water vapor

Water vapor and the climate, why carbon dioxide is a very minor player

Water vapor is a powerful greenhouse gas, but its net effect in the atmosphere is to lower temperatures.  Proponents of anthropogenic global warming (AGW) and most IPCC climate models assume the opposite:

AGW hypothesis: Carbon dioxide, a weak greenhouse gas, begins warming the planet.  This warming evaporates water and so puts water vapor into the atmosphere which amplifies the warming effect.  This is called a positive feedback.

At first look, this proposition seems logical and reasonable.  But other properties of water vapor reduce temperatures and the net effect is a strong negative feedback.  A positive feedback tends to destabilize a system, whereas, a negative feedback tends to keep a system in check.  Just think for a minute, if water vapor had a net positive feedback effect, this planet would have had run-away global warming long ago.  That alone should falsify the positive feedback hypothesis.  But let’s look at some observational evidence for a negative feedback.

The graphic below  (from Brehmer) compares four pairs of cities, each at about the same latitude so that each pair receives about the same amount of sunlight, and the cities are inland, away from possible tempering by sea breezes. The data is from the National Weather Service  (the temperatures have been corrected for elevation differences). The difference between the pairs is that one city is in an arid climate, the other is in a humid climate.  We see that the more humid city in each pair has a lower average annual temperature.  The addition of water vapor to the atmosphere has a cooling effect in spite of water vapor being a greenhouse gas much more powerful than carbon dioxide.


I’ll show another piece of observational evidence (from Sweger).  This shows humidity versus temperature measured daily at 3p.m., the hottest part of the day.  Again we see a negative correlation of temperature with humidity, i.e., higher temperatures occur when humidity is lowest.


I will mention one more example, Al Gore’s favorite graph.  This is the sawtooth graph showing the rise and fall of temperatures and carbon dioxide during the last few glacial-interglacial periods.  I ask, if rising carbon dioxide is further amplified by water vapor, why would the carbon dioxide and temperature rise terminate?  Could it be that other natural forces are much stronger than the effect of carbon dioxide?

Conventional greenhouse theory holds that the sun’s energy passes through a relatively transparent atmosphere (in absence of clouds and aerosols) and heats the surface of the earth.  The surface radiates energy in infrared wave lengths back into the atmosphere.  Some of that energy is absorbed by greenhouse gases, including carbon dioxide and water vapor, and some is re-radiated back toward the surface, thereby raising temperatures.  Why then, does the presence of water vapor, a strong greenhouse gas, cause cooling?  The answer lies in the properties of water.

First, it takes heat energy to turn liquid water into water vapor.  This is the latent heat of vaporization which absorbs heat energy without changing temperature. Likewise is the latent heat of fusion.  It takes heat energy to change ice to liquid water, again without changing temperature.

Second, is the great heat capacity of water and water vapor, i.e., the ability to store heat energy. Since hot air rises, the contained water vapor transports heat away from the surface by convection.

The third property is related to the second.  Water vapor is relatively transparent to incoming solar radiation in the visible part of the spectrum, but is an excellent absorber of infrared radiation.  This capacity warms the water vapor and heat is transported away by convection.  Sweger notes, “that the IR absorption of carbon dioxide  on solar irradiance  is of marginal effect since the absorption by water vapor is virtually 100% without any contribution from carbon dioxide.”  That may be overstating it a bit, but not by much since there is usually at least 60 times more water vapor in the atmosphere compared to carbon dioxide.

And, of course, more water vapor in the atmosphere produces clouds which reflect incoming sunlight back into space, so it never reaches the surface to contribute to the greenhouse effect.

We see from direct observation that water vapor tends to moderate global temperatures.  As Dr. Roy Spencer notes, “The question of how much warming will result from adding carbon dioxide to the atmosphere is what we skeptics are skeptical of. The climate system is amazingly complex, and the IPCC position that elements within the climate system (especially clouds) will change in ways which amplify the resulting small warming tendency is highly questionable, to say the least. If the climate system instead acts to reduce the warming, then anthropogenic global warming (AGW) becomes for all practical purposes a non-issue.”


Earth’s climate engine by Dr. Daniel M. Sweger [link]

The greenhouse effect…explored, by Carl Brehmer [link]

The Alabama two step by Dr. Roy Spencer [link]

Carbon Dioxide and the Greenhouse Effect

The “greenhouse effect,” very simplified, is this: solar radiation penetrates the atmosphere and warms the surface of the earth. The earth’s surface radiates thermal energy (infrared radiation) back into space. Some of this radiation is absorbed and re-radiated back to the surface and into space by clouds, water vapor, methane, carbon dioxide, and other gases. Water vapor is the principle greenhouse gas; the others are minor players. Without the greenhouse effect the planet would be an iceball, about 34 C colder than it is. The term “greenhouse effect” with respect to the atmosphere is an unfortunate usage because it is misleading. The interior of a real greenhouse (or your automobile parked with windows closed and left in the sun) heats up because there is a physical barrier to convective heat loss. There is no such physical barrier in the atmosphere.

Carbon dioxide is a “greenhouse” gas, so let’s examine its theoretical and actual effect on temperature.

co2greenhouse3Even the IPCC agrees that the hypothetical capacity of carbon dioxide to change temperature is given by the formula: Tc = áln(C2/C1), where Tc is the change in temperature in degrees Centigrade and the term ln(C2/C1) is the natural logarithm of the CO2 concentration at time two divided by the concentration at time one. The constant á (alpha) is sometimes called the sensitivity and its value is subject to debate. This relationship was proposed by Svante August Arrhenius, a physicist and chemist, around 1896. This logarithmic formula produces a graph in the form shown at the left. This shows that as the concentration of carbon dioxide increases, its effects have less and less influence. This graph is the pure theoretical capacity of carbon dioxide to warm the atmosphere in absence of any confounding feedbacks. The different curves represent different values of alpha.

