rev 12/31/20,2/26/21,8/31/21
Water Vapor vs CO2 for Planet Warming
Dan Pangburn, P.E. (ret), MSME, ASME life member

During at least the time period when water vapor (WV) and carbon dioxide (CO2) have been accurately measured worldwide, 1988-now, and apparently for centuries, WV increase has been responsible for the human contribution to Global Warming with no significant net contribution from CO2 or any other greenhouse gases.

A useful insight to the influence of WV on planet warming can be obtained from understanding why cloudless nights cool faster and farther when absolute water vapor content of the atmosphere is lower; especially when there is no dew or frost. This simple observation demonstrates that water vapor is infrared electromagnetic radiation (IR) active which makes it a so-called greenhouse gas (ghg), thermalization takes place (ghg molecules absorbing radiant heat from the surface or other ghg and sharing the absorbed energy with surrounding molecules), and that the misleadingly named greenhouse effect (GHE) exists i.e. the planet surface is warmer with the presence of WV than it would be without it.

The words ‘water vapor’ can be a bit misleading. WV is a transparent gas. If something is visible, like steam or a cloud, it is not WV but is condensed liquid water droplets or tiny bits of ice.

The term ‘vapor pressure’ has different meanings in different disciplines. In meteorology it means the partial pressure of WV in the atmosphere. In most other disciplines and general use, it means the pressure developed by the liquid as a result of its impetus to change phase and become a gas. This impetus depends only on the temperature of the liquid water. It is unambiguously called saturation vapor pressure. In this document, the saturation vapor pressure is sometimes referred to simply as vapor pressure (VP). The pressure of WV in the atmosphere might be identified as its partial pressure.

Another difference in term usage is the meaning of the word ‘feedback’. In engineering it usually refers to feedback factor, a dimensionless number which is the ratio minus one of response with feedback to the response if there were no feedback. In most science disciplines it refers to the magnitude of the response to a forcing which contributes to the cause of the forcing. In Climate Science it is measured in W/m2.

1. Atmospheric structure
At a scale of the size of atoms the atmosphere consists of spinning and vibrating gas molecules, with empty space between them, bouncing elastically off surfaces and each other. Activity of all of the gas molecules determines properties which can be measured such as temperature and pressure. Only certain gas molecules significantly contribute to radiation heat transfer. These are IR active in the wavelength range of earth temperatures, are misleadingly called greenhouse gasses (ghg) and include CO2 and WV. The radiation energy travels from ghg molecule to ghg molecule (or between surface and ghg) at the speed of light but dwells in each molecule for a few microseconds making the molecule warmer. The dwell time is also called the relaxation time and cumulative dwell time is what causes the GHE.

Approximately 99% of dry atmospheric molecules are non-ghg; nearly all nitrogen and oxygen with about 1% argon. Near the surface, they are substantially warmed by thermalization of the earth-source thermal radiation energy absorbed by the ghg molecules and, at higher altitudes (starting a few meters or less above the surface), cooled by pressure decline and reverse-thermalization back to the ghg molecules. Increasingly with altitude, because of decline of the population of WV molecules, lower pressure, and wider spacing between ghg molecules, outward directed radiation from WV molecules can make it all the way to space.

All molecule species are fairly well mixed throughout the atmosphere with the exception of WV. WV molecule population declines with altitude from average of about 10,000 ppmv (parts per million by volume) at sea level to, because of the low temperature (~ -50 °C), a maximum of about 32 ppmv at the tropopause. The 10,000 ppmv (1%) average increases to about 4% in the tropics. The tropopause altitude averages about 10 km (32,808 ft) with up to 16 km or so at the equator. In addition to the population decline of WV molecules due to temperature decline, is the decline due to pressure decline. The combination results in an overall average WV molecule population decline up to the tropopause of about 1200 to 1.

2. Thermalization
Thermalization and/or reverse-thermalization occur continuously throughout the atmosphere. The combination of thermalization and the steep gradient of WV molecules causes much of the energy absorbed by CO2 to be shared with (redirected to) WV molecules which radiate it to space. This energy transfer and WV molecule population decline of about 1200 to one with altitude produce the notch and ‘hash’ in Top-of-Atmosphere (TOA) graphs of radiation flux vs wavenumber/cm. A typical example of such a graph, showing 18 W/m^2 being redirected to WV, is at Figure 1.

