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The Claim: Water is a much stronger and more plentiful greenhouse gas than carbon dioxide. Therefore changes in carbon dioxide make negligible contributions to the greenhouse effect.

**__Table of Contents:__**
 * 1) **"Not all Molecules are Created Equal," the Differences Between CO2 and Water**
 * 2) The Differences in IR Absorption for Water and CO2
 * 3) Dispelling the "98%" Notion
 * 4) CO2's Persists in the Atmosphere, Water Does Not
 * 5) **The Significance of the Differences Between CO2 and Water**
 * 6) Understanding Positive Feedback and Climate Forcing
 * 7) The "Water Vapor Feedback" Effect
 * 8) **CO2 and Water in the Earth's Atmosphere**
 * 9) Life Span of a CO2 Molecule
 * 10) Life Span of Water
 * 11) **Conclusion**
 * 12) ** Sources **

Claims that water vapor is abundant, plentiful, and a powerful greenhouse gas are irrefutable, but the prevalence of water vapor in the atmosphere hardly justifies considering the effects of other greenhouse gases "negligible." In fact, changes to CO2 levels in the atmosphere will lead to global warming. ** "Not all molecules are created equal" The Differences between CO2 and Water ** The most fundamental difference between CO2 and water vapor, as greenhouse gases, is the way that they absorb infrared radiation off of earth's surface. Shown below is a graph of absorption vs. wavelength for many important greenhouse gases. In the graph, it is clear that the wavelengths of light that CO2 and H2O absorb are different [|(Watkins)]. There is some IR light that evades being absorbed by a specific gas that is absorbed by different gases, conversely there is some IR light that can be absorbed by many different greenhouse gases. This is important because a specific gas has the capacity to absorb and re-emit a lot more IR if there are no other gases in the vicinity absorbing the same wavelength. Although water clearly absorbs the most wavelengths of IR, there are many times that water concentration in the air (humidity) is very low. When humidity is low, carbon dioxide is the sole greenhouse gas absorbing many different wavelengths of IR light. In this sense, there is not a single "dominant" greenhouse gas, it is better to view greenhouse gases as a different entities, that happen to have the same effect [|(Watkins)]. Taken from: http://www.iitap.iastate.edu/gccourse/forcing/images/image7.gif  ** Dispelling the "98%" misconception ** According to Richard Lindzen in an 1991 report, 98% of the greenhouse effect is accounted for by water. This number of 98% was quoted by Lindzen from the 1990 IPCC but it's accuracy is questionable to say the least (as is Lindzen's claim that it was in the 1990 IPCC). This is not to under exaggerate the effects of water, water vapor accounts for about 90% of greenhouse gases in volume and 80% in mass and it accounts for anywhere between 36% and 66% of the greenhouse effect Including the effect of clouds, water accounts for anywhere between 66 and 85% of the greenhouse effect. CO2 although it accounts for far less mass and volume than water still accounts for nearly 9% and 26% of the greenhouse effect [|(Gavin 2005)]. The disparity between Lindzen's estimate of CO2 accounting for <2% of the greenhouse effect and the actually figure, which states that CO2 accounts for at least 9% of the greenhouse effect, is massive.
 * The Differences in IR Absorption Spectra of Water and CO2 **

**The Significance of the Differences Between CO2 and Water** ** Understanding Positive Feedback and Climate Forcing   ** In order to understand exactly how and why carbon dioxide has the capacity to be so devastating, it is important to understand the concept of **positive feedback**. Simply put, a positive feedback is when you make a small change to something, and this small change triggers a series of changes which all add to the original change (Wolfson 2008 ). One great example of positive feedback is "the snowball effect." If you push a snowball ever so slightly and it starts to roll down a hill, your small push leads to a slow roll, then a fast roll, and the snowball adds more and more snow to itself. The small push that started the small ball rolling leads to a massive, faster moving snowball. The act of the ball rolling down the hill is considered positive feedback, but when one considers how the snowball ended up so big, it was not really the rolling that caused the ball to get so big, but rather, the initial small push. This push can be considered a **forcing**.The name "forcing" is logically based on the physical concept of a force. Just as applying a force to a system nets a change, so too does climate forcing net a change. Both of these phenomena, forcing and positive feedback are very important to climate change. But it is worth reiterating that if something is at equilibrium, feedback can not occur unless some external forcing occurs first (Wolfson 2008 ).

