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Wednesday, July 27, 2016

The water vapour penny precipitates, almost, for Willis Eschenbach at WUWT

Sou | 8:37 PM Go to the first of 7 comments. Add a comment

There's a new article at WUWT by Willis Eschenbach (archived here, latest here). It took him almost twenty years, but he's finally found the RSS total precipitable water (TPW) record. For years Willis has been arguing that Earth's climate can't change much because of thunderstorms. It seems that he may have finally woken up to the fact that there is more water vapour in a warmer atmosphere than in a cooler one and that this is contributing to the greenhouse effect. He wrote (about the dataset that was reported 20 years ago in 1996): "One of my great pleasures is to come across a new dataset."

I've been meaning to write about global warming, water vapour and precipitation for some time. There have been several papers on the subject (see below). What's happening is:
  • the amount of water in the air is increasing as the world warms,
  • the water cycle is intensifying,
  • therefore there is more rain (and snow), and
  • more greenhouse warming because water vapour is a strong greenhouse gas.

Warning: This is a rather long article, with no apologies :)

Water in the atmosphere is increasing - and so is rain

The amount of water in the atmosphere is increasing as we heat up our world. It has been estimated that it will increase at a rate of 7% per degree Celsius of surface warming. That's the total water in the air troposphere not just vapour.

Here is an introduction to the topic from RSS (my emphasis):
Over 99% of the atmospheric moisture is in the form of water vapor, and this vapor is the principal source of the atmospheric energy that drives the development of weather systems on short time scales and influences the climate on longer time scales.

Water vapor is a critical component of Earth's climate systems.  It is the Earth's primary greenhouse gas, trapping more heat than carbon dioxide.  Movement of water vapor, and its associated latent heat of vaporization, is also responsible for about 50% of the transport of heat from the tropics to the poles.  The movement of water vapor is also important for determining the amount of precipitation a region receives.

Total column water vapor is a measure of the total gaseous water contained in a vertical column of atmosphere. It is quite different from the more familiar relative humidity, which is the amount of water vapor in air relative to the amount of water vapor the air is capable of holding. Atmospheric water vapor is the absolute amount of water dissolved in air.  When measured in linear units (millimeters, mm), it is the height (or depth) the water would occupy if the vapor were condensed into liquid and spread evenly across the column. Using the density of water, we can also report water vapor in kg/m2 = 1 mm or g/cm2 = 10 mm.

How much more water is in the air?

There is atmospheric water data available on the RSS website. Below is a chart showing the water anomaly over time, using data from RSS. The data are only for the oceans, and between the latitude bands as shown on the chart - tropics only, as well as almost global. I can say "almost" global for the 60°s-60°N band, because the air is quite dry at the poles. The data are reported in kg/m2 and plotted as an anomaly from the average of 1988 to 2007.
Figure 1 | Precipitable water anomaly for the period January 1988 to June 2016 for the tropics and (almost) global. The base period is 1988-2007. Data source: RSS June 2016.

As you can see in Figure 1, precipitable water is increasing over time. The data also show that the tropics are more variable than the globe overall. Below is an image from RSS showing the trends in total precipitable water (mm/decade) over the oceans over the period 1988 to 2015. Click to enlarge it:

Figure 2 | Trend in total precipitable water over the ocean. RSS Version-7 microwave radiometer. Source: RSS

There was a paper published last year in J Climate, by Thomas M. Smith and Phillip A. Arkin. They wrote about historical changes in total precipitable water going way back to the nineteenth century. I don't have the full paper (anyone?). [Dr Smith has just sent me a copy - which is much appreciated.] The abstract states (my emphasis):
A global mode indicates multidecadal increases in TPW since the nineteenth century, with strongest increases in the tropics and adjacent to land monsoon regions. Strongest multidecadal changes in the global mode are 1910–40 and since 1980. An ENSO mode for the extended period indicates a trend since the 1980s, opposite to the tendency in the global mode. There is no apparent multidecadal variation in the ENSO mode before 1980, suggesting that its multidecadal relationship with the global mode can change. Analysis of SST over the same period shows climate modes consistent with the TPW modes, and for the satellite period there are consistent variations in the satellite data, showing the strong link between SST and oceanic TPW.
The chart below is from the paper, and shows the change in total precipitable water since about 1870. The data are derived from different sources - an extended dynamic reanalysis (R20C) and a reconstruction based on historical sea surface temperature, with RSS data for recent years.