Figure 1: Thermal radiation from below assessed from top-of-atmosphere. Lower wavenumber photons are lower energy. (Original graph is from NASA [1])

3. Relative influence of CO2 and WV on climate
Hitran [2] using Quantum Mechanics calculates, besides many other things, the relative absorb/emit intensity of water vapor molecules vs CO2 molecules. Comparison at zero altitude is shown in Figure 2. Comparison by the ratio of the summation of the multiplication products intensity times wavenumber for each transition (vertical lines on the graph) for each molecule species is (Ξ£ Ii * wni)WV / (Ξ£ Ij  * wnj)CO2 ≈ 1520/46 = 33. On average at ground level, according to the comparatively low populations used by Hitran, WV molecules outnumber CO2 molecules by about 8,000/330 ≈ 24 to one. After accounting for molecule count, each WV molecule is still 33/24 ≈ 1.37 times more effective at warming (absorb/emit of thermal radiation) than a CO2 molecule. 
Figure 2: At zero altitude, CO2 absorb/emit is barely discernable compared to WV.

The relative effectiveness of the increases of WV and CO2 over the last 30 years is calculated as follows:
CO2 increase in 3 decades [3], 1988 to 2018: 407 - 348 = 59 ppmv

Average global water vapor increase trend from Figure 5, which is a graph of NASA/RSS TPW data, is 0.04268/28.9 * 100 * 10 = 1.47 % per decade.

From Figure 3, at 30 degrees latitude (area to pole = area to equator) average global WV ≈ 10,000 ppmv. WV increase in 3 decades = .0147 * 10,000 * 3 = 441 ppmv.
Figure 3: Water vapor declines with latitude and rapidly with altitude. [4] (original from NASA)

Therefore, WV increase has been 441/59 * 1.37 ≈ 10 times more effective at increasing ground level temperature than CO2 increase 1988-2018. (Much of the world human population has been falsely indoctrinated)

A similar assessment made at an altitude of 5 km gives the following results:
WV transition line sum of products = 187.63
CO2 transition line sum of products = 25.47
Ratio of transition line sum products = 187.63/25.47 = 7.37
WV molecules at 5 km per Hitran atmosphere browser, std atm = 1420 ppmv
CO2 molecules at 5 km per Hitran atmosphere browser, std atm = 330 ppmv
WV molecules outnumber CO2 molecules 1420/330 = 4.30 to one.
At 5 km, after accounting for molecule count, each WV molecule is 7.37/4.30 = 1.71 times more effective than a CO2 molecule at thermalization.
WV increase at 5 km in 3 decades = 1420/8000 * 441 = 78.28 ppmv.
At 5 km WV increase has been 78.28/59 * 1.71 = 2.27 times more effective at absorb/emit than CO2 increase 1988-2018.

More importantly, the outward directed emission of WV below wavenumber 600 and from below the tropopause makes it all the way to space while outward directed emission from CO2 molecules in the wavenumber range 600-740 is absorbed by CO2 molecules above. Nearly all of this energy absorbed by CO2 is redirected via thermalization to the WV molecules.

Well above the tropopause, radiation emitted from molecules there to space is primarily from CO2 molecules (indicated in Figure 1 by the spike at wavenumber 667). If you ignore the increase in water vapor (big mistake), near the surface, WV averages about 10,000 ppmv. The increase in absorbers at ground level since 1900 is then about 10,410/10,295 ≈ 1%. WV above the tropopause is limited to about 32 ppmv because of the low temperature (~ -50 °C) while the CO2 fraction remains essentially constant with altitude at (in 2019) about 410 ppmv; up from about 295 ppmv in 1900. The increase in emitters to space at high altitude (~> 20 km, 0.055 atm), and accounting for the lower atmospheric pressure, is (410 + 32)/(295 + 32) * 0.055 ≈ 7.4%. This explains why CO2 increase does not cause significant warming (except near the poles). The result being that Climate Sensitivity (the temperature increase resulting from doubling the CO2 level) is not significantly different from zero.

