So this is a thread suggested by stewgreen. I'm not sure whether it is worth posting, as it seems certain that the subject has been thrashed out many times. Anyway, here's the history. It all started on the "Here we go again" thread, when one Radical Rodent said

I, too, was a firm believer of the AGW lie, though was puzzled as to how such a small change of a very minor component of the atmosphere could create such havoc.

I responded, somewhat provocatively:

So did no one tell you that 99% of the atmosphere is unaffected by IR? And that therefore the "very minor component" is actually a major component of what is left. Or did they tell you and you didn't understand or accept what you were given?

Quick as a flash, the Rodent shot back

What about water and methane, two “greenhouse gasses” that are more effective than CO2, and exist in far greater quantities? There is the point that, though these gasses can be made to show “greenhouse effects” in the controlled conditions of a laboratory, what evidence is there that they are as effective in the chaos of the open atmosphere conditions?

And I deflected that with:

Your GHE denial may win you respect from the odd few here but my guess is that most have got beyond that. It is so 20th century. You might also polish up your ideas of methane concentrations - CH4 does not "exist in far greater quantities" than CO2. Water vapour varies from much less than CO2 to much, much more, but its overall effect is only somewhat over twice that of CO2 despite there being much more of it on average.

Rodent was ready for that though and polished his greenhouse effect doubting instead of his methane concentrations, with:

since when has showing doubt about an unproven hypothesis been “denial”? Point me to any evidence that can be shown to correlate to any gas showing “greenhouse effect” in the atmosphere, and I might adjust my views.

At this point, from the shadows stepped David S with an astute and polite defense of the Rodent that I really cannot answer:

It is clearly theoretically possible that a change in the concentration of a minor component of the atmosphere could have dramatic effects, and an instinctive suspicion that this is unlikely is merely that, a suspicion. The real question is whether there is any clear evidence that the rise in global temperatures between 1975 and 2000 had a different cause from the almost identical rise in temperatures between 1910 and 1940. The "97% consensus" seems to require that the earlier rise can be explained only by natural variation, but the later one only by greenhouse gases.

and stewgreen appeared from nowhere with bundles of empathy:

- I guess by GHE you mean greenhouse effect
what did @Radical Rodent mean to say about the atmosphere ? he was wrong to say "water and methane, two “greenhouse gasses” that are more effective than CO2, and exist in far greater quantities?"
Employing empathy it seems to me that in a rush he mangled 3 concepts
1. quantity : water is in far greater quantities than CO2, but there is far greater CO2 than methane
2. per molecule effect : It something like CO2 is said to have effect something like 6 times that of water
whereas methane was said to have 21x & now said to have 30x
3. absolute effect : My understanding there is absolutely masses more water vapor, and a much bigger effect , but that it is considered to be not very variable when totalled up over the year ..whereas the total amount of CO2 has gone up by 20% in the last 50 years ..so that's where a lot of people expect the change in effect to come from.
- Is it really the case that for water " its overall effect is only somewhat over twice that of CO2" like you say ?

My reply to David, which was probably too nuanced and maybe too long:

David S, yes there are many puzzles about the temperature history (early 20th C etc) and about current temperatures. Many people study the subject and there are numerous explanations for any aspect. And the degree of certainty we can have about much of what we believe to be true in the past is clearly nowhere near 100%. When I look at the amount of work that goes into constructing global average temperature estimates (HadCRUT, GISS etc) from thousands of sensors - and the estimates still vary somewhat - I have to conclude that estimates of historical temperatures from relatively few measurement are likely not all that accurate. That is why they have error margins. But we have what we have. And we know for sure the characteristics if CO2, Methane, etc, even though some fertile minds can be persuaded to the contrary.

And my reply to stewgreen was on the short side:

The number I quoted was for water vapour, not water (ie. not including clouds) - see Kiehl 1997 (pdf)

And that brings us almost up to date, where stewgreen empathised some more:

@Raff "what did @stewgreen mean by water"
If you apply empathy you'll guess he was referring to "water vapour"
- If you have evidence, show me where in that report or another where your "overall effect is only somewhat over twice that of CO2" figure was used..please open up a new discussion thread and tell us

So in answer, I was quoting the abstract of the paper, where it said:

The authors find that for the clear sky case the contribution due to water vapor to the total longwave radiative forcing is 75 W m−2, while for carbon dioxide it is 32 W m−2. Clouds alter these values...

But clouds are water, not water vapour. I make 75 somewhat over twice 32. In fact it is 2.3 times 32, which I think is consistent with "somewhat". It is quite possible that I have misunderstood the quote, so do feel free to correct me.

Now, put what you 'know' into a model and get it to hindcast the last 10,000 years, 4,000 years, 1,000 years, 500 years and the next 30 and we'll get back to you 30 years from now. If the historical models are really good we might be interested sooner. We are well past the point where disconnected bits of science tell us anything useful.