Figure 3 | Global ocean area annual average TPW anomalies from the superensemble (merged, solid black) and the indicated individual historical analyses fR[WSST] (dashed black) and R20C (dashed red)g. The satellite-based RSS analysis is also shown for comparison (dashed green). Units are kilograms per meter squared. Source: Smith & Arkin 2015

The water cycle is intensifying with global warming

Nine years ago, Frank J. Wentz, Lucrezia Ricciardulli, Kyle Hilburn, Carl Mears wrote a paper in Science where they demonstrated that the intensity of the water cycle has been increasing. They worked out from observations that the hydrological cycle is increasing at a rate of about 6% per degree Celsius.

This finding was different to what was predicted by the then available global coupled ocean-atmosphere models. The authors wrote:
More than 99% of the total moisture in the atmosphere is in the form of water vapor. The large increase in water is due to the warmer air being able to hold more water vapor, as dictated by the Clausius-Clapeyron (C-C) relation under the condition that the relative humidity in the lower troposphere stays constant. So according to the current set of global coupled ocean-atmosphere models (GCMs), the rate of increase in precipitation will be several times lower than that for total water. This apparent inconsistency is resolved in the models by a reduction in the vapor mass flux, particularly with respect to the Walker circulation, which reinforces the trade winds (3, 7). Whether a decrease in global winds is a necessary consequence of global warming is a complex question that is yet to be resolved (8).
The authors explained the analysis very clearly. I won't go into all the detail here (you can read it for yourself). The bottom line is that if the rate of increase in precipitation is going to be lower than the rate of increase in atmospheric water vapour, something has to give. That is, according to the equation below (from the supplement). This is equation S1 referenced below:

 Equation S1

where κ is von Karman’s constant, ρ is the surface air density, τ0 is surface wind stress, q0 and qz are the specific humidities at the surface and at height z. The terms z0 and ψ are the roughness length and profile for latent heat flux.

The thing that would have to give is that there would need to be a decrease in global winds. However that's not what the scientists found. For those who want a more detailed explanation, the authors explain (my paras and emphasis):
The GCMs [global coupled ocean-atmosphere models] Global indicate that E [evaporation] should increase about 1 to 3% K–1 of surface warming. However, according to the bulk formula (eq. S1) (8), evaporation increases similarly to C-C [Clausius-Clapeyron] as the surface temperature warms, assuming that the other terms remain constant. For example, a global increase of 1 K in the surface air temperature produces a 5.7% increase in E (8). To obtain the muted response of 1 to 3% K–1, other variables in the bulk formula need to change with time.
The air-sea temperature difference and the near-surface relative humidity are expected to remain nearly constant (8), and this leaves τ0 [surface wind stress] as the one variable that can reduce evaporation to the magnitude required to balance the radiation budget in the models. To bring the bulk formula into agreement with the radiative cooling constraint, Math would need to decrease by about 4% K–1. Thus, a muted response of precipitation to global warming requires a decrease in global winds (2, 3, 7).
When the authors looked at observations (19 years of wind data), they found the opposite to what the models indicated. They found that there was a 1%/decade increase in wind over all oceans, not a decrease. (You might recall the 2014 paper by Matthew England and co about an observed intensification of trade winds over the Pacific.)

There is a lot more detail in the paper. For example, the authors looked at what happened in the tropics as well as what happened overall. The bottom line was that they found that the hydrological cycle accelerated at a rate of about 6% per degree Celsius, and ocean winds increased at around 1% a decade.

The authors left some questions on the table, including the explanation for the difference between the GCMs and observational data (for which they offered some suggestions).

Intensified water cycle as shown by changes in ocean salinity

There have been other papers on the subject, too. For example, in 2012, Paul J. Durack and colleagues had a paper in Science, in which they looked at ocean salinity patterns over the second half of last century (1950-2000) and found that the water cycle has intensified. The rate of intensification they found was slightly higher than the Wentz team (but within the margin of error). They found the rate was 8 ± 5% per degree of surface warming - again considerably higher than GCMs at the time. (There's also a slide presentation with lots of figures.)

Observed vs modeled water vapour

In 2007, Ben Santer and colleagues had a fingerprint study on water vapour published in PNAS. They reported observations from the satellite-based microwave sensor SSM/I, and modeled water vapour. The paper includes the figure below (click to enlarge it). Note that the chart A is for latitudes 50°N–50°S:

Figure 4 | Anomaly time series of monthly mean 〈W o〉, the spatial average of total atmospheric moisture over near-global (50°N–50°S) oceans (A) and an estimate of the stratospheric aerosol optical depth (SAOD) (21) (B). Observations of 〈W o〉 are from the SSM/I data set (20). Simulated 〈W o〉 data are from 49 realizations of 20th century climate change that included combined anthropogenic and natural external forcing (ALL), performed with 12 different models. Vertical lines denote the times of maximum SAOD after major volcanic eruptions. Source: Santer07

