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:
Caldiera states above "it is possible that climate change is causing 'contrails' to persist for longer periods than ye used to". Lets examine this statement. For "condensation trails" to increase and last longer there MUST BE more relative humidity in the atmosphere, but is this the case? The global RH graph below could not be more clear, atmospheric RH has been going down for over 60 years.
Content from External Source
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:
What does the full body of evidence tell us about humidity?
To claim that humidity is decreasing requires you ignore a multitude of independent reanalyses that all show increasing humidity. It requires you accept a flawed reanalysis that even its own authors express caution about. It fails to explain how we can have short-term positive feedback and long-term negative feedback. In short, to insist that humidity is decreasing is to neglect the full body of evidence.
Content from External Source
So, ironically, Dane used climate change denialist propaganda to support his point.
 
Last edited by a moderator:
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.
 
Last edited by a moderator:

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:



GB: Can one say that global warming is potentially responsible for \
an increase in persistent contrails and contrail induced cirrus cloud banks?

Minnis: I doubt that slightly higher temperatures due to climate change are responsible for an increase in persistent contrails and induced cirrus.

Higher temperatures in the troposphere would raise the altitude necessary for contrail formation, so that there would likely be slightly fewer contrails rather than more.

GB: If there is more moisture in the high troposphere and lower stratosphere because of warming . . . and the temperature of the upper troposphere is warmer and the stratosphere is colder . . . so can this means .. . more persistent contrails in the stratosphere but fewer in the troposphere. . . ? Seems very negligible to me . . . I have never seen any publication which included the above discussion or speculation about the persistence of contrails based on global warming . . .see research cites below . . .

Minnis: Persistent contrails rarely ever form in the stratosphere because it is too dry. A slight increase in humidity will not be sufficient to change this negligible frequency. More moisture in the upper troposphere could possibly lead to thicker contrails, but not more of them.
Content from External Source
 

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:



GB: Can one say that global warming is potentially responsible for \
an increase in persistent contrails and contrail induced cirrus cloud banks?

Minnis: I doubt that slightly higher temperatures due to climate change are responsible for an increase in persistent contrails and induced cirrus.

Higher temperatures in the troposphere would raise the altitude necessary for contrail formation, so that there would likely be slightly fewer contrails rather than more.

GB: If there is more moisture in the high troposphere and lower stratosphere because of warming . . . and the temperature of the upper troposphere is warmer and the stratosphere is colder . . . so can this means .. . more persistent contrails in the stratosphere but fewer in the troposphere. . . ? Seems very negligible to me . . . I have never seen any publication which included the above discussion or speculation about the persistence of contrails based on global warming . . .see research cites below . . . sometimes.. rarely ever form in the stratosphere because it is too dry. A slight increase in humidity will not be sufficient to change this negligible frequency. More moisture in the upper troposphere could possibly lead to thicker contrails, but not more of them.
Content from External Source

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:


2.5.5.3 Satellite
AR4 reported positive decadal trends in lower and upper tropospheric
water vapour based on satellite observations for the period 1988–2004.
Since AR4, there has been continued evidence for increases in lower
tropospheric water vapour from microwave satellite measurements of
column integrated water vapour over oceans (Santer et al., 2007; Wentz
et al., 2007) and globally from satellite measurements of spectrally
resolved reflected solar radiation (Mieruch et al., 2008). The interannual
variability and longer-term trends in column-integrated water vapour
over oceans are closely tied to changes in SST at the global scale and
interannual anomalies show remarkable agreement with low-level specific
humidity anomalies from HadCRUH (O’Gorman et al., 2012). The
rate of moistening at large spatial scales over oceans is close to that
expected from the Clausius–Clapeyron relation (about 7% per degree
Celsius) with invariant relative humidity (Figure 2.31). Satellite measurements
also indicate that the globally averaged upper tropospheric
relative humidity has changed little over the period 1979–2010 while
the troposphere has warmed, implying an increase in the mean water
vapour mass in the upper troposphere (Shi and Bates, 2011).
Interannual variations in temperature and upper tropospheric water
vapour from IR satellite data are consistent with a constant RH
behavior
at large spatial scales (Dessler et al., 2008; Gettelman and
Fu, 2008; Chung et al., 2010). On decadal time-scales, increased GHG
concentrations reduce clear-sky outgoing long-wave radiation (Allan,
2009; Chung and Soden, 2010), thereby influencing inferred relationships
between moisture and temperature. Using Meteosat IR radiances,
Brogniez et al. (2009) demonstrated that interannual variations in
free tropospheric humidity over subtropical dry regions are heavily
influenced by meridional mixing between the deep tropics and the
extra tropics. Regionally, upper tropospheric humidity changes in the
tropics were shown to relate strongly to the movement of the ITCZ
based upon microwave satellite data (Xavier et al., 2010). Shi and
Bates (2011) found an increase in upper tropospheric humidity over
the equatorial tropics from 1979 to 2008. However there was no significant
trend found in tropical-mean or global-mean averages, indicating
that on these time and space scales the upper troposphere has seen
little change in relative humidity over the past 30 years. While microwave
satellite measurements have become increasingly relied upon for
studies of upper tropospheric humidity, the absence of a homogenized
data set across multiple satellite platforms presents some difficulty in
documenting coherent trends from these records (John et al., 2011).
Content from External Source
 
Last edited by a moderator:
Top