[Paper Review] Clouds in the atmospheres of extrasolar planets. IV. On the scattering greenhouse effect of CO2 ice particles: Numerical radiative transfer studies

Daniel Kitzmann, A. B. C. Patzer+1 more|arXiv (Cornell University)|Jun 17, 2013

Atmospheric aerosols and clouds37 references17 citations

TL;DR

This study investigates the scattering greenhouse effect of CO₂ ice clouds in exoplanet atmospheres using high-order radiative transfer modeling. It finds that two-stream approximations overestimate the greenhouse effect, revealing that CO₂ ice clouds have a much weaker net warming impact than previously thought—only effective for a narrow range of particle sizes and optical depths.

ABSTRACT

Owing to their wavelengths dependent absorption and scattering properties, clouds have a strong impact on the climate of planetary atmospheres. Especially, the potential greenhouse effect of CO2 ice clouds in the atmospheres of terrestrial extrasolar planets is of particular interest because it might influence the position and thus the extension of the outer boundary of the classic habitable zone around main sequence stars. We study the radiative effects of CO2 ice particles obtained by different numerical treatments to solve the radiative transfer equation. The comparison between the results of a high-order discrete ordinate method and simpler two-stream approaches reveals large deviations in terms of a potential scattering efficiency of the greenhouse effect. The two-stream methods overestimate the transmitted and reflected radiation, thereby yielding a higher scattering greenhouse effect. For the particular case of a cool M-type dwarf the CO2 ice particles show no strong effective scattering greenhouse effect by using the high-order discrete ordinate method, whereas a positive net greenhouse effect was found in case of the two-stream radiative transfer schemes. As a result, previous studies on the effects of CO2 ice clouds using two-stream approximations overrated the atmospheric warming caused by the scattering greenhouse effect. Consequently, the scattering greenhouse effect of CO2 ice particles seems to be less effective than previously estimated. In general, higher order radiative transfer methods are necessary to describe the effects of CO2 ice clouds accurately as indicated by our numerical radiative transfer studies.

Motivation & Objective

• To assess the radiative impact of CO₂ ice clouds on exoplanet climates, particularly their potential to extend the outer habitable zone via scattering greenhouse effect.
• To resolve discrepancies between previous studies that used simplified two-stream radiative transfer methods and more accurate high-order methods.
• To determine the dependence of the greenhouse effect on particle size, optical depth, and stellar type, especially for cool M-type stars.
• To evaluate whether CO₂ ice clouds can significantly warm planetary surfaces, particularly in early Mars and tidally locked exoplanets.
• To provide a more accurate radiative transfer framework for future climate modeling of exoplanets with CO₂ ice clouds.

Proposed method

• Employed Mie theory to compute the wavelength-dependent extinction, scattering, and absorption cross-sections of CO₂ ice particles across a range of effective radii (0.1–200 μm).
• Used the discrete ordinate method (DISORT) for high-order, accurate radiative transfer calculations in a single atmospheric layer with zenith angle of 60°.
• Compared results from DISORT with simpler two-stream radiative transfer approximations to quantify systematic errors in prior studies.
• Simulated spectral reflectance and transmittance for stellar and thermal radiation across various optical depths and particle sizes.
• Incorporated realistic refractive index data for CO₂ ice from Hansen (1997, 2005) and Warren (1986) to ensure accurate optical property modeling.
• Evaluated net radiative effects under different stellar spectral types, focusing on M-type dwarfs with cooler effective temperatures.

Experimental results

Research questions

• RQ1 How does the scattering greenhouse effect of CO₂ ice clouds vary with particle size and optical depth in exoplanet atmospheres?
• RQ2 To what extent do two-stream radiative transfer approximations overestimate the greenhouse effect of CO₂ ice clouds compared to high-order methods?
• RQ3 What is the net climatic effect of CO₂ ice clouds around cool M-type stars, where stellar radiation peaks in the infrared?
• RQ4 Under what conditions does CO₂ ice cloud formation lead to net surface warming rather than cooling or radiative neutrality?
• RQ5 How do surface albedo and Rayleigh scattering influence the effectiveness of the scattering greenhouse effect?

Key findings

• High-order DISORT simulations show that the scattering greenhouse effect of CO₂ ice clouds is significantly weaker than predicted by two-stream models, especially for cool stars.
• For M-type stars, CO₂ ice clouds with effective radii around 1 μm produce a net cooling effect, while only particles near 1 μm and optical depths between 1 and 10 yield a weak net warming effect.
• Particles with effective radii of 0.1 μm are radiatively neutral even at high optical depths, particularly for cool stars.
• Two-stream methods overestimate transmitted and reflected radiation, leading to an overestimation of the greenhouse effect by up to a factor of two in some parameter regimes.
• The net greenhouse effect increases with stellar effective temperature, but remains negligible or neutral for stars with T_eff < 3500 K.
• The results imply that the outer boundary of the habitable zone should be closer to the host star than previously estimated, especially when combined with revised CO₂ collision-induced absorption.