In the conclusions, the authors wrote:
These findings, together with related work on continental-scale river runoff (15), zonal mean rainfall (16), and surface specific humidity (17), suggest that there is an emerging anthropogenic signal in both the moisture content of earth's atmosphere and in the cycling of moisture between atmosphere, land, and ocean. Detection and attribution studies have now moved beyond “temperature-only” analyses and show physical consistency between observed and simulated temperature, moisture, and circulation changes. This internal consistency underscores the reality of human effects on climate.
That was almost ten years ago now. Since then there have been other studies such as the ones I mentioned above. (There have been more recent studies, too. However I'm yet to get access to some of the full papers.)

What the models suggested

An earlier paper on the subject was by Isaac Held and Brian Soden, which was published in the AMS Journal of Climate back in 2006.  Their paper, as you'd probably expect, was about modeled changes. They wrote:
We are confident that lower-tropospheric water vapor will increase as the climate warms. We can predict, with nearly as much confidence, that certain other changes will occur that are coupled to this increase in water vapor. In this article we describe some of these robust hydrological responses to warming.
The Held and Soden paper might be of interest in that it puts forward rationale for the results from the GCM's. I don't know if the more recent papers I described above, which on the face of it aren't in line with models (observations show a more vigorous hydrological cycle with warming), have resulted any changes in the models themselves.

ENSO and rain

I'd best point out that Willis' analysis was only about total precipitable water over the oceans, not over the land surfaces. He did some arithmetic trying to work out what the data meant in terms of watts/m2 - I didn't bother with that but I suspect he got it wrong. Below is one of his charts. It shows the data for total precipitable water over the ocean areas without ice:

Figure 5 | Total precipitable water. I've added pink (El Nino) and blue (La Nina) bars to show the timing of ENSO events. Source: Willis Eschenbach at WUWT and BoM.

One thing he commented on was the impact of ENSO events. He wrote:
Some things of interest. First, in the bottom panel you can see the effect on TPW of the El Nino episodes in 1997/98, 2010/11,  and 2015/16. You can also see that we haven’t quite recovered from the most recent episode.
Willis spoke about El Nino, but I don't know if he understood what he was seeing, going by what he has written previously. During El Nino water falls more on the oceans. In La Nina's, water is shifted from the oceans to the land surfaces particularly in the early stages of La Nina. You can see a hint of that in Willis' chart above, which is of total precipitable water over the oceans.

You might recall the dramatic fall in sea level with the 2010/11 La Nina, when huge amounts of water was shifted from the seas to Australia and South America in particular. Carmen Boening, Josh K. Willis, Felix W. Landerer, R. Steven Nerem and John Fasullo wrote a paper about that for GRL. As stated in the abstract:
Global mean sea level (GMSL) dropped by 5 mm between the beginning of 2010 and mid 2011. This drop occurred despite the background rate of rise, 3 mm per year, which dominates most of the 18-year record observed by satellite altimeters. Using a combination of satellite and in situ data, we show that the decline in ocean mass, which explains the sea level drop, coincides with an equivalent increase in terrestrial water storage, primarily over Australia, northern South America, and Southeast Asia. This temporary shift of water from the ocean to land is closely related to the transition from El Niño conditions in 2009/10 to a strong 2010/11 La Niña, which affected precipitation patterns world-wide.
Here's a chart of detrended global mean sea level with the multivariate ENSO index, provided by the U Colorado Sea Level Research Group. The seasonal signals have been removed from the 60-day smoothed sea level plot:

Figure 6 | Global mean sea level and ENSO. The Multivariate ENSO Index (MEI) is the unrotated, first principal component of six observables measured over the tropical Pacific (see NOAA ESRL MEI, Wolter & Timlin, 1993,1998).  To compare the global mean sea level to the MEI time series, we removed the mean, linear trend, and seasonal signals from the 60-day smoothed global mean sea level estimates and normalized each time series by its standard deviation. The normalized values plotted above show a strong correlation between the global mean sea level and the MEI, with the global mean sea level often lagging changes in the MEI. Source: U Colorado Sea Level Research Group

The sea level is highest during El Nino (positive MEI) and drops most in a La Nina (negative MEI). Since water doesn't stay in the air very long (a matter of hours to days), it means that the water must shift from the oceans to the land surfaces when MEI goes strongly negative. That's what happened with a vengeance in the Big Wet of 2010/11. (It could be happening again now, with all the major flooding around the world these past few weeks.)

