Rstuart - that is the nub of the problem. They have a very poor understanding of exactly what drives the water cycle, which is exactly why the models are garbage at the moment.

However, you can infer from the fact that the planet has never over-heated from a runaway water vapour effect(which is what some of the more extreme AGW hysterics are effectively postulating), that it's not going to happen this time either.

Spencer and Christie have found that the clouds are not forming in the ways predicted, and there is very recent CSIRO research showing that relative humidity is not increasing.

So I'm quite comfortable with the potential rise in temperature driven by CO2 alone - it's well within natural variability and is occurring more slowly than in previous times. Would be good if it could move us to a temperature that would make ice ages less likely.

rstewart

I must respond to Graham Young's comments - it is easy to misinterpret or distort the science, intentionally or otherwise.

For what it’s worth (reproduced here) the water vapour feedback has long been expected to strongly amplify climate changes because of the expectation that the atmosphere's relative humidity would remain roughly constant. Ergo, specific humidity would increase at the rate of the equilibrium vapour pressure, which rises rapidly with temperature. However, observational evidence has been harder to come by and the effect has been controversial (much to Graham’s and his ilk’s delight). Much of that controversy can be laid to rest, thanks to new observations.

Ten/twenty years ago there was little observational or theoretical understanding of atmospheric humidity and how it varied with global climate. As a result, debate raged over whether the water vapour feedback would really occur, with some very influential proposals (championed by Dick Lindzen) that it would not. In particular, many believed that atmospheric humidity and the water vapour feedback were controlled by cloud dynamics and microphysical processes that are not sufficiently well understood and inadequately represented in climate models (this time championed Graham’s heroes, Spencer and Christie).

Successive reports from the IPCC have suggested increasing confidence in our understanding of the water vapour feedback, but they have remained cautious in defending its magnitude. However, recent advances in computing power and satellite data gathering have placed the traditional view of the (positive) water vapour feedback on a stronger footing than is widely appreciated, something that Graham cannot seem to grasp – he is an amateur, after all.

The water vapour feedback mainly results from changes in humidity in the tropical upper troposphere, where temperatures are far below that of the surface and the vapour is above most of the cloud cover. The distribution of humidity in this region is well reproduced by "large-scale control" models, in which air leaves stormy regions in a saturated condition, but with negligible ice or liquid content.

Cont’d

Cont’d

Water vapour is then transported by the large-scale circulation, which conserves the specific humidity (the ratio of the mass of water vapour to the total mass in a unit volume of air), except during subsequent saturation events, when loss of water occurs instantaneously to prevent super-saturation.

Despite the simplicity of this idea, which entirely neglects detailed microphysics and other small-scale processes, such models accurately reproduce the observed water vapor distribution for the mid and upper troposphere. One recent study (S. C. Sherwood, C. L. Meyer, J. Climate 19, 6278 - 2006) estimated the uncertainty in the water vapour feedback associated with micro-scale process behaviour at less than 5%, as a result of the overwhelming control of humidity by the large-scale wind field.

The water vapour feedback is essentially controlled by the large-scale dynamics and the saturation specific humidity in the outflow of the tropical deep convective systems. Convective outflow temperature should, on average, warm along with the mean atmosphere, thus producing the feedback.

Given these considerations, there are good reasons to expect global climate models to accurately simulate the water vapour feedback: The large-scale wind and temperature fields that mainly control the humidity are explicitly calculated from the basic fluid equations, unlike small-scale processes that must be represented by crude parameterizations.

Although the water vapor feedback is strong in all global climate models, its magnitude varies somewhat due to differences among the models in the amount of upper tropospheric warming (and hence the increase in specific humidity) per unit of surface warming. The spread among models in the water vapour feedback is, however, largely compensated by an opposite spread in the "lapse-rate feedback," a negative feedback that occurs because a warmer atmosphere radiates more power to space, thereby reducing net surface warming.

Cont’d

Cont’d

As a result, the sum of the two feedbacks is insensitive to errors in predicted warming of the upper troposphere, and to quantify the sum accurately, one only needs to know how relative humidity (the ratio of specific humidity to that in a saturated condition) changes as the climate warms. The sum of the feedbacks is also smaller than the water vapour feedback--about half the magnitude--and more consistent among climate models, because no model predicts substantial and systematic changes in relative humidity.

Despite these advances, observational evidence is crucial to determine whether models really capture the important aspects of the water vapour feedback. Such evidence is now available from satellite observations of the response of atmospheric humidity (and its impacts on planetary radiation) to a number of climate variations. Observations during the seasonal cycle, the El Niño cycle, the sudden cooling after the 1991 eruption of Mount Pinatubo, and the gradual warming over recent decades all show atmospheric humidity changing in ways consistent with those predicted by global climate models, implying a strong and positive water vapour feedback. A strong and positive water vapour feedback is also necessary for models to explain the magnitude of past natural climate variations.

Both observations and models suggest that the magnitude of the water vapour feedback is similar to that obtained if the atmosphere held relative humidity constant everywhere. This should not be taken to mean that relative humidity will remain exactly the same everywhere. Regional variations of relative humidity are seen in all observed climate variations and in model simulations of future climate, but have a negligible net impact on the global feedback.

Cont’d

Cont’d

Therefore, although there continues to be some uncertainty about its exact magnitude, the water vapour feedback is virtually certain to be strongly positive, with most evidence supporting a magnitude of 1.5 to 2.0 W/m2/K, sufficient to roughly double the warming that would otherwise occur. To date, observational records are too short to pin down the exact size of the water vapour feedback in response to long-term warming from anthropogenic greenhouse gases.

However, it seems unlikely that the water vapour feedback in response to long-term warming would behave differently from that observed in response to shorter-time scale climate variations. There remain many uncertainties in our simulations of the climate, but evidence for the water vapour feedback, and the large future climate warming it implies, is very strong.

I hope the above posts/comments clears some things up. I am reluctant to engage with the likes of arm-chair wannabe scientists like Graham Young because he is not a sceptic, in the scientific sense anyway. He has adopted and ideological stance based on both his political and religious persuasion – which ‘clouds’ the issue. Besides, it does require time and effort to debunk short, sweet, specious and arcane claims – time of which we can better spend with more important things in our life. However, I do understand OLO is very much a part of Graham’s life – life’s choices I guess.

My choice is now to have a wonderful Christmas break and safe and happy New Year. I wish all else here the same – you too Graham : -)

Well Q&A if you were a real scientist I might have some respect for you, but you're not, so you can copy and paste all you like, it doesn't prove anything.

There is no evidence from the history of earth that there is any sort of a runaway water vapour effect. Otherwise it would have occurred when the temperature was much higher as it has been in the past. That water is a greenhouse gas and modifies climate is non-controversial. But to suggest that it does it to the extent that you claim is. But then, someone who would claim that a warmer atmosphere actually increases cooling of the planet's surface obviously has no understanding of basic physics.

Which is typical of your post which essentially says because large scale convection works more or less as understood, then the models must be right - a complete non sequitur.

A major tactic of alarmists like yourself is to denigrate anyone who asks sensible questions and overcomplicate the issue so that people give up trying to understand it and say "whatever". What you've posted here is a good example.

According to you I apparently always act in bad faith because of my politics and my religion. I think one can gauge your level of intelligence and integrity from that sort of a slur. Particularly given the Christian church's vocal advocacy of your point of view.

I follow this issue from an empirical basis and used to worry about global warming. I don't anymore because all of the real world evidence is that it is not a problem. Perhaps empiricism is an ideology, but if it is, in the real world it is the only one worth following.