Radiative forcing of carbon dioxide and methane in the troposphere of Lower Volga region

According to the international IPCC climate change reports, the concentration of CO₂ and CH₄ in the atmosphere has increased by about a quarter over the past 50 years. The increase in the concentration of greenhouse gases such as CO₂, CH₄, etc. causes a rise in the temperature of the atmosphere and underlying surface and, as a consequence, stronger H₂O evaporation, causing positive forcing, and the processes of cloud and aerosol formation lead to a change in the Earth's albedo (negative forcing). According to the data  [1], the CH₄ forcing   over the industrial period (from 1750 to 2011) is underestimated by about 25% in the IPCC results [2]. One of the factors of this uncertainty is the inaccurate accounting of  overlap of the CH₄ absorption bands with the H₂O bands and continuum, and interaction with clouds. Water vapor is a conservative matter, its concentration depends on the air temperature and the type of underlying surface, therefore the increase of water vapor concentration in the atmosphere is caused by positive feedback. This in turn accelerates the greenhouse effect. Therefore, the study of the connections of instantaneous radiative forcing of CO₂ and CH₄ with the total water vapor content in the atmosphere will allow us to clarify the role of CO₂ and CH₄ in the greenhouse effect.

 
The longwave radiation fluxes were calculated: upward at  upper boundary and downward at  lower boundary of the cloudless atmosphere for typical meteorological conditions of the Lower Volga region. ECMWF ERA-5 – European Reanalysis data for summer conditions of 2021 were used. This year was chosen because the average water vapor content in vertical column of the atmosphere in summer (WH₂O = 2.74 g /cm²) was higher than the average for other years. A sample of 368 vertical profiles (4 realizations per day over three summer months) was used for the simulation. The radiative forcing of CO₂ and CH₄ was calculated as the difference in radiative fluxes at the concentration values ​​of these gases corresponding to 1973 year and the present time.

The modeling results showed that with increasing humidity, the contribution of CO₂ to the radiative forcing on the Earth's surface decreases, which leads to less surface heating, but the heating of the atmosphere increases. The enhancement of the greenhouse effect due to the increase in CO₂ concentration under conditions of high humidity leads to even greater heating of the atmosphere. It turned out that at high humidity the atmospheric transparency window closes, i.e. the dominant role in these processes is played by the continuum of water vapor, and not by selective absorption in the H₂O bands.

It was found that, unlike CO₂, the dependence of the CH₄ radiative forcing on total water vapor content in the troposphere is insignificant, and with a rise of the water vapor concentration, the heating of the atmosphere due to the change in CH₄ concentration  over the past 50 years has not increased. At that, the CH4 radiative forcing in the troposphere was in the range of 0.1-0.2 W/m², while the CO₂ forcing was 0.8-1.4 W/m² [3].

The research was supported by the Ministry of Science and Higher Education of the Russian Federation (IAO SB RAS).

1. M. Etminan, G. Myhre, E.J. Highwood,  K.P. Shine.// Geophys. Res. Lett. 2016. V.43. N.24. P.12,614–12,623 

2. IPCC report Climate Change 2013, The Physical Science Basis 

https://ipcc.ch/report/ar5/wg1/

3. K.M. Firsov, T.Yu. Chesnokova,  A.A. Razmolov // AOO. 2024. V.37, N.6. P.689–697