The exception at the poles is because it’s cold there at ground level so WV molecule count is already low. Therefore, transfer of energy to WV molecules which radiate it to space is negligible.

4. Water vapor increase is a cause and also a result of warming
WV increase is a cause of warming (average global temperature increase) because it is a ghg. WV increase is a result of warming because its vapor pressure increases with temperature. The vapor pressure increase causes an increase in the rate of WV molecules being forced into the atmosphere (when the atmosphere at ground level is less than saturated with WV which is usually the case).

Liquid water has a saturation vapor pressure which depends ONLY on the temperature of the liquid water. The relation is available from multiple sources. Numerical values are graphed at Figure 4 along with three different approximations. The approximations are 2nd and 3rd order polynomials fit to the data by EXCEL and the empirical approximation derived by Bolton which is used by meteorologists [25].

 All of these approximations appear to match the measured saturation vapor pressure vs temperature quite well over the temperature range of interest for earth climate. The first derivative (slope) divided by the pressure (dp/dT/p vs T) is the per unit (1/1) feedback effect of temperature change. As shown in the bottom graph of Fig 4 the polynomial approximations appear to be deficient especially at low temperature. The Bolton equation for saturation vapor pressure in kPa vs temperature in C which is

p = 0.6112 * e^(17.67 * T / (T+243.5))

 appears to be plausible and is used for subsequent work in this document.

Figure 4: Saturation vapor pressure of water and fractional rate of change vs liquid water temperature.

The rate-of-change of saturation vapor pressure is a measure of the vapor pressure increase from temperature increase resulting from the warming from added water vapor. This is the slope of the saturation pressure-temperature curve which is obtained by taking the derivative (as in calculus) of the equation for saturation vapor pressure vs temperature. The per unit change (%/100 = 1/1) is obtained by dividing by the vapor pressure at that temperature.

 This shows that, although the vapor pressure of water increases progressively with temperature, the percent increase of VP declines and therefore the influence of higher temperature actually declines.

5. Measured water vapor increase
The accelerated increase in WV which is expected from the surge in irrigation has also been measured. Average measured global atmospheric water vapor (total from surface to TOA) over the years is provided here at Figure 5.

Clear sky water vapor measurements over the non-ice-covered oceans in the form of total precipitable water (TPW) have been made since Jan 1988 by Remote Sensing Systems (NASA/RSS) [5]. A graph of this measured ‘global’ average anomaly data, with a reference value of 28.73 added [24], is shown in the left graph of Figure 5. This data is extrapolated earlier using CO2 level as a proxy, with the expression kg/m^2 TPW = 4.5118 * ppmvCO2^0.31286. The result is the right-hand graph of Figure 5 which shows approximately 7% increase 1960-2005. 

 Figure 5: Average clear air Total Precipitable Water over all non-ice-covered oceans as measured by NASA/RSS using satellite based instrumentation and with extrapolation by me. (Left graph is by month, right graph is by year average.). Estimated near future minimum is the average since mid-2016. Estimated near future maximum is the slope of the recent monthly trend.

6. World Sources of Increased Water Vapor
Irrigation, industrialization, and, increasing population have been causing the rise in atmospheric WV above that from feedback due to liquid water temperature increase. A survey of available on-line literature provides direct and indirect quantification of significant global sources of the extra increase.

Transportation fuel, linearly interpolated to 2017, amounts to 113E15 BTU/y [6]. Energy content of a typical liquid fuel is 115,000 BTU/gal [7]. Liquid fuels weigh about 6.073 lb/gal = 2.75 kg/gal. Therefore transportation fuels amount to
113E15 * 2.75/115000 = 2.7E12 kg fuel/y                 (a)

About 1.42 kg of WV is produced for each kg of liquid fuel [7] so the amount of WV produced by transportation is
2.7E12 * 1.42 = 3.8E12 kg WV/y                  (b)

World electricity generation is now about 25,000 TWH/y [8]. At an average efficiency of 50% and ignoring non-thermal sources this requires a thermal input of 50,000 TWH/yr. Fuel source fractions of energy [9] in 2017 are approximately 0.38 coal, 0.36 natural gas and 0.26 non fossil fuel.