If however you update your predictions every few years and say 'see it looks just like reality' we will give you a derisive smile and walk away. The secret is not to demonstrate we don't understand how the climate works but to prove you do.

stewgreen, I don't know of other estimates or error margins. There clearly are margins of error, so if the real ratio was 1.5:1, 3:1, 4:1 or 5:1 (water vapour:CO2) instead of 2.3:1 what would that mean to you? How would you evaluate it? What would it change in terms of your (or Radical Rodent's) understanding of whether CO2 can have a significant effect on temperature? Quibbling about the exact number seems fruitless.

Why is 75 "good" in your view, as in "too good to be true"? If they has said 75.1, then I might agree that it was suspect. Clear skies means there are no clouds, doesn't it? Is there something wrong with there being a condition? If you do an experiment do you not set and report the conditions? If not how does anyone know how to interpret your results? And if you want to report the effect of CO2 relative to water vapour, would you not study it in the absence of confounding factors (clouds)?

Your criticisms make little sense to me. They seem just like arguing for the sake of it. If you really think that CO2 does not make a major contribution to the energy balance of Earth (a view that Radical Rodent clearly holds), then say it and give some evidence.

Stewgreen, if researchers want to figure the effect of CO2 vs water vapour, they would be sensible to do it with clear skies. Don't you think? But since you are so interested in real world atmospheres, if the real ratio was 3:1, 5:1 or 10:1 (water vapour and water to CO2) instead of 2.3:1 what would that mean to you? How would you evaluate it? What would it change in terms of your (or Radical Rodent's) understanding of whether CO2 can have a significant effect on temperature?

I am content to say I don't know instead of jumping to any conclusions. It's not that I have certainty of some other predicition, ... it's just that warmists have not justified the certainty they have that future climate patterns will be a negative thing for humanity ie likely catastrophe

You can't decide whether there really is a connection between CO2 and the energy budget until someone proves that future climate patterns will be negative? Er, ... wow!

Radical: "less than 0.04% – in other words, equivalent to less than 4p in £100"

So did no one tell you that 99% of the atmosphere is unaffected by IR? And that therefore your 0.04% is actually a major component of what is left? Wait a minute, we've been here before. You don't learn, do you?

"each molecule would have to be trapping an awful lot of heat" - how much, do you think? Or it could mean that there are a lot of molecules - how many, do you think? And careful with "trapping", some people will get very upset about that. You could be in for a lecture from Martin about mice walking into his shed.

"People should not be afraid to say 'I don't know'"

Of course. If you said, "I don't know whether CO2 acts as a GHG and whether that will result in a temperature rise and by how much", I would be delighted. If you also said, "Because I don't know what effect CO2 will have, I also cannot know what the risks are and whether we should do something about it. Therefore I will defer to people who do know", I would give you a hug.

But that is not what sceptics say. Nobody here or at WUWT "doesn't know". You all just know - there is certainty all around. Climate scientists couch all their work in confidence intervals and conditionals, but in an earlier discusson I could get nobody here to admit that the possibility of something bad happening climate-wise was not zero. You all just know! The earth isn't gaining heat, the climate hasn't changed, the seas haven't changed pH, glaciers are not shrinking, sea levels are not rising, etc, etc - or if they are it's all natural and nothing to do with us. And of course you know there will be no big global social or economic losses unless we do something about CO2, in which case we are stuffed. I'm exaggerating of course, but that is the impression you give.

So, your recommendation for people to say, "I don't know", comes back to you.

Mike Jackson

All the changes you describe have taken place, with organisms moving, adapting or becoming extinct in response.

The current situation is unique in two ways.

1) The rate of change is unprecedented. Organisms which would have adapted or moved before are dying because there is no time .

2) An industrial civilization of 7 billion people, based mostly on coastal plains, has built up during the last few millennia of relative climate stability. This has no experience of rapid adaption to large changes in sea level, rainfall or temperature.
Consider the problems of drought in California and Sao Paulo, or the rise in sea level affecting Florida and New York.

Mike Jackson

Unprecedented?

Read our old friend Marcott et al 2013. By definition this could only be falsified. In years of reading I have not encountered a global temperature rise exceeding 0.8C in 130 years or a global average sea level rise exceeding 3.2mm/year. Perhaps you have examples?

extinction?

Adaption?

Read your history. Research The Mayans, the Anasazi, Easter Island andd Angkor Wat as examples.

Drought?

Read news coverage on their current water supply problems. Also papers such as this .

Sea level rise?

Try this or this .

Mike Jackson

I don't think the drought link worked. Try this . There's a link through to the original paper.

stewgreen, there you are, immediate proof. Mike Jackson, a stalwart of the blog, just knows what has happened to the temperature, climate, glaciers the seas. He just knows it is not anthropegenic. No doubt, no confidence intervals, no possibility that he is wrong. Think Like A Freak you said. Spot on!

Please suggest some published papers from the climate sciences that state without equivocation (doubt, confidence intervals, probabilities etc) that any of the controversial aspects of the science are as rock solid certain as MJ believes "it's not us".