Eric T. Wolf

Fractal organic hazes provided an ultraviolet shield for early Earth.

The Past Looks Hazy During the first couple of billion years of Earths history, the Sun is thought to have been 30% less luminous than today, yet the surface of the planet was warm enough to prevent glacier formation and for early life to become established. Why were temperatures so high, despite the lower flux of solar energy? Wolf and Toon (p. 1266; see the Perspective by Chyba) propose that the presence of a photochemical haze with a fractal size distribution was the reason. Such a haze, unlike one composed of spherical particles assumed in previous models, could have been opaque enough to block ultraviolet radiation that would have hindered or prevented life from arising, but transparent enough in the shorter wavelengths to keep the atmosphere warm. A fractal organic haze layer kept the early Earth warm, despite the faintness of the Sun. The Archean Earth (3.8 to 2.5 billion years ago) was probably enshrouded by a photochemical haze composed of fractal aggregate hydrocarbon aerosols. The fractal structure of the aerosols would have had a strong effect on the radiative properties of the haze. In this study, a fractal aggregate haze was found to be optically thick in the ultraviolet wavelengths while remaining relatively transparent in the mid-visible wavelengths. At an annual production rate of 1014 grams per year and an average monomer radius of 50 nanometers, the haze would have provided a strong shield against ultraviolet light while causing only minimal antigreenhouse cooling.

The inner edge of the habitable zone for synchronously rotating planets around low-mass stars using general circulation models

Terrestrial planets at the inner edge of the habitable zone of late-K and M-dwarf stars are expected to be in synchronous rotation, as a consequence of strong tidal interactions with their host stars. Previous global climate model (GCM) studies have shown that, for slowly-rotating planets, strong convection at the substellar point can create optically thick water clouds, increasing the planetary albedo, and thus stabilizing the climate against a thermal runaway. However these studies did not use self-consistent orbital/rotational periods for synchronously rotating planets placed at different distances from the host star. Here we provide new estimates of the inner edge of the habitable zone for synchronously rotating terrestrial planets around late-K and M-dwarf stars using a 3-D Earth-analog GCM with self-consistent relationships between stellar metallicity, stellar effective temperature, and the planetary orbital/rotational period. We find that both atmospheric dynamics and the efficacy of the substellar cloud deck are sensitive to the precise rotation rate of the planet. Around mid-to-late M-dwarf stars with low metallicity, planetary rotation rates at the inner edge of the HZ become faster, and the inner edge of the habitable zone is farther away from the host stars than in previous GCM studies. For an Earth-sized planet, the dynamical regime of the substellar clouds begins to transition as the rotation rate approaches ~10 days. These faster rotation rates produce stronger zonal winds that encircle the planet and smear the substellar clouds around it, lowering the planetary albedo, and causing the onset of the water-vapor greenhouse climatic instability to occur at up to ~25% lower incident stellar fluxes than found in previous GCM studies. For mid-to-late M-dwarf stars with high metallicity and for mid-K to early-M stars, we agree with previous studies.

Hospitable Archean Climates Simulated by a General Circulation Model

Evidence from ancient sediments indicates that liquid water and primitive life were present during the Archean despite the faint young Sun. To date, studies of Archean climate typically utilize simplified one-dimensional models that ignore clouds and ice. Here, we use an atmospheric general circulation model coupled to a mixed-layer ocean model to simulate the climate circa 2.8 billion years ago when the Sun was 20% dimmer than it is today. Surface properties are assumed to be equal to those of the present day, while ocean heat transport varies as a function of sea ice extent. Present climate is duplicated with 0.06 bar of CO2 or alternatively with 0.02 bar of CO2 and 0.001 bar of CH4. Hot Archean climates, as implied by some isotopic reconstructions of ancient marine cherts, are unattainable even in our warmest simulation having 0.2 bar of CO2 and 0.001 bar of CH4. However, cooler climates with significant polar ice, but still dominated by open ocean, can be maintained with modest greenhouse gas amounts, posing no contradiction with CO2 constraints deduced from paleosols or with practical limitations on CH4 due to the formation of optically thick organic hazes. Our results indicate that a weak version of the faint young Sun paradox, requiring only that some portion of the planets surface maintain liquid water, may be resolved with moderate greenhouse gas inventories. Thus, hospitable late Archean climates are easily obtained in our climate model.

Delayed onset of runaway and moist greenhouse climates for Earth

As the Sun slowly grows brighter over its main sequence lifetime, habitability on Earths surface will eventually become threatened probably leading to moist and then runaway greenhouse climates. One-dimensional climate models predict that a catastrophic thermal runaway will be triggered by a 6% increase in the solar constant above its present level. However, here simulations using a three-dimensional climate model with fixed carbon dioxide and methane indicate that surface habitability may be maintained at significantly larger solar constants. A 15.5% increase in the solar constant yields global mean surface temperatures of 312.9 K, well short of moist and runaway greenhouse states. Numerical limitations prevent simulation of climates much warmer than this. Nonetheless, our results imply that Earths climate may remain safe against both water loss and thermal runaway limits for at least another 1.5 billion years and probably for much longer.