Coal combustion produces about 0.4 kg WV/kg coal [10]. Energy content of bituminous coal is about 8200Wh/kg [11]. The amount of WV resulting from burning coal to generate electricity is then
50E15 * 0.38 * 0.4/8200 = 0.93E12 kg WV/y                       (c)

The amount of WV produced by natural gas (nearly all methane, CH4) is readily calculated from the dominant chemical reaction
CH4 + 2O2 => CO2 + 2H2O                (d)

Where a mole of methane weighs about 16 g and the two moles of WV weigh about 18 g each.
Natural gas energy content is about 15,400 Wh/kg [11]. The amount of WV resulting from burning natural gas to generate electricity is then
50E15 * 0.36 *36/16/15400 = 2.6E12 kg WV/y                    (e)

The total WV from all fossil fuel used to generate electricity is then
0.9E12 + 2.6E12 = 3.5E12 kg WV/y                         (f)

Waste energy during electricity generation can be approximately accounted for by evaporation of water in cooling towers, etc. At 50% efficiency the waste energy is the same as the energy in the electricity produced, 25,000 TWH/yr = 25E12 kWh/y.
Latent heat of water = 2257 kJ/kg = 0.627 kWh/kg = 1.594 kg/kWh.
The amount of WV from waste heat (cooling tower, etc.) during electricity generation is then
25E12 * 1.594 = 39.8E12 kg WV/y                           (g)

Irrigation is by far the largest source of WV. The increase in irrigation is indicated by the increase in withdrawal for agriculture as shown in Figure 6 [12].
Figure 6: Global water withdrawal includes both ground water and surface water [12]

The total agricultural area equipped for irrigation in 2012 was 324E10 m2 [13]. Estimate 80% were actually being irrigated. Estimating an increase of 2% to 2017, the total area being irrigated is now about
324E10 * 0.8 * 1.02 = 266E10 m2                              (h)
This is more than 4 times the area of France.

Total annual fresh water withdrawal (both ground and surface) is now 3,986 km3 = 3.986E15 kg/y [14]. Of this, about 70% is for agricultural use [15]. This works out to
3.986E15 * 0.7/266E10 = 1052 kg/m2/y ≈ 1 m/y                   (i)
which appears reasonable because average rainfall for the planet is about 1 m/y.

Evapotranspiration, WV from plants and landscape, is discussed in the ‘thematic discussion’ of Aquastat [12]. From there, the amount of precipitation on land is 110,000 km3 /y of which the fraction evapotranspirated is 0.56 + 0.05 = 0.61. Given the planet surface area of 510.1E6 km2, and land fraction of 0.29 this results in the equivalent liquid depth of the total amount of water leaving the surface as WV as
110,000 * 0.61/0.29/510.1E6 = 0.00045 km = 0.45 m/y                    (j)

Water weighs 1000 kg/m3 so evapotranspiration amounts to 450 kg/m2 /y.

Worldwide about 86% of irrigated area is flood irrigated [20]. To simplify calculation, assume all irrigation is flood irrigation approximated as furrow type [16]. Optimum frequency is to flood the furrows about every 10 days [17]. Thus about half the area is covered by water 10% of the time where evaporation from the water is about one meter per year [18] and the rest of the time, the additional evaporation is assumed to be according to the calculated evapotranspiration. Evapotranspiration prior to irrigation must have been low or irrigation would not be done. Evapotranspiration with irrigation, to be cost effective, is most likely to be much more than calculated. These two uncertainties are assumed to approximately cancel each other. A further assumption is that, on average, irrigation is applied for about 1/3 year. The total amount of WV resulting from irrigation is then

[(0.1 * (1 + 0.45)/2 + 0.9 * 0.45) * 266E10]/3 = 42.3E10 m3 = 42.3E13 kg/y           (k)


These calculations are summarized in Table 1


Water vapor source

E13 kg/y




90.0 %

Transportation fuel


0.8 %

Fossil fuel for electricity generation


0.7 %

Cooling towers, etc. for electricity generation





100 %

Table 1: Summary of contributions to atmospheric water vapor.

