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. 2013 Aug;13(8):715-39.
doi: 10.1089/ast.2012.0961. Epub 2013 Jul 15.

The effect of host star spectral energy distribution and ice-albedo feedback on the climate of extrasolar planets

Aomawa L Shields  1 , Victoria S Meadows, Cecilia M Bitz, Raymond T Pierrehumbert, Manoj M Joshi, Tyler D Robinson
Affiliations

The effect of host star spectral energy distribution and ice-albedo feedback on the climate of extrasolar planets

Aomawa L Shields et al. Astrobiology. 2013 Aug.

Abstract

Planetary climate can be affected by the interaction of the host star spectral energy distribution with the wavelength-dependent reflectivity of ice and snow. In this study, we explored this effect with a one-dimensional (1-D), line-by-line, radiative transfer model to calculate broadband planetary albedos as input to a seasonally varying, 1-D energy balance climate model. A three-dimensional (3-D) general circulation model was also used to explore the atmosphere's response to changes in incoming stellar radiation, or instellation, and surface albedo. Using this hierarchy of models, we simulated planets covered by ocean, land, and water-ice of varying grain size, with incident radiation from stars of different spectral types. Terrestrial planets orbiting stars with higher near-UV radiation exhibited a stronger ice-albedo feedback. We found that ice extent was much greater on a planet orbiting an F-dwarf star than on a planet orbiting a G-dwarf star at an equivalent flux distance, and that ice-covered conditions occurred on an F-dwarf planet with only a 2% reduction in instellation relative to the present instellation on Earth, assuming fixed CO(2) (present atmospheric level on Earth). A similar planet orbiting the Sun at an equivalent flux distance required an 8% reduction in instellation, while a planet orbiting an M-dwarf star required an additional 19% reduction in instellation to become ice-covered, equivalent to 73% of the modern solar constant. The reduction in instellation must be larger for planets orbiting cooler stars due in large part to the stronger absorption of longer-wavelength radiation by icy surfaces on these planets in addition to stronger absorption by water vapor and CO(2) in their atmospheres, which provides increased downwelling longwave radiation. Lowering the IR and visible-band surface ice and snow albedos for an M-dwarf planet increased the planet's climate stability against changes in instellation and slowed the descent into global ice coverage. The surface ice-albedo feedback effect becomes less important at the outer edge of the habitable zone, where atmospheric CO(2) could be expected to be high such that it maintains clement conditions for surface liquid water. We showed that ∼3-10 bar of CO(2) will entirely mask the climatic effect of ice and snow, leaving the outer limits of the habitable zone unaffected by the spectral dependence of water ice and snow albedo. However, less CO(2) is needed to maintain open water for a planet orbiting an M-dwarf star than would be the case for hotter main-sequence stars.

