warmair,

No one's doubting that the natural level of CO2 has a significant effect on the earth's temperature. The issue is whether the increase in CO2 has a significant effect.

Your analogy about painting windows is a little simplistic, but adequate for this purpose.

So you paint out all the windows. Fully agree that that would cause an increase in the houses internal temps. Let's call that the natural level of CO2.

Now paint the windows again. Does that have a significant effect on the temps. Does the temp rise by 50% of the rise from the first two temps.

Let's say the temp does still rise by some amount. Now paint them again...does the temp rise again and does it rise by the same amount as with the third coat. And so on....

Mhaze,

The issue is how thick or effective the initial layer of paint is. It is easy to envisage a situation where the initial cover reduces the amount of radiation passing through a window by say 12.5%. The next coat could reasonably reduce the radiation by a further 12.5%. If at each application, you put on twice as much paint as the last time, it would take 3 applications to completely block all the radiation.

http://www.climatedata.info/Forcing/Emissions/files/BIGw08-greenhouse-effect.gif.gif

CO2 currently represents 0.039% of the atmosphere, It is well established that without the greenhouse effect the global temperature would be some 33 Deg C cooler. It is also been established that water vapour is responsibly for some 60% plus of the warming. Co2 accounts for around 29% of the remaining heating effect.

http://www.climatedata.info/Forcing/Emissions/files/BIG09-percentage.gif.gif

In the end what we know that a further doubling of CO2 will directly cause near surface heating of 0.8 to 1.2 Deg C with a best estimate of 1 deg C, which when we include the effect of water vapour leads a likely figure around 3 deg C warming for a doubling of CO2.

Below is a more rigorous explanation
http://www.physicsforums.com/showthread.php?t=30768

Well yes we could postulate that each coat catches 12.5% but in the real world that doesn't happen. Using your own figures which I'll accept for the sake of argument, CO2 accounts for around 30% of total GHG warming and total GHG warming is around 30 deg C. So CO2 accounts for around 9 deg C. (I know its not that simple but its in that ballpark).

So a doubling of CO2 increases temps by around 1 deg C. So in your painting scenario the second coat is only 10% as effective at increasing temps as the first coat and so on.

"when we include the effect of water vapour leads a likely figure around 3 deg C warming for a doubling of CO2."

Well this is the so-called positive feedback mechanism that I mentioned in a previous thread and which you didn't then understand. All models postulate a +ve feedback which is the only way they can get anything scary out of a doubling of CO2 (ie converting a 1 deg C into a 3 or more deg C increase.) The only problem is that there is next to no evidence for positive feedbacks and it seems that the IPCC in AR5 will be admitting that, not only don't they know anything about the extent of postive feedbacks, they aren't even sure they are positive. Many scientists have suggest that at least some of the feedbacks will be shown to be negative ie offsetting the increase on CO2 warming.

Indeed it may well turn out that the unwarming of the last 2 decades is caused by these postulated negative feedbacks.

The situation is straight forward higher temperatures mean higher levels of water vapour in the atmosphere.

Many studies have confirmed this in theory:-

Cess, 1989;
Elliott, 1995; Zhang et al, 1995
Manabe & Wetherald, 1967
Sohn & Schmetz, 2004
Stephens et al, 2004
Bony et al, 2006
Held & Soden, 2000
McCarthy & Toumi, 2004

The earth's surface is about 70% ocean and and numerous studies show an increase in moisture levels over the oceans. The measured figure is 5.7% per 1 deg C increase in temperature. The increase over all the oceans for the period 1988 to 2004 has been established at 1.2%.

Numerous studies have confirmed the effect is real in practice:-
Robinson, 2000
Wang and Gaffen, 2001
Philipona et al, 2004
Auer et al, 2007
Ishii et al, 2005

All the evidence points to the conclusion that water vapour increases in line with temperature and that the feedback is positive.

On the claim that temperatures have not increased over the last 2 decades, I think you might like to explain the dramatic reduction in the volume and area of the Arctic ice sheet.

http://nsidc.org/arcticseaicenews/files/2000/08/Figure3-350x261.png

I don't doubt that its very likely that a warmer planet means more water vapour all else being equal. But that's not the end of the story. Clouds, at least certain types of clouds, have a negative effect on temps by reflecting sunlight back into space long before it has any heating effect. Svensmark and others are working on ideas that the level of cloud cover is determined by solar activity. He might be right and he might be wrong but its definitely wrong to say we know all there is about such things.

If you are right that higher temps lead to more water vapour which leads to higher temps what causes that cycle to break. We know that temps were higher in the past than now. Why didn't they just keep rising. Why did the Medieval Warm Period stop?

Could it be that more water vapour leads to more clouds which leads to lower temps? We and they don't know. But they are prepared to uproot society despite this ignorance of the whole story.

"On the claim that temperatures have not increased over the last 2 decades,". Its not a claim. Its fact..or at least fact if the temp records are right. Even the IPCC and the CRU now admit it.

As to the arctic...we have records going back to 1979 only. But there is very good data that this decline in sea ice occurred in the past and even in the recent past eg around 1850. And if the rising temps are causing the arctic to melt why aren't they doing the same to the Antarctic?

Just came across this spiral graph on Arctic ice loss.

http://haveland.com/share/arctic-death-spiral-1979-201301.png

West Antarctica?

http://www.realclimate.org/index.php/archives/2012/12/the-heat-is-on-in-west-antarctica/