Radiative equilibrium temperature profiles in the middle atmosphere at high CO2 concentrations

The middle atmosphere can turn to the stage of warming with increase in well-mixed greenhouse gasses (WMGHGs) if the atmospheric WMGHGs' concentrations are very high. Here we investigate the characteristics of radiative equilibrium temperature profiles at high CO₂ concentrations, which are within the range of the CO₂ level in the Neoproterozoic Snowball Earth era (∼2×10⁴ Pa of CO₂, i.e., ∼2.0×10⁵ ppm CO₂ with defining the ppm as the proportion of CO₂ number density to the present air number density). We set 7 model atmospheres whose concentrations of CO₂ are 330, 660, 10³, 5×10³, 10⁴, 5×10⁴, 10⁵ ppm, and whose concentrations of the other radiatively active gases (such as O₃, H₂O, and other four gasses) are fixed values taken from the US standard atmosphere (1976). We calculated radiative equilibrium temperature profiles with employing the Rapid Radiative Transfer Model (RRTM) for heating rate calculations. As the results, we found that the higher CO₂ concentration is, the more upward the isothermal layer extends in the lower stratosphere when the CO₂ concentration is less than 10⁴ ppm, and the typical shape of the temperature maximum in the middle atmosphere (i.e., the stratopause) almost disappears when the CO₂ concentration is 5×10³ and 10⁴ ppm. In the 5×10⁴ and 10⁵ ppm CO₂ atmospheres, however, a portion of the stratosphere ranged from around 40 to 50 km heights turns into the stage of warming with further increase in CO₂, thus a small stratospheric temperature maximum is generated at around 50 km height, probably owing to the larger longwave heating by CO₂ in addition to the shortwave heating by O₃. Since our calculations exceed the expected range of temperatures and pressures in the RRTM's look-up table for the absorption coefficient calculation, we attempt to improve these problems at the presentation. Besides, introducing the convective adjustment and water vapor feedback may be taken into account at the next step.