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Explaining how the water vapor greenhouse effect works

What the science says...

Select a level... Basic Intermediate

Increased CO2 makes more water vapor, a greenhouse gas which amplifies warming

Climate Myth...

Water vapor is the most powerful greenhouse gas

“Water vapour is the most important greenhouse gas. This is part of the difficulty with the public and the media in understanding that 95% of greenhouse gases are water vapour. The public understand it, in that if you get a fall evening or spring evening and the sky is clear the heat will escape and the temperature will drop and you get frost. If there is a cloud cover, the heat is trapped by water vapour as a greenhouse gas and the temperature stays quite warm. If you go to In Salah in southern Algeria, they recorded at one point a daytime or noon high of 52 degrees Celsius – by midnight that night it was -3.6 degree Celsius. […] That was caused because there is no, or very little, water vapour in the atmosphere and it is a demonstration of water vapour as the most important greenhouse gas.” (Tim Ball)

When skeptics use this argument, they are trying to imply that an increase in CO2 isn't a major problem. If CO2 isn't as powerful as water vapor, which there's already a lot of, adding a little more CO2 couldn't be that bad, right? What this argument misses is the fact that water vapor creates what scientists call a 'positive feedback loop' in the atmosphere — making any temperature changes larger than they would be otherwise.

How does this work? The amount of water vapor in the atmosphere exists in direct relation to the temperature. If you increase the temperature, more water evaporates and becomes vapor, and vice versa. So when something else causes a temperature increase (such as extra CO2 from fossil fuels), more water evaporates. Then, since water vapor is a greenhouse gas, this additional water vapor causes the temperature to go up even further—a positive feedback.

How much does water vapor amplify CO2 warming? Studies show that water vapor feedback roughly doubles the amount of warming caused by CO2. So if there is a 1°C change caused by CO2, the water vapor will cause the temperature to go up another 1°C. When other feedback loops are included, the total warming from a potential 1°C change caused by CO2 is, in reality, as much as 3°C.

The other factor to consider is that water is evaporated from the land and sea and falls as rain or snow all the time. Thus the amount held in the atmosphere as water vapour varies greatly in just hours and days as result of the prevailing weather in any location. So even though water vapour is the greatest greenhouse gas, it is relatively short-lived. On the other hand, CO2 is removed from the air by natural geological-scale processes and these take a long time to work. Consequently CO2 stays in our atmosphere for years and even centuries. A small additional amount has a much more long-term effect.

So skeptics are right in saying that water vapor is the dominant greenhouse gas. What they don't mention is that the water vapor feedback loop actually makes temperature changes caused by CO2 even bigger.

Basic rebuttal written by James Frank


Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

 

Last updated on 5 July 2015 by pattimer. View Archives

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Comments 26 to 50 out of 378:

