[Paper Review] Dependence of Earth's Thermal Radiation on Five Most Abundant Greenhouse Gases

W. A. van Wijngaarden, W. Happer|arXiv (Cornell University)|Jun 4, 2020

Atmospheric and Environmental Gas Dynamics28 references22 citations

TL;DR

This study uses line-by-line radiative transfer calculations with over 330,000 HITRAN spectral lines to quantify the radiative forcing of Earth's five most abundant greenhouse gases—H₂O, CO₂, O₃, N₂O, and CH₄—under current and optically thin atmospheric conditions. It finds that per-molecule forcing is suppressed by up to four orders of magnitude for H₂O and CO₂ due to saturation, while less abundant gases like CH₄ retain significantly higher per-molecule forcing, and doubling CO₂, N₂O, or CH₄ increases forcing by only a few percent.

ABSTRACT

The atmospheric temperatures and concentrations of Earth's five most important, greenhouse gases, H$_2$O, CO$_2$, O$_3$, N$_2$O and CH$_4$ control the cloud-free, thermal radiative flux from the Earth to outer space. Over 1/3 million lines having strengths as low as $10^{-27}$ cm of the HITRAN database were used to evaluate the dependence of the forcing on the gas concentrations. For a hypothetical, optically thin atmosphere, where there is negligible saturation of the absorption bands, or interference of one type of greenhouse gas with others, the per-molecule forcings are of order $10^{-22}$ W for H$_2$O, CO$_2$, O$_3$, N$_2$O and CH$_4$. For current atmospheric concentrations, the per-molecule forcings of the abundant greenhouse gases H$_2$O and CO$_2$ are suppressed by four orders of magnitude. The forcings of the less abundant greenhouse gases, O$_3$, N$_2$O and CH$_4$, are also suppressed, but much less so. For current concentrations, the per-molecule forcings are two to three orders of magnitude greater for O$_3$, N$_2$O and CH$_4$, than those of H$_2$O or CO$_2$. Doubling the current concentrations of CO$_2$, N$_2$O or CH$_4$ increases the forcings by a few per cent. These forcing results are close to previously published values even though the calculations did not utilize either a CO$_2$ or H$_2$O continuum. The change in surface temperature due to CO$_2$ doubling is estimated taking into account radiative-convective equilibrium of the atmosphere as well as water feedback for the cases of fixed absolute and relative humidities as well as the effect of using a pseudoadiabatic lapse rate to model the troposphere temperature. Satellite spectral measurements at various latitudes are in excellent quantitative agreement with modelled intensities.

Motivation & Objective

• To quantify the radiative forcing of Earth’s five most abundant greenhouse gases—H₂O, CO₂, O₃, N₂O, and CH₄—under realistic atmospheric conditions.
• To assess how per-molecule radiative forcing is suppressed by saturation and spectral overlap among greenhouse gases.
• To evaluate the impact of doubling CO₂, N₂O, and CH₄ concentrations on top-of-atmosphere thermal radiation and surface temperature.
• To compare model-simulated top-of-atmosphere spectral intensities with actual satellite measurements for validation.
• To investigate the role of water vapor feedback and atmospheric temperature profiles in determining climate sensitivity.

Proposed method

• Line-by-line radiative transfer calculations were performed using over 330,000 rovibrational spectral lines from the HITRAN database with intensities as low as 10⁻²⁷ cm.
• The atmosphere was divided into 500 altitude segments, with temperature and gas concentration profiles defined using standard midlatitude atmospheric data.
• Radiative-convective equilibrium was modeled to estimate surface temperature changes under CO₂ doubling, incorporating fixed absolute and relative humidity scenarios.
• A pseudoadiabatic lapse rate was used to model tropospheric temperature profiles, accounting for latent heat release from condensing water vapor.
• Top-of-atmosphere (TOA) spectral fluxes were computed and compared to satellite measurements across diverse climate zones, including the Sahara, Mediterranean, and Antarctica.
• The absence of a CO₂ or H₂O continuum in calculations was validated by comparing modeled TOA intensities with observed satellite spectra, showing excellent agreement.

Experimental results

Research questions

• RQ1 How does per-molecule radiative forcing vary for H₂O, CO₂, O₃, N₂O, and CH₄ under optically thin versus current atmospheric conditions?
• RQ2 To what extent is radiative forcing suppressed by saturation and spectral overlap among greenhouse gases?
• RQ3 What is the change in surface temperature due to CO₂ doubling when accounting for radiative-convective equilibrium and water vapor feedback?
• RQ4 How well do line-by-line model calculations reproduce satellite-measured TOA spectral intensities without including a CO₂ or H₂O continuum?
• RQ5 How does climate sensitivity vary under fixed absolute humidity, fixed relative humidity, and pseudoadiabatic lapse rate assumptions?

Key findings

• Per-molecule radiative forcing for H₂O and CO₂ is suppressed by four orders of magnitude at current concentrations due to extreme saturation, while O₃, N₂O, and CH₄ experience less suppression.
• For current concentrations, the per-molecule forcing of O₃, N₂O, and CH₄ is two to three orders of magnitude greater than that of H₂O or CO₂.
• Doubling CO₂, N₂O, or CH₄ concentrations increases radiative forcing by only a few percent due to saturation effects.
• The model-simulated top-of-atmosphere spectral intensities show excellent agreement with satellite measurements across diverse latitudes, validating the line-by-line approach.
• Climate sensitivity is estimated at 1.4 K for fixed absolute humidity and 2.2–2.3 K for fixed relative humidity with a pseudoadiabatic lapse rate, consistent with other studies.
• The absence of a CO₂ or H₂O continuum in the calculations has negligible impact on TOA flux, as confirmed by satellite data agreement.