The water vapor feedback loop

The standard IPCC models assume constant relative humidity as temperature increases. This assumption makes modeling calculations easy since it allows the Clausius-Clapyeron equation to be used. Keeping constant relative humidity requires a good deal of energy to maintain it in the real atmosphere where a gravitational field exists. The modelers have ignored the energy requirement resulting in a vast overestimation of the water vapor feedback warming.

Recently the increase in humidity has been measured accurately for the first time and is found to be much less than the models assumed. A press release summarizing the new measurements is reproduced below:

Satellite finds warming 'relative' to humidity

“A NASA-funded study found some climate models might be overestimating the amount of water vapor entering the atmosphere as the Earth warms. Since water vapor is the most important heat-trapping greenhouse gas in our atmosphere, some climate forecasts may be overestimating future temperature increases.”

“In response to human emissions of greenhouse gases, like carbon dioxide, the Earth warms, more water evaporates from the ocean, and the amount of water vapor in the atmosphere increases. Since water vapor is also a greenhouse gas, this leads to a further increase in the surface temperature. This effect is known as "positive water vapor feedback." Its existence and size have been contentiously argued for several years.”

“Ken Minschwaner, a physicist at the New Mexico Institute of Mining and Technology, Socorro, N.M., and Andrew Dessler, a researcher with the University of Maryland, College Park, and NASA's Goddard Space Flight Center, Greenbelt, Md, did the study. It is in the March 15 issue of the American Meteorological Society's Journal of Climate. The researchers used data on water vapor in the upper troposphere (10-14 km or 6-9 miles altitude) from NASA's Upper Atmosphere Research Satellite (UARS).”

“Their work verified water vapor is increasing in the atmosphere as the surface warms. They found the increases in water vapor were not as high as many climate-forecasting computer models have assumed. "Our study confirms the existence of a positive water vapor feedback in the atmosphere, but it may be weaker than we expected," Minschwaner said.”

“"One of the responsibilities of science is making good predictions of the future climate, because that's what policy makers use to make their decisions," Dessler said. "This study is another incremental step toward improving those climate predictions," he added.”

“According to Dessler, the size of the positive water vapor feedback is a key debate within climate science circles. Some climate scientists have claimed atmospheric water vapor will not increase in response to global warming, and may even decrease. General circulation models, the primary tool scientists use to predict the future of our climate, forecast the atmosphere will experience a significant increase in water vapor.”

“NASA's UARS satellite was used to measure water vapor on a global scale and with unprecedented accuracy in the upper troposphere. Humidity levels in this part of the atmosphere, especially in the tropics, are important for global climate, because this is where the water vapor has the strongest impact as a greenhouse gas.”

“UARS recorded both specific and relative humidity in the upper troposphere. Specific humidity refers to the actual amount of water vapor in the air. Relative humidity relates to the saturation point, the amount of water vapor in the air divided by the maximum amount of water the air is capable of holding at a given temperature. As air temperatures rise, warm air can hold more water, and the saturation point of the air also increases.”

“In most computer models relative humidity tends to remain fixed at current levels. Models that include water vapor feedback with constant relative humidity predict the Earth's surface will warm nearly twice as much over the next 100 years as models that contain no water vapor feedback.”

“Using the UARS data to actually quantify both specific humidity and relative humidity, the researchers found, while water vapor does increase with temperature in the upper troposphere, the feedback effect is not as strong as models have predicted. "The increases in water vapor with warmer temperatures are not large enough to maintain a constant relative humidity," Minschwaner said. These new findings will be useful for testing and improving global climate models.”

“NASA's Earth Science Enterprise is dedicated to understanding the Earth as an integrated system and applying Earth system science to improve prediction of climate, weather and natural hazards using the unique vantage point of space. NASA plans to launch the Aura satellite in June 2004. Along with the Terra and Aqua satellites already in operation, Aura will monitor changes in Earth's atmosphere.”

Further comments:

A few years ago on my former website I said the water vapor feedback could not occur as it was formulated in the models:


"The water vapor feedback loop. According to model output results, an increase of 300 ppm of CO2 will cause the mean water vapor content to increase from about 14,000 ppm to 14,600 ppm. The added 600 ppm of water vapor will add to the greenhouse effect and hence it is called the "enhanced" greenhouse effect for this reason. According to Ramanathan, the carbon dioxide doubling will increase the surface radiation flux by 1 W/m2. However, I would like to point out that 600 ppm of water vapor when summed up over the globe represents a considerable tonnage. This mass must be lifted an average of about 5 kilometers and replaced weekly. To accomplish this lifting cycle requires slightly more than 1 W/m2. It is my conclusion that the water vapor feedback loop is energetically impossible. It appears that the modelers are neglecting the gravitational energy flux. Including gravitational energy in this feedback loop will reduce its strength by more than 90%. In brief, the water vapor feedback loop is impossible as presently formulated. There is simply not enough energy available to accomplish the reported warming, reported evaporation and reported altitude rise in water vapor."


Elsewhere on the old website in discussing overall climate sensitivity, I said:


"Water vapor feedback coefficient of 0.4 from models [0.4 is a factor in the denominator of the climate sensitivity equation giving the strength of the positive water vapor feedback loop], or assuming constant relative humidity.

Comment 1: The unproven assumption of constant relative humidity is incorrect. If you use ALL the physics, you find that the humidity increases by a much smaller portion so that the feedback strength is not 0.4, but more like 0.02. The figure 0.4 assumes that the laws of physics in a closed static laboratory system (where gravity can be safely ignored) are applicable to the Earth's atmosphere which is a dynamic open system in a gravitational field.

Comment 2: The 0.4 figure assumes the additional 1 W/m2 at the surface arising from a doubling of carbon dioxide is about 15 times more effective in evaporating and transporting water vapor as the pre-industrial 388 W/m2 at the surface.

Comment 3: If you include biology, this feedback could even be negative since increased carbon dioxide causes plants to have less transpiration.

Comment 4: There are no observations of increased moisture in the atmosphere as would be predicted if the number 0.4 were correct. Stream flow observations also have not increased as predicted if 0.4 were the correct number."

Looking at the Minschwaner paper, the following points can be made concerning the increase in water in ppm in the troposphere:


1.8 to 4.2 ppm/C with an average of 3 ppm/C (Minschwaner, all data; p.13)

-0.2 to 3.2 ppm/C with an average of 1.5 ppm/C (Minschwaner, excluding 1997-98 El Nino data)

8.5 to 9.5 ppm/C (Minschwaner model)

25 ppm/C (climate models used by IPCC)

1.7 ppm/C (my model results reported in earlier posts, using 7 days; perhaps about 2.4 ppm/C using 10 days)


My old model (from 1980) agrees with Minschwaner observations where the El Nino is excluded. The feedback seems to be slightly positive, but is less than 10% as strong as what the IPCC models predict. Perhaps outside the tropics and outside where Minschwaner looked, downward subsidence drys the atmosphere and further reduces the vapor feedback effect as argued by Ellsaesser (1984).

The net effect is that the IPCC range of 1.5 to 4.5 C for a carbon dioxide doubling to 0 to 3 C.


Ellsaesser, H. W., 1984. The Climatic Effect of CO2: A Different View. Atmos. Env., 18, 431-434.

Minschwaner, K., and A. E. Dessler, 2004. Water vapor feedback in the tropical upper troposphere: Model results and observations. Journal of Climate.

The preprint is available from <>

Links with further discussion of this paper:

By Pat Michaels.

By James Taylor.

By Vincent Gray.