 Radiation transmitted by atmosphereThe reason it works this way is because carbon dioxide can absorb only a few specific wavelengths of thermal radiation. The current concentration of carbon dioxide has absorbed almost all available radiation in those wavelengths so there is little left for additional carbon dioxide to absorb. Notice too, that water vapor absorbs many of the same wavelengths of thermal radiation. Also notice that in a certain part of the spectrum there is an open window of no absorption.

We see, therefore, that increasing levels of carbon dioxide in the atmosphere will have a decreasing hypothetical effect on temperature. That is also why our proposed attempts to decrease atmospheric carbon dioxide will have almost no effect on temperature.

The IPCC says that warming will produce more water vapor which will enhance greenhouse warming, a positive feedback. All their climate models are based on this assumption. Sounds reasonable except in the real world, it doesn’t happen. Increased water vapor produces more clouds which block the sun thereby inducing cooling, a negative feedback.

Dr. Roy Spencer explains here why doubling the carbon dioxide concentration in the atmosphere will add only 3% to Earth’s greenhouse effect. Spencer has further discussion here in which he says, “that about 50% of the surface warming influence of greenhouse gases has been short-circuited by the cooling effects of weather.”

The atmosphere is not static; we have weather which tends to dissipate heat into space. According to real world measurements, the negative feedbacks overwhelm the theoretical positive feedback posed by the IPCC.

An example of negative feedbacks:

In 2001, a paper by M.I.T. researchers proposed that warming dissipated high-altitude cirrus clouds which had the effect of dumping heat into space, thereby helping to regulate earth’s temperature. This paper was controversial because it went against the orthodoxy of global warming and there were many detractors. However, in 2007 researchers from the University of Alabama, using NASA satellite data found evidence to support the theory. In 2009, the original M.I.T. researchers, using National Centers for Environmental Prediction’s 16-year (1985-1999) monthly record of sea surface temperature, together with corresponding radiation data from the Earth Radiation Budget Experiment, found more real world evidence in support of the theory (see PDF). It might be noted that 11 major climate models used by the IPCC assume positive feedback, but real world data shows a temperature-moderating negative feedback. However, the role of clouds is still poorly-understood and more real-world data is needed.

What happens on other planets:


Venus has a surface temperature of about 900 F and an atmosphere composed of 96% carbon dioxide. The temperature is the same from equator to poles, from day to night (Venus rotates on its axis in 2,802 hours rather than 24 hours). Venus is often touted as the extreme example of run-away greenhouse warming. But, there is almost no greenhouse warming on Venus because little, if any, direct sunlight gets to the surface. The atmosphere is too thick. In 1975, the Russian Venus lander Venera 9 measured clouds that were 30–40 km thick with bases at 30–35 km altitude. The surface air pressure on Venus is about 92 times greater than that on Earth. The high pressure alone can explain most of the high surface temperature. Although Venus gets almost twice the solar irradiation of Earth, Venus’ high albedo reflects back 65% of the sunlight.

 Venus has almost no water vapor in the atmosphere (about 0.002%), and therefore lacks the major greenhouse gas that Earth has.


Mars has an atmosphere composed of 95% carbon dioxide and only a trace of water. Its atmosphere is very thin. Its surface pressure is about 2% that of Earth. The temperatures on the two Viking landers, measured at 1.5 meters above the surface, range from + 1° F, ( -17.2° C) to -178° F (-107° C). However, the temperature of the surface at the winter polar caps drop to -225° F, (-143° C) while the warmest soil occasionally reaches +81° F (27° C) as estimated from Viking Orbiter Infrared Thermal Mapper (NASA data). Again, no water vapor, no greenhouse effect.


The greenhouse model is a simplified story that helps explain how our atmosphere works. However, the real world is very complicated and still not fully understood. Even global warming alarmist James Hansen of NASA’s Goddard Institute for Space Studies, had this to say: “The forcings that drive long-term climate change are not known with an accuracy sufficient to define future climate change.” — James Hansen, “Climate forcings in the Industrial era”, PNAS, Vol. 95, Issue 22, 12753-12758, October 27, 1998.

And even the IPCC once admitted, “In climate research and modeling, we should recognize that we are dealing with a coupled non-linear chaotic system, and therefore that the prediction of a specific future climate state is not possible.” — Final chapter, Draft TAR 2000 (Third Assessment Report), IPCC.

Human carbon dioxide emissions are 3% to 5% of total carbon dioxide emissions into the atmosphere, and about 98% of all carbon dioxide emissions are reabsorbed through the carbon cycle. (Source )

Although Earth’s atmosphere does have a “greenhouse effect” and carbon dioxide does have a limited hypothetical capacity to warm the atmosphere, there is no physical evidence showing that human carbon dioxide emissions actually produce any significant warming. If you disagree with that statement, then produce some physical evidence to refute it.

UPDATE March 3, 2011: A new paper in Geophysical Research Abstracts (Vol. 13, EGU2011-4505-1, 2011) reports that detailed spectrographic analysis found that because of the overlap absorbance of the much more abundant water vapor for long wave radiation, the effective sensitivity of carbon dioxide and methane as greenhouse gases is only one-seventh that claimed by the IPCC and used in climate models.