If you read many articles and comments in the blogosphere you would think that “skeptics” have discovered something hidden. Or highlighted an important truth that climate science is trying to hide.
Water vapor is actually the dominant “greenhouse” gas
This is true.
If only climate science actually realized it and stopped pretending that CO2 was the most important “greenhouse” gas..
If Only They Wrote it Larger..
For terrestrial radiation, water vapor is the most important single constituent of the lower atmosphere, although carbon dioxide is always significant..
Atmospheric Radiation: Theoretical Basis, Goody & Yung, Oxford University Press (1989, 2nd edition)
Water vapor is the most important atmospheric greenhouse gas.. Carbon dioxide is the second most important greenhouse gas..
Radiation and Climate, Vardavas & Taylor, Oxford University Press (2007)
Generally speaking, water vapor is the single most important atmospheric absorber in the IR band..
No other atmospheric constituent is better known to the general public as a “greenhouse gas” than CO2. In actuality, water vapor has a larger overall impact on the radiative energy budget of the atmosphere..
A First Course in Atmospheric Radiation, Grant Petty, Sundog Publishing (2006)
Water vapor is the most important gas for the transfer of radiation in the atmosphere..
Global Physical Climatology, Hartmann, Academic Press (1994)
Table 6 shows the relative contributions of H2O, CO2 and O3 to reducing the outgoing longwave flux, from which it is seen that the longwave effect of H2O is significantly larger than the effects of CO2 and O3..
Climate Modeling through Radiative-Convective Models, Ramanathan & Coakley, Reviews of Geophysics and Space Physics (1978)
The importance of water vapor in regulating climate is undisputed. It is the dominant greenhouse gas, trapping more of Earth’s heat than any other gaseous constituent..
The Radiative Signature of Upper Tropospheric Moistening, Soden, Jackson, Ramaswamy, Schwarzkopf & Huang, Science (2005)
The dominant role of water vapor as a greenhouse gas has long been noted..
The Importance and Nature of the Water Vapor Budget in Nature and Models, Lindzen, Climate Sensitivity to Radiative Perturbations: Physical Mechanisms and Their Validation (1996)
The authors find that for the clear sky case the contribution due to water vapor to the total longwave radiative forcing is 75 W/m², while for carbon dioxide it is 32 W/m²..
Earth’s Annual Global MeanEnergy Budget, Kiehl & Trenberth, Bulletin of the American Meteorological Society (1997)
Water vapor is the dominant greenhouse gas, the most important gaseous source of infrared opacity in the atmosphere..
Water Vapor Feedback and Global Warming, Held & Soden, Annual Review Energy Environment (2000)
In fact, it’s so well-known that most times in papers it isn’t repeated. No one involved in atmospheric physics is confused about the subject.
Why the focus on CO2 in that case?
Water vapor arguably lies at the heart of all key terrestrial atmospheric processes. Humidity is essential for the development of disturbed weather, influences (directly and indirectly through cloud formation) the planetary radiative balance, and influences surface fluxes and soil moisture. Water vapor is the only radiatively important atmospheric constituent that is sufficiently short‐lived and abundant in the atmosphere so as to be essentially under purely natural control..
Tropospheric Water Vapor, Convection & Climate, Sherwood, Roca, Weckwerth & Andronova, Review of Geophysics (2010)
The point is that water vapor responds to climate – and therefore influences climate as a feedback. The concern is that humans adding CO2 to the atmosphere will cause a change to the climate and water vapor will have a feedback effect (see, for example, Clouds and Water Vapor – Part One and subsequent articles).
It is quite difficult for humans to add water vapor to the atmosphere. The oceans are a vast source of water, and just above the surface of the ocean the atmosphere is saturated
Why isn’t there more focus on water vapor?
There is a huge focus on water vapor because it is a difficult subject. CO2 is well-mixed in the atmosphere so the application of radiative transfer theory to CO2 is not in debate (in scientific circles).