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Figures

FIG. 1.
FIG. 1.
Top: the SEDs for F-, G-, K-, and M-dwarf stars, normalized by the peak flux. Bottom: the spectral distribution of fine-grained snow, blue marine ice, and 25%, 50%, and 75% mixtures of the two end-members. Ocean and land spectral distributions used in the radiative transfer and energy balance models are also plotted.
FIG. 2.
FIG. 2.
Wavelength-dependent reflectivity of a planet with an Earth-like atmosphere, an underlying snow surface, clear sky conditions (red), 100% cirrus cloud cover (green), and 100% stratocumulus cloud cover (blue), calculated with SMART. The empirical spectrum for fine-grained snow (from Fig. 1) is plotted here (black) for reference.
FIG. 3.
FIG. 3.
Mean ice line latitude and global surface temperature as a function of obliquity, calculated using a seasonal EBM. Earth's northern hemisphere ice line latitude and global mean surface temperature at its present obliquity of 23.5° (vertical dashed line) are verified to within six degrees in latitude and 3°C, respectively. Ocean and land surfaces were assigned broadband albedos of 0.32 and 0.41, respectively (including atmosphere, for 36% clear sky, 40% stratocumulus cloud cover, and 24% cirrus cloud cover). Regions where the surface temperature fell below −2°C were assigned a broadband albedo of 0.46, which was calculated by using the same percentages of clear sky and cloud cover, with the spectrum corresponding to the 25% mixture of blue marine ice and fine-grained snow in Fig. 1. Random overlap between the two cloud layers is assumed. Color images available online at www.liebertonline.com/ast
FIG. 4.
FIG. 4.
Broadband planetary albedos calculated with upwelling and direct downwelling stellar flux outputs from SMART for ice, snow, ocean, and land surfaces given the SEDs of F-, G-, K-, and M-dwarf stars. For each star: Left—no atmospheric gas absorption, but Rayleigh scattering is included; middle—no gases or Rayleigh scattering (broadband F-dwarf planetary albedos for ice and snow surfaces are still larger than those for G-, K-, or M-dwarf planets, even after the effects of Rayleigh scattering are removed); right—Rayleigh scattering, gas absorption, and clouds are included. These values are listed in Table 2. Color images available online at www.liebertonline.com/ast
FIG. 5.
FIG. 5.
Top-of-atmosphere upwelling flux divided by the downwelling stellar flux (which is a measure of the planetary albedo) as a function of wavelength for an ocean-covered planet with a surface pressure of 1 bar and 0.1 mbar orbiting an F-dwarf star at an equivalent flux distance to Earth around the Sun, calculated with SMART. No atmospheric gas absorption is included here. The rise in top-of-atmosphere flux (and therefore planetary albedo) evident in the 1-bar atmosphere case at λ<0.7 μm is due to Rayleigh scattering. At 0.1 mbar, the Rayleigh scattering tail is absent and matches the empirical albedo spectrum of the ocean surface from Fig. 1 (dotted black). Color images available online at www.liebertonline.com/ast
FIG. 6.
FIG. 6.
Mean ice line latitude (top) and global mean surface temperature (bottom) in the northern hemisphere as a function of percent of modern solar constant are calculated by using a seasonal EBM, at present Earth obliquity (23.5°) for aqua planets (land and ocean fraction 0.01 and 0.99, respectively) orbiting F-, G-, and M-dwarf stars at an equivalent flux distance. Below-freezing surfaces encountered during the EBM runs (where the temperature is less than −2°C) were given broadband planetary albedos for ice and snow of varying grain size calculated from SMART at 1-bar surface pressure. EBM simulations were run with an initial warm start, with an approximate Earth-like zonal mean temperature distribution. The results can be assumed to be similar for the southern hemisphere, given an assumed eccentricity of zero. The present atmospheric level of CO2 was used (F-dwarf planet in blue, G-dwarf planet in black, and M-dwarf planet in red). Asterisks denote the minimum ice line latitude before collapse to the equator and global ice coverage.
FIG. 7.
FIG. 7.
Top-of-atmosphere absorbed shortwave radiation minus outgoing longwave radiation as a function of latitude, calculated in the GCM (left) and the EBM (right) for a G-dwarf planet receiving 100% of the modern solar constant. When averaged over a few years or more, the net incoming heat flux must be equal to the divergence of heat from each grid cell in order to yield a net surface flux of zero locally and globally in our slab ocean model. The EBM shows a large jump in the net incoming heat flux near the poles, due to the abrupt change in albedo for ice-covered areas in the EBM, and lack of parameterized clouds (beyond our SMART treatment). A smoother transition in net incoming heat flux as a function of latitude is visible in the GCM, due to the presence of full-scale atmospheric dynamics, including clouds.
