Debunked: Decline in Global Relative Humidity [It's Increasing]

skephu

Senior Member.
I have noticed that Dane Wigington has added a new section to his article criticizing the new study:
This graph was prepared and popularized by the group named "Friends of Science", which seems to be a climate change denialist group. It is known to be an oil-industry funded astroturf group.

The graph is essentially incorrect and shows a false trend. The problems with the graph are discussed in environmental scientist Jim Milks' blog here:
James Taylor versus relative humidity and specific humidity.
Basically, the data used to make the graphs are unhomogenized, i.e. changes of the location and type of the instruments have not been accounted for. This produces a false trend. The data come from the "NCEP Reanalysis" data set, which has been criticized in the literature, see:
Dessler, A. E., and S. M. Davis (2010), Trends in tropospheric humidity from reanalysis systems, J. Geophys. Res., 115, D19127, doi:10.1029/2010JD014192.

In fact the relative humidity in the upper troposphere has been increasing, as confirmed by a number of published studies.
The graph below from Dessler and Davis compares several different reanalyses:
upload_2016-8-19_3-31-46.png
This shows the specific humidity at 300 hPa in the tropics. The solid line is the criticized "NCEP Reanalysis" data set, and this is the only one showing a declining trend. The other lines are other reanalyses, showing a slightly increasing trend.

See also:
Chung, E. S., Soden, B., Sohn, B. J., & Shi, L. (2014). Upper-tropospheric moistening in response to anthropogenic warming. Proceedings of the National Academy of Sciences, 111(32), 11636-11641. http://www.pnas.org/content/111/32/11636.full

Also, from skepticalscience.com:
So, ironically, Dane used climate change denialist propaganda to support his point.
 
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I enquired about this some time ago. According to my tame scientist, "they say" relative humidity is decreasing, and specific humidity is increasing. The increasing upper tropospheric temperature accounts for the discrepancy.. Personally I would not assume tropical or global values to apply to, say, the uk.
 
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skephu

Senior Member.
According to my tame scientist, "they say" relative humidity is decreasing, and specific humidity is increasing. The increasing upper tropospheric temperature accounts for the discrepancy.
Yes, I also thought of that. Sounds plausible. But on the NCEP Reanalyis graph, the specific humidity is also decreasing.
On the other hand, for contrails, it's the relative humidity that matters, so if that is decreasing then it cannot make contrails more persistent.
 
Average values of relative humidity, as such, do not affect persistent contrails. On the other hand persistent contrails can lower average humidity, (by removing water by gravity) but you have to have the contrails to start with.. There is some paper that suggests making larger ice crystals to reduce warming!
 

Mick West

Administrator
Staff member
Average values of relative humidity, as such, do not affect persistent contrails.

Why not exactly? It would seem like higher RH would lead to more ice-supersaturated regions, and hence more frequent persistent contrails. It would also (on average) give more available moisture to the contrail during growth, making larger (and/or more) ice crystals, which would make the contrails thicker and spread more. So on a simple level it would seem to change both the frequency and morphology of persistent contrails.

In a previous thread in 2012 @George B contacted NASA contrail expert Patrick Minnis on this question, and Minnis replied:

 

Trailblazer

Moderator
Staff member
In a previous thread in 2012 @George B contacted NASA contrail expert Patrick Minnis on this question, and Minnis replied:
I have read that global warming is actually linked with lower temperatures in the stratosphere. But, as Minnis points out, the humidity declines sharply above the tropopause, so contrails will still be unlikely. Although having said that, polar stratospheric clouds do seem to have become more common in recent years (but they are at much higher altitudes than contrails).
 
Why not exactly? It would seem like higher RH would lead to more ice-supersaturated regions, and hence more frequent persistent contrails. It would also (on average) give more available moisture to the contrail during growth, making larger (and/or more) ice crystals, which would make the contrails thicker and spread more. So on a simple level it would seem to change both the frequency and morphology of persistent contrails.

In a previous thread in 2012 @George B contacted NASA contrail expert Patrick Minnis on this question, and Minnis replied:


I can be a bit nit-picky sometimes. The persistence of contrails depends only on local conditions. They don't care about gobal average conditions. You cannot assume that the distibution of RH is unchanging, and not affected by the distibution of aircraft, for instance.
 
I have read that global warming is actually linked with lower temperatures in the stratosphere. But, as Minnis points out, the humidity declines sharply above the tropopause, so contrails will still be unlikely. Although having said that, polar stratospheric clouds do seem to have become more common in recent years (but they are at much higher altitudes than contrails).

Good point. I will have to check my facts! Vertical convection is driven by the greenhouse effect, so the height of the tropopause will increase according to herd science, but the temperature at a given height in the troposphere?
 
Good point. I will have to check my facts! Vertical convection is driven by the greenhouse effect, so the height of the tropopause will increase according to herd science, but the temperature at a given height in the troposphere?

The information I was given was based on the IPCC report:


 
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