  1. Hmmm...You've made some odd misinterpretations of my post again. If you read the two sentences following "THREE" in my post just above (#24) you can see that water vapour makes a significant effect. Since this is water vapour that re-equilibrates in the atmosphere in response to anthropogenic CO2-induced warming it can be classed as anthropogenic water vapour (AWV). And I certainly didn't say that "the atmospheric total of WV remains more or less constant because of precipitation". If you read the two sentences following "THREE" in my post just above, it states: "AS the entire troposphere warms under the influence of cumulatively enhanced CO2 concentrations, so the atmospheric water vapour concentration rises." I'm not sure how you can interpret a statement that the WV rises as meaning that "the WV remains constant"! Of course the atmospheric WV fluctuates. It varies according to the local atmospheric temperature (and pressure). However on average the amount of atmospheric water vapour rises as the atmospheric temperature rises on average in a warming world. This can be measured in the real world. And any water vapour that doesn't partition into the atmosphere according to the local temperature and pressure just falls right out again as precipitation. If we doubled the amount of water that we pumped into the near ground atmosphere that doesn't change the fact that it is ultimately the local atmospheric temperature and pressure that governs the amount of water vapour in the atmosphere. Anything else that's pumped up there just falls right out again as precipitation. The NOAA site gives a pretty basic description although it's a bit out of date with respect to the determination of tropospheric water vapour in response to greenhouse warming.
  2. some sums.............contribution to greenhouse effect: 'natural' 'man added' WV 94.999% 0.001% CO2 3.502% 0.117% CH4 0.294% 0.066% NO2 0.903% 0.047% rest 0.025% 0.047% So A.CO2 contributes around 0.117% of total greenhouse effect.....33 x 0.117% = .038 degC Total A GG's contribute around 0.28%...33 x 0.28% = 0.09 degC
  3. Source please Mizimi for those columns of faux-"numerology". Where did you cut and paste them from and what do the columns mean?
  4. Apologies for the format, the spaces that should have been there seem to have been ignored in uploading. try again: The figures given indicate the relative contribution of GG's to global warming as a % of the whole, ie +33C WV from natural sources 94.999%: manmade 0.001% CO2 from natural sources 3.502%: manmade 0.117% CH4 from natural sources 0.294%; manmade 0.066% NO2 from natural sources 0.903%: manmade 0.047% Rest ....................0.025%: manmade 0.047% The basic data is available from the DOE website ( other than they ignore WV as a GG) "Current Greenhouse Gas Concentrations"; " Greenhouse Gases and Global Warming Potentials" From the IPCC "Warming Potentials of Halocarbons and Greenhouses Gases"
  5. Mizimi those numbers are nonsense. I don't believe they're from the IPCC reports at all. Why not simply give us the url from the site that you cut and pasted them from? And how can you say one the one hand that "they ignore WV as a GG"... ...and on the other show data that indicates that water vapour is 94.999% of the whole? ...???????
  6. The numbers are not cut and pasted but taken from the sites mentioned. Check them out yourself. The DOE site does not include WV in their GG listing so I take that as ignoring its effect as a GG. The 95% WV quoted is the 'high' side figure from various sites; the 'low' side figure generally quoted is 90% from other sources and one can rerun the sums using the 90% if desired. The rest is simple maths. I included WV in the calcs for obvious reasons.
  7. I have checked them out. Your numbers are nonsense and don't come from the DOE or from the IPCC. The data on Greenhouse Gas concentrations from the US government CDIAC refer to anthropogenic greenhouse gas and so don't include water vapour. Is that what you're on about? e.g. http://cdiac.esd.ornl.gov/pns/current_ghg.html But we don't really know what you're talking about unless you give us a link to the data that you cut 'n pasted those weird numbers from.
  8. Try: http://yosemite.epa.gov/oar/globalwarming.nsf/UniqueKeyLookup/SHSU5BUM9T/$File/ghg_gwp.pdf which gives warming potentials; (and they acknowledge WV is a GG and address their tables to 'selected' GG's) and: http://cdiac.esd.ornl.gov/pns/current_ghg.html (which doesn't acknowledge WV...and their data is NOT just AGG's........unless you deem we are responsible for all the pre-1750 gases ???) If you check the math from table 2 in the EPA site ( which can be checked against the DOE site) CO2 accounts for ~72% of GG warming EXCLUDING WV. CH4 about 7% and N2O 19%, the rest is CFC's etc. Now factor in WV at 95% (topend) and CO2 is responsible for 72% of 5%...3.6% of the total, or 1.19C Take the lowend 90% WV and you get 7.2% of the total or 2.38C Now run the increases due to man's contributions alone and you will get the numbers I quoted in #29