WV increase above that due to feedback from liquid water temperature increase results about 90 % from irrigation. WV added by irrigation might be particularly influential because it is added at locations where natural WV is low and because the liquid water is shallow, it is quickly and substantially warmed by the sun.

The area of the oceans, much of them quite cold, produced the WV responsible for the basic GHE temperature increase of 33 K. The added WV, mostly from irrigation in previously warm dry areas, has contributed an additional approximately 0.8 K [19] of the total average global temperature increase since 1700. Ref [19] identifies the three contributing factors, the data sources, and the algorithm which calculates the temperatures which match measured average global temperatures 96+% 1895-now (thru 2020).

Given the earth area of 510E12 m^2 and average annual precipitation of about a meter or 1000 kg/m^2 the increased water use, mostly for irrigation, results in 42.3E13/5.1E17 = 0.0008 = 0.08 % equivalent increase in global precipitation.

7. Comparison of measured WV with WV increase calculated from feedback over a long time period.

Some people have asserted that WV content increases with air temperature. That would be true if the air was all continuously at saturation. Of course it is not. The driving factor for WV increase from temperature increase is the temperature increase of the liquid water source. Increase of air temperature does add accommodation for the added WV.

As described in Section 4, as the temperature of liquid surface water increases, its vapor pressure increases which forces more WV molecules into the atmosphere. This contributes to a net feedback from all factors which caused the temperature to increase. A conservative value for WV increase (actual WV increase from feedback will be less than calculated) can be calculated from available measured data. The large effective thermal capacitance of the planet is the main contributor to the conservatism. The temperature rise is in response to the time-integral of the forcing so it will lag the feedback forcing. The percent increase is obtained from the measured WV per-unit increase (%/100) as shown in Figure 4. With compounding (arguable because of time lag due to effective thermal capacitance) the 6.33 % increases to 0.0633 + 0.06332 + 0.06333 + … = 0.0676 = 6.76 %/K.

 The file for calculated change is generated in EXCEL where each row contains:

 WVn = WV(n-1) + (Tn – T(n-1))* R * WV(n-1)


WVn = calculated WV in month n, kg/m^2

Tn = temperature anomaly in month n, K

R = effective rate of WV increase resulting from feedback of temperature increase, 0.067/K (= 6.7 %/K)

 The starting calculated WV is adjusted to make the starting trends the same. This has no effect on the slope.

 The results of this algorithm are shown in Figure 7 along with the actual measured WV anomaly measured and reported by NASA/RSS [5] (plus 28.73). The measured WV is 0.0431/0.0237 = 1.82 or about 82% steeper than the calculated trend using HadCRUT4 Jan 1988 – June 2021 temperature trend and 6.7%/K.

 Figure 7: Measured WV vs calculated WV based on HadCRUT4 reported average global temperatures and compounded feedback.

The GCMs calculate the WV within the models with the result that relative humidity is approximately constant. The linear trends assuming three different values for average relative humidity remaining constant with increasing air temperature are included on Figure 7.1.

Figure 7.1: Same as Fig. 7 but with trends based on constant relative humidity added.

The observation that the actual measured trend is steeper than calculated trends demonstrates that, on the long term, measured WV is increasing faster than possible from just temperature increase of the liquid water (feedback). The steepest slope calculated for the constant RH cases is (29.36 – 28.242)/34 = 0.0329 kg/m^2/y. The measured trend is then 0.0431/0.0329 = 1.31 or about 31% steeper than the estimated trend calculated in GCMs. 

An even more basic consideration is that determining the influence of increased WV from just feedback from temperature increase is too low because it does not include the WV added by human activity. The only valid consideration for the influence of WV increase is to use the measured WV increase.

A corroboration of the long-term temperature trend is as follows: Assume that at the beginning of the warm up the temperature increase was caused by something else. Then the WV increase can be calculated from that temperature increase using the saturation vapor pressure vs temperature for water and the assumption that % increase in WV = % increase in saturation vapor pressure. But the WV has increased more than that so there has to be an additional source of WV. The additional source of WV (about 90% from irrigation) is the something else that produced the initial warming. 