Probably the single most important reason to fear water as a greenhouse gas is the fact that if the world heats up even a little bit, water has the capacity to begin a positive feedback process. Suppose there is an increase in the amount of CO2 in the atmosphere. This higher concentration of CO2 will enhance the greenhouse effect and result in the warming of the surface of the earth. This surface warmning thereby increases the evaporation of surface waters by water vapor that is already in the air; the more water vapor that is in the air further increases the greenhouse effect and the temperature of the earth rises more than it would have if this positive feedback didn't exist (Wolfson 2008). While water vapor feedback is difficult to measure, computer climate models suggest that water vapor increases the earth's response to radiative forcing by about 50 percent more than what would occur without this feedback, showing that this greenhouse gas plays a very significant role in the greenhouse effect (Wolfson 2008).
 * The "Water Vapor Feedback" Effect **

Taken from: http://earthobservatory.nasa.gov/Newsroom/NasaNews/ReleaseImages/20040315/02_humiditym.jpg The video below, excerpts, from an interview with noted IPCC scientist Richard Wasdell helps to explain the concept of positive feedback in terms of climate change: media type="youtube" key="LFHDQs19ilk" height="344" width="425"

 **CO2 and Water Vapor in the Earth's Atmosphere** **Life-span of a CO2 molecule ** The average life span of a CO2 molecule in the earth's atmosphere is approximately 1,000,000 years, as shown in the chart below ([|Blasing 2008]). It is emitted through the natural carbon cycle and through human activities such as the burning of fossil fuels. Every CO2 molecule emitted by the combustion of petroleum, coal, and natural gas remains in the atmosphere for many years. Fortunately, the earth has natural ways of removing billions of tons of atmospheric CO2 with oceans and plants, also known as "sinks". These sinks annually emit and absorb carbon dioxide in to and out of the atmosphere. When the emissions and absorptions are relatively equal, the natural sources are said to be in "balance" ([|"Carbon Dioxide"]). Humans, on the other hand, have no plausible way to absorb any carbon dioxide they put in to the atmosphere. These billions of tons of anthropogenic CO2 have significantly increased the atmosphere's CO2 concentration and in return, increased the greenhouse effect. With more and more burning of fossil fuels, the inability of natural sinks to counteract the human contribution of CO2, and the lengthy life-span of each molecule, changes in carbon dioxide make significant contributions to the greenhouse effect. Taken from: http://cdiac.ornl.gov/pns/current_ghg.html
 * ~ GAS ||~ Pre-1750 tropospheric concentration1 ||~ Current tropospheric concentration2 ||~ GWP3(100-yr time horizon) ||~ Atmospheric lifetime4(years) ||~ Increased radiative forcing 5 (W/m2) ||
 * ~ Concentrations in parts per million (ppm) ||
 * Carbon dioxide (CO2) || 2806 || 383.97 || 1 || ~ 1004 || 1.66 ||
 * ~ Concentrations in parts per billion (ppb) ||
 * Methane (CH4) || 7008 || 18579/17359 || 25 || 124 || 0.48 ||
 * Nitrous oxide (N2O) || 27010 || 3219/3209 || 298 || 1144 || 0.16 ||
 * Tropospheric ozone (O3) || 251 || 344,1 || n.a.4 || hours-days || 0.354 ||
 * ~ Concentrations in parts per trillion (ppt) ||
 * CFC-11 (trichlorofluoromethane) (CCl3F) || zero || 2469/2439 || 4,750 || 45 || 0.063 ||
 * CFC-12 (CCl2F2) || zero || 5419/5379 || 10,900 || 100 || 0.17 ||
 * CF-113(CCl2FFClF2) || zero || 779/779 || 6,130 || 85 || 0.024 ||
 * HCFC-22(CHClF2) || zero || 1979/1759 || 1,810 || 12 || 0.033 ||
 * HCFC-141b(CH3CCl2F) || zero || 219/179 || 725 || 9.3 || 0.0025 ||
 * HCFC-142b(CH3CClF2) || zero || 209/179 || 2,310 || 17.9 || 0.0031 ||
 * Halon 1211 (CBrCIF2) || zero || 4.49/4.29 || 1,890 || 16 || 0.001 ||
 * Halon 1301 (CBrCIF3) || zero || 3.29/3.19 || 7,140 || 65 || 0.001 ||
 * HFC-134a(CH2FCF3) || zero || 499/419 || 1,430 || 14 || 0.0055 ||
 * Carbon tetrachloride (CCl4) || zero || 909/889 || 1,400 || 26 || 0.012 ||  ||
 * Methyl chloroform (CH3CCl3) || zero || 12.79/12.19 || 146 || 5 || 0.0011 ||
 * Sulfur hexafluoride (SF6) || zero || 6.409,11/6.039,11 || 22,800 || 3200 || 0.0029 ||
 * Other Halocarbons || zero || Varies by substance ||  ||   || collectively 0.021 ||

