The American Chemical Society’s ACS Climate Science Toolkit cuts right to the chase in an excellent analysis on its website,

…water vapor is the largest contributor to the Earth’s greenhouse effect…However, water vapor does not control the Earth’s temperature, but is instead controlled by the temperature…If there had been no increase in the amounts of non-condensable greenhouse gases (like carbon dioxide), the amount of water vapor in the atmosphere would not have changed with all other variables remaining the same. The addition of the non-condensable gases causes the temperature to increase and this leads to an increase in water vapor that further increases the temperature. This is an example of a positive feedback effect. The warming due to increasing non-condensable gases causes more water vapor to enter the atmosphere, which adds to the effect of the non-condensables.

The atmosphere is self-limiting in terms of water vapor. If a parcel or volume of air becomes saturated as it is cools, a cloud forms. Professor Adam Sobel of Columbia University provides a nice analogy,

Saying water vapor is a more important greenhouse gas than carbon dioxide is like saying the amplifier in a sound system is more important than the volume dial for producing the sound. It’s true, in a literal sense, but very misleading. CO2 and other long-lived greenhouse gases are the volume dial on the climate, and the water vapor amplifies the warming that they produce.

CO2 and other long-lived greenhouse gases are the volume dial on the climate, and the water vapor amplifies the warming that they produce.

 

Texas A&M’s Andrew Dessler’s work on the importance of water vapor in climate warming also discusses these complex interactions and amplifications.

Greenhouse effect: Part 1, incoming shortwave radiation. Source: Barb Deluisi, NOAA.

Let’s step back and review the Greenhouse Effect. Ironically, the most abundant gases (nitrogen and oxygen) exhibit virtually no warming affect. A small collection of minor gases representing about 0.43% of the atmospheric constituency allows us to live on Earth.   Greenhouse gases (GHGs) include: water vapor, carbon dioxide, methane, nitrous oxide, and sulfur hexaflouride. If teaching Atmospheric Physics or Radiative Transfer (staples of good meteorology/climate curricula), I would refer to them as selective absorbers. Selective absorbers absorb and emit energy at preferential wavelengths of the electromagnetic spectrum. For example, in the stratosphere, ozone is a selective absorber of ultraviolet (UV) radiation (thank goodness for us). GHGs selectively absorb in the infrared portion of the spectrum. A refresher of blackbody radiation, the foundation of the statement that “good absorbers are good emitters” can be found here.

Greenhouse effect. Part 2, Absorption and emission of infrared radiation. Source: Barb Deluisi,… [+]

The sun drives the climate system, but I often see its influence misrepresented due to lack of understanding of the complexity of the climate’s radiative system. The sun’s energy is mostly shortwaveradiation (visible and UV). Much of it gets reflected or absorbed by the clouds, atmosphere, or Earth’s surface. The absorbed energy is emitted from the surface as longwave radiation or infrared radiation (heat). Wien’s Law says that objects at different temperatures emit at different wavelengths. The Earth is cooler than the sun so emits at longer wavelengths (i.e., there is an inverse relationship between temperature and maximum wavelength of emission). The longwave radiation from Earth is emitted to space or absorbed by GHGs. GHGs reemit the longwave energy to space, other gases, or back to Earth. The astute reader will note that the atmospheric “greenhouse” effect is really a series of absorption-emission processes rather than just heat entrapment (as a real greenhouse does).

According to NASA, water vapor accounts for about 50+% of the absorption. Carbon dioxide accounts for 20%. Clouds also account for roughly 25%. Atmospheric physics theory dating back to Arrhenius (1895) and Callendar (1938), respectively, have pointed out how relatively small contributions of CO2 can warm the atmosphere. Yet, many mistakenly latch on to the fact that water vapor is dominant and not the aforementioned feedback processes. Dr. Gavin Schmidt writes,

…as temperature rises, the maximum sustainable water vapor concentration increases by about 7% per degree Celsius. Clouds too depend on temperature, pressure, convection and water vapor amounts. So a change in CO2 that affects the greenhouse effect will also change the water vapor and the clouds. Thus, the total greenhouse effect after a change in CO2 needs to account for the consequent changes in the other components as well. If, for instance, CO2 concentrations are doubled, then the absorption would increase by 4 W/m2, but once the water vapor and clouds react, the absorption increases by almost 20 W/m2 — demonstrating that (in the GISS climate model, at least) the “feedbacks” are amplifying the effects of the initial radiative forcing from CO2 alone.

People are often surprised to learn that moist air is less dense than dry air or that deserts can actually get cold at times. Both of these simple assertions are wrong. This article cautions readers that oversimplification is very dangerous.