Does water vapour not carbon dioxide rule global warming? One frequently-made challenge by so-called ‘climate skeptics’ to the hypothesis that human-caused carbon dioxide (CO2) emissions will/are significantly contributing to global warming) is that:

• compared to water vapour, CO2 is a minor Greenhouse gas, and
• not fully factoring water vapour into the equation hugely exaggerates the possible role of human activities in global warming

Geocraft’s Monte Hieb summarises the issue: “Just how much of the Greenhouse Effect is caused by human activity? About 0.28% if water vapour is taken into account, about 5.53% if water vapour is (left out of the equation)”.

Other information from Monte’s website includes:

Water vapour accounts for about 95% of Earth’s Greenhouse Effect [1] and is 99.999% of natural (i.e. not human) origin

Despite the above, the US Department of Energy [2] chooses not to include water vapour in its comparison of Greenhouse gases. Taking only CO2, methane, nitrous oxide and ‘miscellaneous (minor greenhouse) gases” into account caused the department to allocate 72.37% to CO2. Had they included water vapour, CO2’s share would have been just 3.618%

The relatively small amounts of the powerful greenhouse gases methane and nitrous oxide are also mostly of natural origin. The human contributions to these are 18% and 5% respectively

If one multiplied the adjusted CO2 contribution to the Greenhouse effect (3.618%) by the proportion of atmospheric CO2 thought to be due to human activities (3.225%), one might deduct that only a tiny part (0.117%) of the Greenhouse Effect due to CO2 is currently due to human activity. The share in the Greenhouse Effect of the other well-known Greenhouse gases are similarly recalculated and summarised in the table below.

Research by geochemist Dr. Wallace Broecker, a geochemist at Columbia’s Lamont-Doherty Earth Observatory, led him to conclude that water vapour may have been the single most important factor in much of the climate change over the last 10,000 years. He found, for instance, that air reaching glaciers during the last Ice Age had less than half the water vapour content of today [3]

TABLE 4a. Natural/Anthropogenic (human-made) contribution to the Greenhouse Effect of major greenhouse gases

                     Greenhouse Effect  Natural source  Human source
                             %                        %                    %

Water vapour       95.000                94.999               0.001
Carbon Dioxide      3.618                 3.502                0.116
Methane               0.360                  0.294               0.066
Nitrous Oxide        0.950                  0.903               0.047
Other gases          0.072                  0.024               0.048
Total                 100.000                99.722               0.278

Ed.- (i) Pointing out that the ability of humans to influence atmospheric water vapour concentrations is negligible, Monte wonders whether this could be the reason the significance of water vapour has been played down. In one prominent place the IPCC does, after all, focus its mission to studying “the risk of human-induced climate change”.

(ii) During the Late Ordovician Period around 450 million years ago there was an Ice Age when atmospheric CO2 concentrations were nearly twelve times higher than today (4,400 ppm). Clearly, other factors besides atmospheric CO2 were also influencing temperatures and global warming.

(iii) According to Professor Robert Essenhigh, the radiation absorption coefficients (warming potentials) of water vapour and carbon dioxide particle by particle are almost identical, but the Earth’s atmospheric water vapour concentration averages out at 10,500 parts per million (ppm)3 compared to CO2’s 380ppm. This means that water vapour is causing on average around 27 times the warming of CO2, very close to the figures used by Monte above. Furthermore, Robert informs us, the average radiation absorption coefficient of the atmosphere as a whole varies between 5.6 and 7.6µm, which is what one would expect if atmospheric water vapour concentrations varied between 60% and 80%.

Please note that neither Monte Hieb nor Robert Essenhigh claim to be climate change experts. For some this rules out their thinking and conclusions. For others it makes them more objective, given the reluctance of most climate scientists to challenge the new orthodoxy demonising carbon dioxide and (rightly for other reasons) short-sighted human behaviour.

(iv) The amount of water vapour the atmosphere can hold is almost purely a function of temperature - the warmer the air gets, the more vapour it is able to hold. We know, for example, that the atmospheric water vapour concentration over the slowly warming oceans has increased by 0.41 kilograms per square meter every ten years since 1988

(v) According to the IPCC, if CO2 emissions were to double, that and the additional water vapour the warmer air would be able to hold would probably turn a predicted 2oC rise in average global temperature into a 3.6oC rise