8. Over a short time period, water temperature drives WV.
Surface water temperature fluctuates as shown in an animation [21]. A particularly dominant fluctuation is in the equatorial Pacific and is tracked and reported weekly as el Nino. Fluctuations in el Nino affect short term global average WV and average global temperature as shown in Figure 8.

Figure 8: On the short term, local water temperature fluctuations drive global WV and average global temperature.

9. Influence of WV increase on HadCRUT4 average global temperature.
Figure 9 is a plot of the measured WV vs measured HadCRUT4 data. It shows the short term scatter as well as the long term trend of the influence of WV on average global temperature.

Figure 9: Scatter graph of WV vs HadCRUT4 measured data.

This provides fertile ground for those motivated to mislead to cherry-pick periods where the increasing side of fluctuations drives both WV and outgoing-longwave-radiation up.

10. Energy Redirection
Figure 10 shows TOA radiation flux intensity vs wavenumber as calculated by Modtran [22]. This calculated radiation flux profile has been corroborated by satellite measurements [23]. Superimposed on the graph are plots of black body radiation flux at specific temperatures. Black body radiation imposes an upper limit on radiation intensity at characteristic wavenumbers of each ghg.
Figure 10: Typical TOA radiant emission

Standard atmosphere tables show temperature vs altitude so the bb radiation curves are also altitude curves. The curves are very nearly equally spaced (the lapse rate) up to the tropopause. From this it is seen for example that all radiation emitted in the range 500-600/cm is from between the altitudes 2 – 6 km and that the outward directed radiation in this range makes it all the way to space.

Essentially all of this radiation comes from WV molecules. The result is energy absorbed by CO2 molecules at this altitude range is shared with surrounding molecules via gaseous conduction and the fraction radiated outward by WV molecules makes it all the way to space. Effectively radiation energy absorbed by CO2 is redirected to WV where it is radiated to space.

This process applies to all radiation absorbed by CO2 molecules up to the tropopause and accounts for the energy missing from below the tropopause in the wavenumber range 600-740/cm.

Humanity’s contribution to planet warming is from increased atmospheric water vapor resulting nearly all from increased irrigation. The increased CO2 has negligible effect on warming. Climate Sensitivity, the temperature increase from doubling CO2, is not significantly different from zero.

5. NASA/RSS TPW  (they only report data which includes the latest month available, 201910 means thru October, 2019)
9. Fuel sources for electricity generation
11. Energy content of bituminous coal
17. Frequency of furrow irrigation
19. Climate change drivers
20. Surface irrigation 86%:
21. Ocean surface temperature animation
23. Modtran comparison with measured:
24. Atmospheric absorption of water vapor is logarithmic:
25. Bolton equation for water saturation p T


  1. Very interesting paper and thanks for sharing it Dan.

  2. Agree, very important investigation.
    The key is to simplify.
    We know the black body temperature steady state temperature of the earth which can easilly be calculated to -18 Celsius, meaning without atmosphere.
    We know aveage earth temperature today which is + 14 C Celcius.
    I am trying to corelate the two and need discussion about CO2 and WV H2O. Most important how many Watt -1 Tonn of each backscatter..

  3. Without the Atmosphere as we know today, the black body steady state temperature of the earth would be different from -18 C due to Oceans.

    1. The -18 C assumes albedo of 0.3 which would not hold if no atmosphere. Also, if no atmosphere there would be no water vapor and no oceans.

  4. So what do we do about the irrigation problem?

    1. Water vapor, and warming from increased WV, is limited by how much WV the atmosphere will hold. The amount of warming possible from increased WV appears to be less than a degree K and not a problem. Global cooling, portended by the quiet sun, is a far greater threat. The warming from increased WV is countering that, will slow and possibly prevent it

    2. This comment has been removed by the author.

  5. Great article (I'm a retired CEng). Nice statement re WV warming balancing quietening sun! Trying to convince youngsters they're not about to die through CO2 climate change is a challenge.