  9. Not really Mizimi: (i) The CDIAC is exactly about anthropogenic greenhouse gases. That's why they reference them with respect to pre 1750 levels (zero for the CFC's but not for CO2, methane, ozone, and nitrous oxide). (ii) both sites (EPA and CDIAC) don't include water vapour because they are considering anthropogenic greenhouse gas forcings. The EPA site has a specific section about water vapour. As they state (and we've already established this point in numerous posts above on this thread), human activities aren't believed to directly affect water vapour concentrations (it's not a forcing, it's a feedback), but the warmer atmosphere from anthropogenic greenhouse gases results in a water vapour feedback that amplifies the anthropogenic forcing from enhanced greenhouse gas concentrations. That's all very straightforward and easy to understand. (iii) Otherwise your numerology is suspect. In fact it's not possible to partition greenhouse effect contributions from the individual greenhouse gases in the manner that you have done, since the greenhouse gases don't act independently, especially when water vapour is considered. removing CO2 from the atmosphere results in a very large cooling, since a significant part of the water vapour contribution arises as a feedback to CO2-induced warming, and if you remove the CO2 you remove a lot of the water. Do you see why that makes a linear, discrete "partitioning" of the greenhouse effect to individual components inaccessible to simple-minded arithmetic? In fact this issue has been dealt with many times through the use of modelling of the effects of removing various components of the atmosphere. An early example is: Ramanathan V, Coakley JA (1978) Climate Modeling Through Radiative-Convective Models. Rev. Geophys. 16, 465-489. For example if you remove CO2 from the atmosphere the greenhouse effect is reduced by 9% and if you remove water vapour it's reduced by 36%. But if you removee CO2 and water vapour it's reduced by more than 45% (the sum of the two). Likewise if you remove everything but CO2, 26% of the longwave IR is still absorbed in the atmosphere. So if one wanted to put numbers to the contribution of CO2, it's somewhere between 9-26% of the greenhouse effect.... If you find Ramanathan and Coakley heavy going, Wikipedia has a goodish account: http://en.wikipedia.org/wiki/Greenhouse_effect#Water_vapour_effects
  10. "a significant part of the water vapour contribution arises as a feedback to CO2-induced warming," If we allow CO2 alone is responsible for a 0.17C rise/decade in GMT as per GISS data, and we allow ALL of the heat associated with that temp rise to evaporate water then an estimate of the increase in WV is around 0.07gm/kg of dry air. The atmosphere contains an estimated 5.135x10E18 kg of dry air (National Centre for Atmospheric research) which gives us a possible 5.135x10E18 x .07x10E-3 kg of WV added by this temp rise. Which ( providing I get my powers sorted out his time!) amounts to some 36x10E13 kg, PER DECADE. The current estimate for mm WV additions is 2360 cubic k's which is 2.36x10E12 kg....PER ANNUM. So over a decadal period we would add WV equivalent to 6.5% of that caused by CO2. Not insignificant. Dismissing AWV on the basis it precipitates out within 7-14 days does not do away with the fact that it does have a warming effect during that time period and it is a continuous effect at that.
  11. Sorry!! got my powers screwed up again, and having re-checked the sums there appears to me to be some overestimates in the AWV basic data, especially in respect of agricultural 'evapotranspiration' and the total for evaporation from reservoirs. So let's drop these out and concentrate on industrial figures which are a lot 'harder'. 90 billion tonnes of WV from industrial use ( excluding water from combustion) is 9 x 10E13 kg /annum or 3 times that produced by CO2 warming. AND it is increasing. There is still the 30% rise in usage recorded from 1980 to 1990 which just happens to coincide with the upturn in GMT. Just a coincidence? Also your figures comparing the 'turnover' of WV in the atmosphere (5 x 10E17kg/annum) only use the WV reckoned to be evaporated from cooling towers, not ALL AWV.
  12. Some figures from the World Water Council on evaporation from human sources (not abstraction which is much much higher) 1940 1950 1960 1970 1980 1990 2000 50...80..155..245.. 285..320..515 figures in cubic k's and exclude agriculture and domestic/municipal usage. Including these quadruples the figures. It is clear from the data that AWV has been steadily increasing since 1940 and thus the average global atmospheric water content will have risen since the atmosphere is not saturated. This addition occurs at low altitudes...precisely where one expects the warming effect of GG's to be the greatest.
  13. The article concerns itself with radiative forcing and positive feedback and does not address basics.... The mass of CO2 in the atmosphere is around 3x 10E15 kg. The mass of WV is around 12.7 x 10E15 kg. The specific heat of CO2 @ 275K is 0.819kj/kg The specific heat of water @ 275K is around 4.2kj/kg Simplistically ( I can see the objections coming!) the heat content increase for a 1C temp increase is thus: CO2: 3x10E15 x 0.819 x 1 = 2.45 x 10E15 kj Water: 12.7 x 10E15 x 4.2 x 1 = 53.3 x 10E15 kj IE: the heat content available from wv is 22 x greater than CO2 disregarding any IR effect.
  14. A fine debate! Thanks to both of you. The questions are probably as important as the answers. If cooling towers are significant contributors to global warming, the alternatives to coal-fired power stations need to be reassessed. Surely there is a complication, though. The effluent from chimneys and (probably) cooling towers forms plumes, which are not only blown down-wind but also rise up through the atmosphere, cooling as they do so, remaining warmer and lighter than the surrounding atmosphere until completely mixed. I have a vague (and unsubstantiated) idea that these plumes rise to considerable altitudes – so is it possible that a large part of the AWV (and CO2) mixes into the atmosphere at the middle or top of the troposphere? Here, the influence of the AWV might be more significant… I have limited knowledge of climate modelling, so find it difficult to progress this idea. Any comments would be gratefully received.