In 2007 there was another paper in J Climate, by Guojun Gu of GISS NASA and co. They looked at monthly data from the 27 year (1979–2005) Global Precipitation Climatology Project (GPCP) dataset. In that dataset they didn't find a change in the linear precipitation trend globally, although they did see an increase in tropical rainfall (0.06 mm day−1 decade−1), and a weaker decrease in the mid-latitudes of the Northern Hemisphere. The authors describe the dataset:
The monthly precipitation dataset from the Global Precipitation Climatology Project (GPCP) is a community-based analysis of global precipitation under the auspices of the World Climate Research Program (WCRP) from 1979 to the present. On a global 2.5° × 2.5° grid, the data are combined from various information sources: microwave-based estimates from the Special Sensor Microwave Imager (SSM/I), infrared (IR) rainfall estimates from geostationary and polar-orbiting satellites, and surface rain gauges.
Although the authors didn't find a change in the global rainfall, they did find changes in the tropics, including with ENSO events. They found the same as Figure 6 suggests, with La Ninas having more water shifting from the ocean to the land, while during El Ninos it didn't (more rain fell over the oceans). They wrote:
Generally, the warm events correspond to more (less) rainfall over ocean (land) with the opposite variation for cold events. Strong variabilities can be seen in tropical total rainfall, but are weaker than rainfall variations over land and ocean separately. This is due to the opposite effects of ENSO on rainfall over land and ocean (e.g., Neelin et al. 2003). 
The researchers also attempted to distinguish between the impact of volcanic eruptions and the impact of ENSO events. This was tricky, because the two volcanic eruptions in the period occurred with El Niño events. What they found was that whereas ENSO events mainly affected where the rain fell (over land or ocean) rather than the amount of precipitation, volcanic eruptions suppressed rainfall.

Where is the warming? It's in the air, on the surface and in the oceans

Now Willis doesn't understand the thermodynamics of the planet. He doesn't realise that the surface and the ocean are part of the system. He tried to convert the increase in precipitable water to w/m-2, and toward the end he wrote:
This leads us to a curious position where we have had a larger change in forcing from water vapor since 1988 than from all the other IPCC-listed forcings since 1750 … so where is the corresponding warming?
Is he not aware that the world has warmed since 1988? Here is a chart of the average of 12 months to June each year. The 12 months to June 2016 averaged 1.01 °C above the 1951-1980 mean and was 0.6 °C hotter than the 12 month period to June 1988:

Figure 7 | Global mean surface temperature anomaly for the 12 months to June each year. The base period is 1951-1980. Data source: GISS NASA

In that time, the oceans have absorbed a huge amount of heat as well.

Figure 8 | Global ocean heat content 0 to 2000 metres. Data source: NOAA

The take-away messages from all this are: a) that water vapour is increasing as the world warms and b), because water vapour is a strong greenhouse gas, it causes the world to warm more.

From the WUWT comments

Bloke down the pub is probably wrong about his friends pointing to Willis' article. However he's right in that an increase in water vapour is a positive feedback:
July 25, 2016 at 11:45 am
I suspect some of our alarmist friends will point to this and say ‘there’s the positive feedback that we said would happen’.

Willis Eschenbach  says he thinks that the surface should have warmed. He doesn't know that it has, and he forgets about the oceans.
July 25, 2016 at 11:51 am
But it’s not positive feedback, as it has been happening during the “pause” in warming …
And in either case my question remains—where is the projected warming claimed to result from the 3.3 W/m2 increase in water vapor forcing?

Jan Kjetil Andersen commented how Willis is starting to see how more water vapour is a positive feedback to greenhouse warming:
July 25, 2016 at 12:10 pm
Great article Willis
You have demonstrated and quantified the positive feedback mechanism of water vapor here.
Some warming produce more water vapor, which produce some additional warming, and so on. Not enough to get a runaway process, but enough to give a positive feedback, which confirm one fundamental element in the IPCC models.

george e. smith shouts his denial, and wrongly claims that water vapour is a negative feedback. He thinks the increased temperature from more water vapour is more than offset by an increase in clouds, but only those that reflect sunlight more than retain heat in the air (ie more low clouds than high clouds). He provides no evidence. Nor does he explain how the world keeps getting hotter.
July 25, 2016 at 1:38 pm
NO ! More water vapor equals more clouds, and less sunlight reaching the surface.
So it is a definite negative feedback. It is the principal regulator of earth’s Temperatures.

TomB wonders if any scientists have published anything about total precipitable water. I don't know where he thinks Willis got his data from.
July 25, 2016 at 1:12 pm
A fascinating analysis and correlation. Much more relevant than the number of pirates. Any idea if this idea has been explored in scientific literature?