  6. Thanks. It seems a higher % of engineers figure it out than others.

    1. Chemical Engineer by training, now missionary to Haiti. In college Univ of Texas at Austin, one of my classes was a heat and material balance of the earth. It stretched my mind.

    2. yes the engineer/scientist divide is often amusingly stark

      i like to joke that a scientist's job is to come up with a thousand ideas in hopes that one might move us a little closer to the truth, whereas an engineers' job is avoid a thousand fatal mistakes in one real-world application so that it works every time

  7. In researching scientific data on climate after retirement from spending 40 years researching and designing jet engines, I discovered that the water vapor component of the atmosphere is about 75 times that of CO2 and became very suspicious of the IPCC claims about CO2. I am fascinated with this in -depth analysis, which shows the dominance of water vapor. Very gratifying!

  8. Hi Dan, I found your paper very interesting... do you have a rough idea how the total WV content of the atmosphere compare to human WV emissions?

    wondered a few years back how much human water usage would have to increase to affect sea levels and after some BOE calcs the answer there seems to be “by a few orders of magnitude” but then of course the hydrosphere masses about 300x what the atmosphere does so that seems like it might be in a relevant ballpark for WV

    at any rate the idea human WV emissions might directly affect temp and thus CO2 levels is certainly intriguing... might help explain several discrepancies in the IPCC view... hope you are able to write more

    1. Not much. In Section 6 it is calculated that humanity adds on average only about 0.00026% to the average precipitation.

    2. Oops, I discovered a factor of 1000 error (Its been fixed). Humanity adds on average only about 0.26% to the average precipitation.

  9. Very interesting. One large-scale climate effect of irrigation I know myself---for the past 30 years, the North India plains (including West Punjab and East Bengal) have seen far more winter fogs than were known previously. This coincides with a great increase in tube-well irrigation and consequent groundwater extraction on a massive scale.

    I think that only the ground water extraction component of the irrigation should be included in your estimate of 3986 km3 of total irrigation. The surface water is merely being shifted from one place to another. Only the groundwater extraction adds to the surface water quantity.

    1. Gyan, My argument for including surface water there is that the area where evaporation takes place is what matters and essentially all of the surface water would be spread out adding to the evaporation area without significantly diminishing the surface area of its source. Realize that this was just a sanity check and did not contribute to the findings shown in Table 1.

  10. How does water vapour account for regular abrupt climate changes of Dansgaard-Oeschger events during the glacial cycle?

    1. It doesn't. Human influence on WV was tiny prior to about 1800 and the rate increased dramatically around 1960.

  11. Dan, most interesting. However, I don't understand where you've gotten your extrapolation of water vapor back to the year 1600 ... what did I miss?

    My best to you,


    1. W,
      You didn't miss anything. I had developed the extrapolation in 2016 for use in The graph appears as Fig 3 there with very brief explanation (and wrong coefficient, I actually use 4.5247, not 4.5118). I used smoothed CO2 as a proxy and the resultant graph looks reasonable to me. I found these notes where I did it:
      TPW = X (CO2 ppmv)^Y

      π‘Œ= (ln⁡(π‘‡π‘ƒπ‘Š1−ln⁡(π‘‡π‘ƒπ‘Š2)/(ln⁡(𝐢𝑂2,1)−ln⁡(𝐢𝑂2,2))

      X = TPW1/CO2,1^Y

      1. Linear eqn thru TPW data
      2. Calc TPW1 & TPW2 from [linear regression] eqn
      3. Select CO2,1 & CO2,2 frm [smothed] data [for same time interval?]
      [That second equation should be: π‘Œ= (ln⁡(π‘‡π‘ƒπ‘Š1)−ln⁡(π‘‡π‘ƒπ‘Š2))/(ln⁡(𝐢𝑂2,1)−ln⁡(𝐢𝑂2,2))

    2. Thanks for the clarification, Dan. What I'm still not understanding is why you think that TPW varies linearly with the log of CO2 ... where is the observational evidence for that relationship?


    3. I don't remember what my thinking was. The resulting curve looked reasonable and that was all that I was after.


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