  15. Wavelength: The answer is : It depends! I live 26km from a coal fired power station/cement works and when the prevailing wind is northerly I can see, out at sea a horizontal band of brown haze..the plume. On other days with higher winds it disperses more quickly and does not appear here. On still winter days smoke from local bonfires rises straight up and flattens out at around 100m and is dispersed by around 200m How high and far the plume goes depends entirely on 'local' weather conditions. Back a few years the scandinavian countries suffered 'acid rain' from british power stations.
  16. Wavelength: Air temperature decreases with altitude..this is called the 'lapse rate' and is approx -6.5C/km, so at around 5km the air temp is -13C and the pressure is about 0.5bar (half surface pressure). As WV content of air is temperature/pressure related then the amount of WV decreases as you ascend. Both these factors influence how high the plume can ascend without 'external' help from air turbulence or other factors. Therefore I would expect a greater warming effect from WV at surface level ( say up to 500metres)and then a steady decline towards zero at around 3000m
  17. Somewhere around 50% of the world’s CO2 and 75% of the AWV is produced by large point sources, in contrast to natural evaporation from oceans or lakes - so to me it seems reasonable to ask if this is likely to affect climate models. I have not seen it mentioned in the reading I have done. Since your previous response, I have looked on the web and found some information, such as: http://ams.confex.com/ams/88Annual/techprogram/paper_136670.htm This shows an average height of plumes above wildfires to be 2.3 km worldwide with 3 km for North America. This presumably only refers to the visible plume from particles/cloud formation, but is quite a bit below the 10-15 km that I had in mind. Also, the updraft from wildfires is probably hotter than from cooling towers, although with a lower relative humidity. The point-source question remains to niggle me. I suspect it is not significant, but still have no definitive argument to dismiss it.
  18. There's a new satellite-based study of the relationship of water vapor to CO2 by Dessler and colleagues on NASA.gov (the Earth section). It nails down the specific feedback effect's size.
  19. Had a quick look at the summary page which talks about measuring total WV content of the atmosphere....but no mention of assigning values to WV from global warming as distinct from WV evaporating from human activities. It;s all lumped together as 'WV'.
  20. "a network of 124 U.S. military weather stations with continuous human observations provides useful information of total cloud cover averaged over the contiguous United States, and suggests an increasing trend (~1.4% of the sky covered per decade) in U.S. total cloud cover from 1976 to 2004, with increases over most of the country except the Northwest." http://www.tiimes.ucar.edu/highlights/fy06/dai.html Clear indication of increasing WV over the time period global warming has 'accelerated'. The question is, how much is due to man's additions and how much to CO2 induced warming?
  21. re #45 It's all due to CO2-induced warming Mizimi.....at least none of it is due to "addition" by man (or ladies for that matter). Man can't "add" water vapour to the atmosphere. The atmospheric water vapour levels are essentially "defined" by the atmospheric temperature and pressure. We've already determined (see post #20) that the amount of water vapour released into the atmosphere by burning stuff, cooling towers and so on is a tiny proportion of the water vapour released into the atmosphere by the natural evaporative cycle (we calculated mankinds contribution is around 0.005% of that released by natural evaporation). What happens to all of that water (e.g the vast amount from natural evaporation)?. It all comes straight out as precipitation. What stays in the atmosphere is what the atmosphere can support in relation to the atmospheric temperature and pressure. In fact the research indicates that the atmosphere tends to maintain a relatively constant relative humidity (i.e. raised absolute humidity with increasing temperature and vice versa). In the paper[***] that Tom Dayton refers to (Tom links to an info summary), the water vapour levels increase right throughout the troposphere to the high altitude/low pressure regions. The authors note that as the atmospheric temperature fluctuates (e.g. the strongish La Nina cooling), so the atmospheric water vapour levels follows (such that a constant relative humidity is roughly maintained). Each of these observations is consistent with our understanding of the response of the atmospheric water vapour concentration to temperature, and its rather short term response to temperature changes (after Pinatubo the atmospheric water vapour levels dropped even ‘though we were still releasing water vapour into the atmosphere from cooling towers!), and is incompatible with the notion that mankind's emissions (a miniscule proportion of the natural evaporative contribution), can somehow affect the atmospheric water vapour levels. [***] A. E. Dessler et al (2008) Water-vapor climate feedback inferred from climate fluctuations, 2003–2008. Geophys. Res. Lett. 35, L20704, abstract: Between 2003 and 2008, the global-average surface