References and further reading

Smith, Thomas M., and Phillip A. Arkin. "Improved Historical Analysis of Oceanic Total Precipitable Water*." Journal of Climate 28, no. 8 (2015): 3099-3121. DOI:

Wentz, Frank J. "A well‐calibrated ocean algorithm for special sensor microwave/imager." Journal of Geophysical Research: Oceans 102, no. C4 (1997): 8703-8718. DOI: 10.1029/96JC01751 (open access)

Boening, Carmen, Josh K. Willis, Felix W. Landerer, R. Steven Nerem, and John Fasullo. "The 2011 La Niña: So strong, the oceans fell." Geophysical Research Letters 39, no. 19 (2012). doi:10.1029/2012GL053055 (open access)

Gu, Guojun, Robert F. Adler, George J. Huffman, and Scott Curtis. "Tropical rainfall variability on interannual-to-interdecadal and longer time scales derived from the GPCP monthly product." Journal of Climate 20, no. 15 (2007): 4033-4046. DOI: (open access)

Santer, Benjamin D., C. Mears, F. J. Wentz, K. E. Taylor, P. J. Gleckler, T. M. L. Wigley, T. P. Barnett et al. "Identification of human-induced changes in atmospheric moisture content." Proceedings of the National Academy of Sciences 104, no. 39 (2007): 15248-15253. doi: 10.1073/pnas.0702872104 (open access)

Wentz, Frank J., Lucrezia Ricciardulli, Kyle Hilburn, and Carl Mears. "How much more rain will global warming bring?" Science 317, no. 5835 (2007): 233-235. DOI: 10.1126/science.1140746 (pdf here)

Durack, Paul J., Susan E. Wijffels, and Richard J. Matear. "Ocean salinities reveal strong global water cycle intensification during 1950 to 2000." Science 336, no. 6080 (2012): 455-458. DOI: 10.1126/science.1212222 (pdf here, presentation here)

Held, Isaac M., and Brian J. Soden. "Robust responses of the hydrological cycle to global warming." Journal of Climate 19, no. 21 (2006): 5686-5699. DOI: (open access)

England, Matthew H., Shayne McGregor, Paul Spence, Gerald A. Meehl, Axel Timmermann, Wenju Cai, Alex Sen Gupta, Michael J. McPhaden, Ariaan Purich, and Agus Santoso. "Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus." Nature Climate Change 4, no. 3 (2014): 222-227. doi:10.1038/nclimate2106

Explaining how the water vapor greenhouse effect works - articles at

Remote Sensing Systems (RSS) and NASA GHRC articles and data

From the HotWhopper archives


  1. Willis has never had anything interesting to say, and wouldn't waste my time on him. He has no background in science and so is unable to understand basic Arrhenius-type of laws.

    More to the point is to go after the "model torturers" such as Richard Lindzen, who have delayed progress in atmospheric science by decades with their misguided theories.

  2. Well I thought so ...
    IPCC AR5 WG1 Chapter 8
    FAQ 8.1 | How Important Is Water Vapour to Climate Change? (p. 666

    WE doesn't understand the difference between external forcing and internal or "emergent" properties of AOGCM's. Simple as that.

    "The maximum amount of water vapour in the air is controlled by temperature. A typical column of air extending from the surface to the stratosphere in polar regions may contain only a few kilograms of water vapour per square metre, while a similar column of air in the tropics may contain up to 70 kg. With every extra degree of air temperature, the atmosphere can retain around 7% more water vapour (see upper-left insert in the FAQ 8.1, Figure 1). This increase in concentration amplifies the greenhouse effect, and therefore leads to more warming. This process, referred to as the water vapour feedback, is well understood and quantified. It occurs in all models used to estimate climate change, where its strength is consistent with observations. Although an increase in atmospheric water vapour has been observed, this change is recognized as a climate feedback (from increased atmospheric temperature) and should not be interpreted as a radiative forcing from anthropogenic emissions."

    1. Somewhat akin to saying the
      steam inside a campoven is caused by temperature, not
      the campfire?

    2. Good one. That analogy would make Willis' head splode due to excessive wonderin

    3. tbf - probably a combination of temperature and Force X

  3. Sou

    What an excellent article. Thank you. Bookmarked.

    A tiny typo:

    (You might recall the 2014 paper by Matthew English and co about an observed intensification of trade winds over the Pacific.)

    Matthew England.

    1. Thanks BBD. I suspect Prof England wouldn't think it was a *tiny* typo :( (I did proof read it but missed that. Fixed now, thanks again.)


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