The evolution of habitable climates under the brightening Sun

On water-dominated planets, warming from increased solar insolation is strongly amplified by the water vapor greenhouse feedback. As the Sun brightens due to stellar evolution, Earth will become uninhabitable due to rising temperatures. Here we use a modified version of the Community Earth System Model from the National Center for Atmospheric Research to study Earth under intense solar radiation. For small (≤10%) increases in the solar constant (S0), Earth warms nearly linearly with climate sensitivities of ~1 K/(W m−2) and global mean surface temperatures below 310 K. However, an abrupt shift in climate is found as the solar constant is increased to +12.5% S0. Here climate sensitivity peaks at ~6.5 K/(W m−2), while global mean surface temperatures rise above 330 K. This climatic transition is associated with a fundamental change to the radiative-convective state of the atmosphere. Hot, moist climates feature both strong solar absorption and inefficient radiative cooling in the low atmosphere, thus yielding net radiative heating of the near-surface layers. This heating forms an inversion that effectively shuts off convection in the boundary layer. Beyond the transition, Earth continues to warm but with climate sensitivities again near unity. Conditions conducive to significant water loss to space are not found until +19% S0. Earth remains stable against a thermal runaway up to at least +21% S0, but at that point, global mean surface temperatures exceed 360 K, and water loss to space becomes rapid. Water loss of the oceans from a moist greenhouse may preclude a thermal runaway.

Assessing the Habitability of the TRAPPIST-1 System Using a 3D Climate Model

The TRAPPIST-1 system provides an extraordinary opportunity to study multiple terrestrial extrasolar planets and their atmospheres. Here we use the National Center for Atmospheric Research Community Atmosphere Model version 4 to study the possible climate and habitability of the planets in the TRAPPIST-1 system. We assume ocean-covered worlds, with atmospheres comprised of N2, CO2, and H2O, and with orbital and geophysical properties defined from observation. Model results indicate that the inner three planets (b, c, and d) presently reside interior to the inner edge of the traditional liquid water habitable zone. Thus if water ever existed on the inner planets, they would have undergone a runaway greenhouse and lost their water to space, leaving them dry today. Conversely the outer 3 planets (f, g, and h) fall beyond the maximum CO2 greenhouse outer edge of the habitable zone. Model results indicate that the outer planets cannot be warmed despite as much as 30 bar CO2 atmospheres, instead entering a snowball state. The middle planet (e) represents the best chance for a presently habitable ocean-covered world in the TRAPPIST-1 system. Planet e can maintain at least some habitable surface area with 0 - 2 bar CO2, depending on the background N2 content. Near present day Earth surface temperatures can be maintained for an ocean-covered planet e with either 1 bar N2 and 0.4 bar CO2, or a 1.3 bar pure CO2 atmosphere.

Controls on the Archean climate system investigated with a global climate model.

The most obvious means of resolving the faint young Sun paradox is to invoke large quantities of greenhouse gases, namely, CO2 and CH4. However, numerous changes to the Archean climate system have been suggested that may have yielded additional warming, thus easing the required greenhouse gas burden. Here, we use a three-dimensional climate model to examine some of the factors that controlled Archean climate. We examine changes to Earths rotation rate, surface albedo, cloud properties, and total atmospheric pressure following proposals from the recent literature. While the effects of increased planetary rotation rate on surface temperature are insignificant, plausible changes to the surface albedo, cloud droplet number concentrations, and atmospheric nitrogen inventory may each impart global mean warming of 3-7 K. While none of these changes present a singular solution to the faint young Sun paradox, a combination can have a large impact on climate. Global mean surface temperatures at or above 288 K could easily have been maintained throughout the entirety of the Archean if plausible changes to clouds, surface albedo, and nitrogen content occurred.

Pale Orange Dots: the Impact of Organic Haze on the Habitability and Detectability of Earthlike Exoplanets

Hazes are common in known planet atmospheres, and geochemical evidence suggests early Earth occasionally supported an organic haze with significant environmental and spectral consequences. The UV spectrum of the parent star drives organic haze formation through methane photochemistry. We use a 1D photochemical-climate model to examine production of fractal organic haze on Archean Earth-analogs in the habitable zonesof several stellar types: the modern and early Sun, AD Leo (M3.5V), GJ 876 (M4V),

Differences in Water Vapor Radiative Transfer among 1D Models Can Significantly Affect the Inner Edge of the Habitable Zone

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Constraints on Climate and Habitability for Earth-like Exoplanets Determined from a General Circulation Model

Eridani (K2V), and