FIG. 8.
FIG. 8.
Mean ice line latitude (top) and global mean surface temperature (bottom) as a function of percent of modern solar constant after a 40-year GCM run for an aqua planet orbiting the Sun (asterisks), M-dwarf star AD Leo (circles), and F-dwarf star HD128167 (triangles) at equivalent flux distances. The slope of each line is a measurement of the climate sensitivity of the planet to changes in stellar flux. The shallower slope of the M-dwarf planet indicates a smaller change in surface temperature and ice extent for a given change in instellation than on the planets orbiting stars with greater visible and near-UV output.
FIG. 9.
FIG. 9.
Surface temperature on an aqua planet receiving 90% of the modern solar constant from an M-dwarf star (red) compared with an aqua planet receiving 100% of the modern solar constant from the Sun, a G-dwarf star (blue dashed), after a 40-year GCM run. Color images available online at www.liebertonline.com/ast
FIG. 10.
FIG. 10.
Precipitation rate on an aqua planet receiving 90% of the modern solar constant from an M-dwarf star (red) compared with an aqua planet receiving 100% of the modern solar constant from the Sun (blue dashed), after a 40-year GCM run. Color images available online at www.liebertonline.com/ast
FIG. 11.
FIG. 11.
GCM comparison of an M-dwarf aqua planet receiving 90% of the modern solar constant compared with an aqua planet receiving 100% of the modern solar constant from the Sun, a G-dwarf star. (a) Shortwave heating in the atmosphere of the G-dwarf planet; (b) shortwave heating in the atmosphere of the M-dwarf planet; (c) increase in shortwave heating in the atmosphere of the M-dwarf planet, calculated by taking the difference between the M-dwarf planet's shortwave heating profile and the G-dwarf planet's shortwave heating profile; (d) zonal mean temperature in the atmosphere of the G-dwarf aqua planet; (e) zonal mean temperature in the atmosphere of the M-dwarf aqua planet; (f) increase in zonal mean temperature in the atmosphere of the M-dwarf aqua planet, calculated by taking the difference between the M-dwarf planet's atmospheric temperature profile and the G-dwarf planet's atmospheric temperature profile.
FIG. 12.
FIG. 12.
GCM comparison of an M-dwarf aqua planet receiving 90% of the modern solar constant compared with an aqua planet receiving 100% of the modern solar constant from the Sun, a G-dwarf star. Blue dotted—clockwise circulation. Red—counterclockwise circulation. (a) Meridional stream function in the atmosphere of the G-dwarf planet; the contours start at 50 kg/s×109, and the contour interval is 25 kg/s×109. (b) Meridional stream function in the atmosphere of the M-dwarf planet; the contours start at 50 kg/s×109, and the contour interval is 25 kg/s×109. (c) Increase in the meridional stream function in the atmosphere of the M-dwarf planet, calculated by taking the difference between the M-dwarf planet's meridional stream function and the G-dwarf planet's meridional stream function; the contours start at 10 kg/s×109, and the contour interval is 5 kg/s×109. The weaker Hadley circulation on the M-dwarf planet results in greater atmospheric temperatures, and less heat transported away from the equator, compensating for the reduced instellation relative to the G-dwarf planet. Color images available online at www.liebertonline.com/ast
FIG. 13.
FIG. 13.
Difference between the cloud fraction in the atmosphere of an aqua planet receiving 90% of the modern solar constant from an M-dwarf star and an aqua planet receiving 100% of the modern solar constant from the Sun after a 40-year GCM run.
FIG. 14.
FIG. 14.
Surface albedo as a function of latitude for an M-dwarf aqua planet receiving 90% of the modern solar constant (red) compared with an aqua planet receiving 100% of the modern solar constant from the Sun, a G-dwarf star (blue dashed), after a 40-year GCM run.
FIG. 15.
FIG. 15.
GCM comparison of an M-dwarf aqua planet receiving 73% of the modern solar constant (993 W/m2) with an aqua planet orbiting the Sun and receiving 92% of the modern solar constant (1251 W/m2). Both planets are completely ice-covered. (a) Zonal mean temperature in the atmosphere of the G-dwarf planet; (b) zonal mean temperature in the atmosphere of the M-dwarf planet; (c) increase in temperature in the atmosphere of the M-dwarf aqua planet, calculated by taking the difference between the M-dwarf planet's atmospheric temperature profile and the G-dwarf planet's atmospheric temperature profile.
