CO2 Versus Water

CO2, water vapour and clouds have the most significant contributions to the greenhouse effect. Various sources give conflicting estimates of the contributions of these components to the greenhouse effect. The infrared absorption spectrum of atmospheric greenhouse gases is very complex. In some regions absorption frequencies of various greenhouse gases overlap, so the contributions of each component do not add linearly. Radiation at a particular frequency can be absorbed by either water vapour or CO2. The concentration of water vapour is dependent on temperature and varies greatly by both latitude and altitude. Also, water changes from a liquid to a gas with heat energy for the latent heat of evaporation required for the transformation.

Most sources put the greenhouse effect at 33 oC. This is the difference between the current air surface temperature (15 oC) and temperature without the greenhouse effect of gases and clouds, but with the clouds continuing to reflect 31% on the incoming solar radiation.

Nature does not allocate the contribution of various greenhouse gases - only the total effect is meaningful. Nevertheless, a rough estimate of the contributions can be made. The relative contribution of water, clouds and CO2 to the greenhouse effect can be estimated in two ways; by estimating from radiation models the change to the greenhouse effect by removing one component, and by estimating the greenhouse effect of having only that one component in the atmosphere. If one removes the water vapour & clouds' greenhouse effect the remaining components would trap 34 percent of the heat, implying that water vapour & clouds would trap 66 percent as shown in the "Heat Not Trapped" column of the table below. The sum of the components calculated this way is only 80% of the greenhouse effect due to overlapping absorbing spectra. Similarly, if one includes only water vapour and clouds (no CO2, O3 or Other), they would trap 85% of the long wave radiation. However, the contributions of each component adds up to 126% of the greenhouse effect.

 It is reasonable to just allocate the overlap proportionally to each component, so the effect is normalized in the "Relative Effect" columns so the sum of the effects equals 100%. This calculation suggests that water vapour & clouds contribute 70% to 80%, and CO2 contributes 10% to 20% of the greenhouse effect as shown in the table below:

                               Change in Long-wave Absorption
Remove Component Heat Trapped
Heat Not Trapped
Relative Effect
Component Only Heat Trapped
Relative Effect
Average of Methods

Water & clouds 34
82.5% 85
CO2 91
11.3% 26
O3 97
3.8% 7
Other 98

This gives a rough estimate of component contribution to the current total greenhouse effect, but this tells us almost nothing of the incremental effect of changing the concentration of a component.

Water vapour is the most important gas of the Greenhouse Effect. Water vapour is usually considered a feedback, while CO2 is considered a forcing because the residence time of a change in water vapour concentration is very short compared to CO2. Human caused water emissions (other than high altitude airplanes) do not have a significant effect on climate, but water can have a significant effect as a feedback on a temperature change initiated by the Sun or CO2 emissions.

If one magically removed 20% of all water vapour in  the atmosphere, water will quickly evaporate from the oceans to replace it so that in 20 days the water concentration will be 99% of the original value as the graph below shows.

Water Removal Effect
Likewise, if humans suddenly doubled our water emissions from the surface, in a few days the increased water vapour will rain out leaving the water vapour concentration almost unchanged. The above graph and absorption values were calculated using the Goddard Institute for Space Studies General Circulation Model. See here.

These calculations do not include the effects of airplanes.  It is so cold at the elevation that airplanes fly that there is virtually no water vapour. The only time water gets that high is when high ground temperatures cause thermal uplift bringing water up with it. It is too cold up there for water to exist as vapour so droplets form and we see this as airplane vapour trails. These are artificial clouds of the type that traps infrared radiation but passes sunlight therefore creating a warming effect. Water vapour injected into the upper atmosphere has a much longer residence time than water injected into the atmosphere near the surface, so it may have a minor effect on climate.