temperature of the Earth varied by 0.6°C. We analyze here the response of tropospheric water vapor to these variations. Height-resolved measurements of specific humidity (q) and relative humidity (RH) are obtained from NASA's satellite-borne Atmospheric Infrared Sounder (AIRS). Over most of the troposphere, q increased with increasing global-average surface temperature, although some regions showed the opposite response. RH increased in some regions and decreased in others, with the global average remaining nearly constant at most altitudes. The water-vapor feedback implied by these observations is strongly positive, with an average magnitude of λq = 2.04 W/m2/K, similar to that simulated by climate models. The magnitude is similar to that obtained if the atmosphere maintained constant RH everywhere. I'm not totally convinced by the clouds snippet you linked to. Again reading the full paper[*****] a couple of things stand out: (i) the large proportion of those 124 stations are in S. Calif, along the Gulf coast and along the Eastern seaboard. So vast regions of the US aren't covered, and it's possible that a small rise in cloudiness in these built up regions may relate to their location in built up areas of the US (??). The authors acknowledge this: "Quantitative estimates of the uncertainties of the U.S. cloud trend during 1976–2004 shown in Fig. 8b are difficult to derive because of the poor coverage by the 124 military stations and the subjective nature of the human observations." and: "Although the 124 U.S. military stations provide useful data for total cloud amount up to the present, they have limited spatial coverage and are inadequate for monitoring regional trends in the western and other parts of the country." (ii) satellite data show a decreasing cloud trend during this period worldwide. So either the military station trend is a localized one..or isn't correct...or the satellite trend is incorrect...or the two series are measuring different things (e.g. low level clouds from ground observations vs high level clouds from satellites). I think one can quite reasonably be skeptical about drawing too many inferences from empirical cloud data so far. The main point of Dai and Trenberth's paper is that replacement of human cloud monitoring (as done in the military bases) by automatic monitoring systems, is a backwards step.. Note that although mankinds water vapour emissions don't have a significnat effect on greenhouse levels of atmospheric water vapour, we can probably influence the hydrological cycle by land use effects. and of course we are indirectly influencing the atmospheric water vapour levels by our CO2, methane, nitrous oxide emissions! [*****]Dai, A., T. R. Karl, B. Sun, and K. E. Trenberth, (2006) Recent trends in cloudiness over the United States: A tale of monitoring inadequacies. Bull. Amer. Meteor. Soc., 87, 597-606
  22. So we don't warm the atmosphere directly? The 14 terawatts of heat released by us annually has no effect on the global temp? It doesn't raise the temp at all and thus increase Tmin? The 6.7x 10E15 watts emitted annualy by the human population just by being alive doesn't affect Tmin either? Or the 3x 10E15 watts emitted by cows? All that heat is dissipated into the atmosphere and provides energy for WV to increase. Bear in mind too, that industrial WV emissions INCLUDE the energy needed to vaporise the water...no heat is required from the atmosphere, unlike 'natural' evaporation. One of the papers you referred to in another post indicated an increase in WV of 0.4gm/kg dry air over the oceans since 1988...far too much for just CO2 induced warming.
  23. PS: Have you considered the net contribution to the earth's energy budget made by Life? All metabolic processes are exothermic, from whales to bacteria. Even decomposition produces heat. And the source of that heat is the sun. Not IR, but mostly visible light in the green/yellow band, converted by photosynthesis into complex organic compounds which are then metabolised....giving out heat. In addition water vapour is emitted by air breathers ( roughly 60% of metabolic heat is emitted in the form of WV)adding to the atmospheric WV total. And whilst you will no doubt argue that compared to IR it is tiny - it is also iterative.
  24. #47....just re-read that paper and confirmed that often we see what we think we should see rather than what is there. The paper actually states 0.4kg/m2 increase in WV through the lower troposphere. Which if you assume is 8km deep allows an increase of 400/8000 gm/m3 or .05gm/m3. Air at 15C/~50%RH contains about 5.5gms/m3, so this increase is pretty insignificant and probably less than background 'noise', especially when you consider this is over a 10yr period.
  25. A few sums ....various sources give our annual energy usage from FF as around 14 terawatts. Looking around the Australian Bureau of Statistics gives the following population figures.... People 21 million Horses 400,000 Kangaroos 23 million Camels 400,000 Cattle (dairy and beef) 26 million Sheep 20 million Rabbits 250 million Simple maths - multiplying numbers by the basal metabolic rate at rest of each species gives a daily heat emission of 315 x 10E9 watts or 114 x 10E12 watts per annum. This figure increases with physical activity. In other words, the small selction of life forms listed from ONE country put 9 times more heat into the atmosphere than man does through FF consumption....and they represent a tiny fraction of the worlds animal species. How does the GW model accomodate this?

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