One surprising aspect of climate science, perhaps more surprising to alarmists than anyone else, is that CO2 on its own doesn’t do a lot of warming in climate models. Instead, it causes a bit of warming that supposedly causes extra water vapour to accumulate in the air and in turn cause a lot of heating. But how do we know extra water vapour should accumulate? It’s the usual “simple physics”. You see, the Clausius-Clapeyron rule says that for every degree warmer the air gets, its potential humidity rises by about 7%. Climate models predict that the air’s water vapour content (called the “specific humidity”) should rise with temperature. Or, more technically, both the specific humidity and the potential maximum humidity (the ratio of which is called “relative humidity”) should remain about constant. But a recent study shows that, especially over the arid and semi-arid regions of Earth, it’s not. As the authors state, “This is contrary to all climate model simulations in which it rises at a rate close to theoretical expectations, even over dry regions. This may indicate a major model misrepresentation of hydroclimate-related processes; models increase water vapor to satisfy the increased atmospheric demand, while this has not happened in reality.” So the simple physics isn’t so simple after all. Again.
The following charts shows the discrepancy over time:
The top panel shows specific humidity in arid/semi-arid regions. The models (blue and purple lines labeled CMIP6, AMIP6, LENS2 and GOGA2) say it should be rising according to the Clausius-Clapeyron rule, but the observations (black line) say it isn’t. In the bottom panel the same outcome is shown in terms of relative humidity (RH). And here the models says RH should be constant while observations show it declining.
The authors point out that they don’t know why RH is declining. Something is preventing water vapour from forming in the expected way, or moving from more humid regions to the arid regions. At the moment scientists don’t know what the problem is, but the authors speculate on some possible reasons, including the possibility that the data are not very good. But they also note that the problem shows up in all models and has a specific pattern, which means it’s probably a flaw in climate models:
“We, therefore, argue that a more likely explanation is that some process is being misrepresented in models... Efforts to distinguish between these and other possibilities must be guided by the fact that the issue is common to all models, the trend discrepancy is most severe in arid/semi-arid regions, and in more humid regions, it is only found in the most arid seasons.”
The authors argue that getting the humidity story wrong means models may give incorrect guidance on things like future wildfire trends. Which is true as far as it goes. But a bigger potential issue is that models may also be getting overall water vapour feedbacks and warming amplification wrong as well.
On the plus side, resolving the humidity problem may help explain why so many climate models over-predict warming. Assuming, of course, that you don’t want the models to over-predict warming to justify radical policy programmes.
The models are hypotheses (not the scientific evidence climateers represent—they aren’t scientists as traditionally defined). When the hypothesis doesn’t match the data, throw out the hypothesis and get a new one—not the other way around. That’s science for all those climateers out there.
The data for annual global average absolute humidity are not very good. Poor spatial coverage between the surface and the satellite heights -- the altitudes with the most water vapor! But data we have show the same as the charts here. AH increasing from 1980 to 2000 but steady from 2000 to 2020 Which means no conclusion is possible. The CC Relation only states the potential increase of water vapor with temperature, not the actual increase. ... Wild guesses about CO2 feedbacks have the estimates of CO x 2 warming ranging from about +0.75 to +5.5 degrees C. ... which obviously means no one knows. At a CO2 rise rate of +2.5 ppm a year, CO2 will double from 420 to 840 ppm in 168 years. Maybe the long term effect of CO2 will be figured out by then? I can't wait.
Richard,I'm not a willing participant in this multi-trillion dollar gamble to try and mitigate supposed negative effects of increasing CO2.Temps have fluctuated wildly even in the relatively recent past,geologically speaking.And the earth was probably at least as warm during the Roman Period and the Middle Ages,as it is today.Besides,the biggest carbon emitters by far,China,India et al are not de-carbonising.Just the opposite.
It seems to me that increased temperature means increased evaporation and increased rainfall which has been noted for the earth's average. Increased humidity would maybe imply that the water stays in the air instead of precipitating out. Since evaporation is about 45W/meter-squared as determined by the average rainfall over the planet, it seems that evaporation might be a negative feedback like a giant heat pump from the oceans to the sky and space which is the ultimate cooling source.
CO2 is a simple molecule compared with H2O, though both have three atoms. This true especially in the context of the climate. Yet, no number for the atmospheric lifetime of the simple CO2molecule is available. A lifetime of 100 years is assumed, as compromise. The real lifetime depends on physical circumstances. The equations are straightforward, but the variables are unmeasured over most of the planet.
So, no one can really say.
In the case of water, more than one feedback process controls absolute humidity. The balance among the processes is not measured at sufficient coverage and spatial resolution, although the basic physics is known. The measurements (e.g. CERES) which do exist show the system to be dynamic, but overall, quite stable. Precipitation is rising slowly over the planet in the period from the end of the little Ice Age, as might be expected in the recovery from a cold period.
The global climate system is working quite well, so let's not try to screw it up, please.
There are a couple of things to consider.
1. Evaporation may be influenced by temperature, but direct sun light plays a vital role. That is not a secret. I have been searching for consistent data on it, but failed so far. Still, with solar intensity being constant, relative humidity should be declining in the face of rising temperatures.
2. We are adding a lot of artificial cirrus clouds to the sky. They not just warm the planet, but also limit the natural water cycle below.
3. If WV feedback is correct in the models is a futile question. There is a far more profound mistake in the assessment of the magnitude of WV feedback, based on pure misinterpretation..
https://greenhousedefect.com/the-holy-grail-of-ecs/the-incredibly-stupid-case-of-water-vapor-feedback