‘activated ‘by CO2. Or at the least, why would water vapor not become already activated by a much smaller CO2 concentration? It sounds like Climate Science says both that water vapor is and is not a ghg……………

JPeden thinks that scientists say that water is "activated" by CO2.  It's an interesting way of looking at it I suppose.

In simple terms, what JPeden is missing is that CO2, being a greenhouse gas, slows the amount radiation leaving earth.  When it's constant that's fine.  As long as CO2 and other greenhouse gases stay fairly constant they keep the earth nice and warm so life can thrive.  When CO2 increases the temperature goes up.  That means that more water evaporates.  The properties of water don't change.  It's just that there's more of it in the atmosphere.  The extra CO2 plus the extra water vapour means the amount of radiation leaving earth is even less.  As long as we're adding more CO2, earth will keep heating up and more water will evaporate so earth will heat up more and so on it goes - until we stop adding more CO2 to the atmosphere than leaves it.

Goodness knows when that will be.  I hope it's before earth gets too hot to handle.

Fortunately, water also leaves the atmosphere fairly readily.  It falls as rain, snow and ice.  Otherwise earth would get really, really hot very quickly.  As it is it's getting hot much too quickly for our good.

The Bureau of Meteorology has a booklet about the greenhouse effect and climate change.

PS I know my readers know all this and many of you know a heck of a lot more than I do about climate.  Occasionally there are strays who land here by accident or design, who might be just starting to learn about climate change.  This post is for them.