Relative importance of greenhouse gases in relation to temperature and humidity

Kiehl and Trenberth estimated in their 1997 paper that the most important greenhouse gases are water vapour (60%) and CO2 (26%), by U.S. Standard Atmosphere from 1976. We expanded their calculations for six different standard atmospheres, three cold and dry Arctic and three southern hot and humid radiosonde measured profiles to see, how the greenhouse gases proportions are related to atmosphere humidity content at cloudless sky conditions. We used radiative transfer model FUTBOLIN to calculate column spectral upward radiances of greenhouse gases for different atmospheric compositions, including double CO2, double CH4 and tenfold CH4 cases. We calculated radiances by removing one greenhouse gas by one from the atmosphere to estimate relative importances.

For the standard atmosphere, we got similar proportions to Kiehl and Trenberth - 62% for H2O and 26% for CO2. It turned out, that H2O and CO2 proportions in greenhouse effect has strong linear relation with the surface temperature - H2O importance increases by 0.35% and CO2 importance decreases by 0.27% with 1 K temperature increase. Relation with atmospheric humidity content is linear with a hogged end. At precipitable water level PW = 1 mm the proportions are 40% for both H2O and CO2 but above level PW = 15 mm the proportions are in all cases about 65% for H2O and 25% for CO2. Due to 10% increase of CO2 during period 2000-2017 CO2 importance as greenhouse gas has increased 0.4%. CO2 increase to double of year 2000 concentration (737 ppmv) would increase CO2 importance by 2.3%. CH4 importance as greenhouse gas would increase by 0.6% to double of CH4 concentration and by 2.8% of tenfold of CH4.

Rising CO2 has the strongest effect in cold regions and seasons. Still, the effect is not big enough to explain the Arctic amplification. Anthropogenic CH4 has strong attention as greenhouse gas, but even at tenfold concentration its radiational contribution to the greenhouse effect would be small.