Water is continually cycling throughout the earth's atmosphere; evaporating from the surface, condensing to form clouds, and precipitating back to the earth, as shown in the diagram below. Infrared radiation coming from the sun causes water to evaporate and heat is expelled once the clouds turn to precipitation ([|Water Vapor in the Climate System]). Water vapor is the crucial greenhouse gas behind the comfortable climate of the earth; without water vapor the earth would be freezing and inhabitable. Similar to CO2, an increase in the concentration of water vapor enhances the greenhouse effect through a positive feedback. An increase in the surface temperature of the earth, due to an increased amount of greenhouse gases, causes an increase in water vapor and an overall warming of the earth. Taken from: http://berkeley.edu/news/media/releases/2006/02/images/watercycle680.jpg
 * Life Span of Water **

** Conclusion ** It is irrifutable that water vapor is the most abundant of the greenhouse gases, but that does not mean changes in the other greenhouse gases make negligible contributions to the greenhouse effect. Every greenhouse gas differs; from absorption of infrared radiation to atmospheric lifetime. Human production of CO2 from the burning of coal and natural gases and the inability to absorb emisions such as carbon sinks do, is subsequently fueling the fire to a warmer earth. Carbon dioxide accounts for 9 percent of the greenhouse effect and with a lengthy lifetime of 1,000,000 years in the atmosphere it is clear that carbon dioxide's contribution to the greenhouse effect cannot be neglected.

**__ Sources: __**

Blasing, TJ. "Recent Greenhouse Gas Concentrations." __Carbon Dioxide Information Analysis Center__. Dec. 2008. 19 Mar. 2009 [].

"Carbon Dioxide." __Climate Challenge - Greenhouse Gas Emissions__. U.S. Environmental Protection Agency. 19 Mar. 2009 [].

Gavin, (2005,4,6). Water vapour: feedback or forcing?. from [].

__Water Vapor in the Climate System__. Rep. no. 0-87590-865-9. 1995. American Geophysical Union. 19 Mar. 2009 [].

Watkins, Thayer Saturation, Nonlinearity and Overlap in the Radiative Efficiencies of Greenhouse Gases. Retrieved March 19, 2009, from applet-magic.com Thayer Watkins Silicon Valley & Tornado Alley USA Web site: http://www.sjsu.edu/faculty/watkins/radiativeff.htm.

Wolfson, Richard (2008). //Energy, Enviroment, and the Climate//. New York, NY: Norton & Co..