FIG. 16.
FIG. 16.
GCM comparison of an M-dwarf aqua planet (red) receiving 73% of the modern solar constant (993 W/m2) with an aqua planet orbiting the Sun and receiving 92% of the modern solar constant (1251 W/m2, blue dashed). Both planets are completely ice-covered. (a) Surface shortwave downwelling flux as a function of latitude; (b) surface albedo; (c) net shortwave flux absorbed by the surface; (d) surface temperature. Color images available online at www.liebertonline.com/ast
FIG. 17.
FIG. 17.
Mean ice line latitude (top) and global mean surface temperature (bottom) as a function of percent of modern solar constant after a 40-year GCM run for an aqua planet orbiting an M-dwarf star, from Fig. 8. Also plotted here are the resulting ice extents and global mean surface temperatures for M-dwarf planets receiving 75% and 85% instellation, with IR and visible-band ice and snow albedos lowered to 0.2 (asterisks). The difference in climates is larger between the M-dwarf planets receiving 75% instellation than between the M-dwarf planets receiving 85% instellation (as indicated by the black vertical lines), exhibiting a shallower change in global mean surface temperature and ice extent for lowered instellation than with the default albedo parameterization. Color images available online at www.liebertonline.com/ast
FIG. 18.
FIG. 18.
Top: The normalized SED of the M3V star AD Leo. Middle: The albedo spectrum of fine-grained snow (solid line), with artificially enhanced albedo values of 0.6 at wavelengths longer than 1.1 μm (dashed line). Bottom: Broadband planetary albedos (0.15 μm≤λ≤2.5 μm) as output from SMART, given an incident M-dwarf spectrum, input actual (plus symbols) and artificially enhanced (triangles) snow albedo spectra, and various concentrations of atmospheric CO2. The concentration of CO2 can be expected to effectively mask the ice-albedo spectral dependence when the broadband planetary albedo for the actual input snow spectrum (with lower near-IR albedo values) matches that for the artificially enhanced snow spectrum (with high values of near-IR albedo), demonstrating that broadband planetary albedo is no longer sensitive to the surface albedo of the planet. This appears to happen at atmospheric concentrations of between 3 and 10 bar of CO2. Color images available online at www.liebertonline.com/ast
FIG. 19.
FIG. 19.
Wavelength-dependent reflectivity of a planet with various concentrations of atmospheric CO2 and an underlying snow surface, calculated with SMART and the actual measured albedo spectrum, and one that was altered to exhibit artificially high albedo values of 0.6 at wavelengths longer than 1.1 μm. Top: 0.1 bar of CO2 and an underlying snow surface matching the fine-grained snow spectrum in Fig. 1 (blue); 0.1 bar of CO2 with a snow surface with an artificially enhanced spectrum (red); 3-bar CO2 atmosphere with the actual snow spectrum (green); 3-bar CO2 atmosphere with the artificially enhanced snow spectrum (black). Bottom: Change in reflectivity between the planets with artificially enhanced vs. actual snow surface albedo spectra. With 3 bar of CO2 in the atmosphere, the difference between the albedos of the planets (orange) has decreased significantly compared to that of the 0.1-bar planets (purple). With 10 bar of CO2 in the atmosphere, the difference in the albedo spectra of the planets (black) is close to zero, due to increased near-IR absorption by CO2 at longer wavelengths.
FIG. 20.
FIG. 20.
Mean ice line latitude (top) and global mean surface temperature (bottom) in the northern hemisphere as a function of percent of modern solar constant are calculated using a seasonal EBM, at present Earth obliquity (23.5°) for aqua planets (land and ocean fraction 0.01 and 0.99, respectively) orbiting F-, G-, and M-dwarf stars at an equivalent flux distance, as in Fig. 6. Here the present atmospheric level (PAL) of CO2 as well as 3 bar of CO2 were used (F-dwarf planet in blue, G-dwarf planet in black, and M-dwarf planet in red). Asterisks denote the minimum ice line latitude before collapse to the equator and global ice coverage. Also plotted here as vertical solid lines are the updated maximum CO2 greenhouse limits for the F-dwarf (blue), G-dwarf (black), and M-dwarf (red) planets (Kopparapu et al., 2013a, 2013b).
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