We use a three-dimensional model to calculate steady-state climates at various intervals in Earth's future, across a parameter space of increasing insolation and decreasing mixing ratio. Comparison with prior results shows an overestimation of warming by one-dimensional models when solar constant is increased and mixing ratio is fixed. We consider two future trajectories as limiting cases: strong weathering, in which surface temperature remains constant but is drawn down; and weak weathering, in which remains constant and surface temperature increases. Under strong weathering, we find the conventional 10 ppm starvation limit for C4 photosynthesis that occurs at 1.35 Gyr; however, we suggest that crassulacean acid metabolism (CAM) photosynthesis could persist below this limit and note that aquatic macrophytes can utilize dissolved bicarbonate if atmospheric is low. If we take the starvation limit at 1 ppm instead, then the vegetative biosphere could continue until 1.84 Gyr. Thermal limits apply instead under weak weathering, in which Earth would be too hot for most land plants at 1.68 Gyr (>323 K) and too hot for all land plants (>338 K) at 1.87 Gyr. These lifetimes approach the moist and runaway greenhouse limits for Earth. We discuss other possible mechanisms for extending the lifetime of Earth's biosphere, noting that both technological intervention and evolutionary processes could enable life to adapt to a brightening sun. Plain Language Summary The ultimate life span of Earth's biosphere is limited due to the steady brightening of the sun as it progresses in age. Earth's long-term carbon cycle may respond by drawing carbon dioxide out of the atmosphere and into carbonate rocks, thereby reducing the greenhouse effect and offsetting the increased sunlight. Most prior studies have argued that this would eventually make carbon dioxide levels too low to sustain photosynthesis, thus marking the end of the biosphere as we know it. In this study, we use a three-dimensional computational climate model to calculate scenarios of Earth's future climate with increasing sunlight and decreasing carbon dioxide. We show that Earth's biosphere could survive for much longer than indicated in most studies, noting that some photosynthetic life on Earth can thrive at very low carbon dioxide levels. We also explore possible trajectories in which carbon dioxide does not decrease but instead remains constant. Earth's vegetative biosphere could survive up to about 1.8 billion years from now, about the same time that Earth would lose its oceans to space. We discuss other ways that biological evolution as well as technological intervention could even further extend the lifetime of Earth's vegetative biosphere.

The search for life beyond Earth will depend on detecting faint signatures in the atmospheres of potentially habitable exoplanets. Interpreting those signatures requires understanding the host planet's climate: the same molecule may signal life on one planet and abiotic chemistry on another. Global climate models (GCMs) provide this understanding, but individual runs can require up to millions of core-hours and substantial domain expert time. Machine-learning emulators could remove this bottleneck, but progress has been limited by the absence of a curated, multi-model exoclimate dataset. We introduce ThousandWorlds, an ML-ready benchmark for exoclimate emulation and for the broader regime of low-data, multi-simulator, parameter-to-field regression. The dataset contains approximately 1800 simulations from five GCMs, mapping eight planet parameters to 3D atmospheric fields including temperature, humidity, winds, clouds, and radiation. Three nested subsets define progressively harder challenges: single-simulator regression, multi-simulator regression with complete observations, and multi-simulator regression with structured missingness. We propose two evaluation protocols: one for ranking methods, and one that measures performance relative to the disagreement between GCMs themselves. We evaluate seven baselines spanning simple methods, deep learning, and Gaussian processes. GP-based methods perform best, suggesting that ThousandWorlds exposes a regime where off-the-shelf deep learning does not yet succeed. Data: https://doi.org/10.57967/hf/8695. Code: https://github.com/edstevenson/ThousandWorlds.

Discoveries of giant planet candidates orbiting white dwarf (WD) stars and the demonstrated capabilities of the James Webb Space Telescope bring the possibility of detecting rocky planets in the habitable zones (HZs) of WDs into pertinent focus. We present simulations of an aqua planet with an Earth-like atmospheric composition and incident stellar insolation orbiting in the HZ of two different types of stars-a 5000 K WD and main-sequence K-dwarf star Kepler-62 (K62) with a similar effective temperature-and identify the mechanisms responsible for the two differing planetary climates. The synchronously rotating WD planet's global mean surface temperature is 25 K higher than that of the synchronously rotating planet orbiting K62, due to its much faster (10 hr) rotation and orbital period. This ultrafast rotation generates strong zonal winds and meridional flux of zonal momentum, stretching out and homogenizing the scale of atmospheric circulation, and preventing an equivalent buildup of thick, liquid water clouds on the dayside of the planet compared to the synchronous planet orbiting K62, while also transporting heat equatorward from higher latitudes. White dwarfs may therefore present amenable environments for life on planets formed within or migrated to their HZs, generating warmer surface environments than those of planets with main-sequence hosts to compensate for an ever shrinking incident stellar flux.

Oxygen and ozone are two of the most important gases in Earth’s atmosphere. These arose as a result of photosynthesis and appeared prominently around 2.3–2.4 billion yr ago. For exoplanets, these species have been proposed both as remote biosignatures and antibiosignatures, depending on the abundances and astrophysical context. ExoCAM, an extension of the Community Earth System Model for deep paleoclimate and exoplanets, has previously been limited to anoxic atmospheres. This work presents a substantial update to the radiative transfer in ExoCAM to include the effects of oxygen and ozone. We describe the implementation of line lists, empirical cross sections, Rayleigh scattering, and collision-induced absorption and test the resulting framework in 1D and 3D for the modern Earth atmosphere. We quantify the changes in flux, temperatures, and circulation due to the two gases.

Geological proxies indicate that early Archean Earth was not in a permanent snowball state. Otherwise, we have limited data on its atmospheric composition, how volatile cycling operated, its evolving land-sea mask, topography and ocean bathymetry, etc. At the same time Archean climate studies provide a relatively large and underexplored parameter space for full complexity General Circulation Models (GCMs). Here we model the climate of the Archean at 3.8Ga when the amount of exposed land is likely very small (e.g. Cawood et al. 2022). The ROCKE-3D (R3D; Way et al. 2017) GCM is used. It is a full-complexity fully coupled atmosphere, land and ocean model. In contrast to previous studies we utilize a full complexity atmosphere, a coupled fully dynamic ocean, and dynamic sea ice. R3D is a child of the Goddard Institute for Space Studies GCM Model_E that is used for climate change studies (Schmidt et al. 2013). We model day lengths of 12 and 18 hours in an aquaplanet setup. We use an N2 dominated atmosphere with differing amounts of CO2 and CH4 (being careful to avoid ratios that lead to climate cooling hydrocarbon hazes). Surface pressures of 1, 0.5 and 0.25 bar are modeled, given various Archean pressure proxy constraints (e.g. Som et al. 2016). We use a solar spectrum from 3.8Ga with a reduction of ~25% from modern day insolation. Perhaps unsurprisingly we discover that the tipping point from a temperate state (similar to modern day Earth) versus a snowball state is very sensitive to greenhouse gas amounts and total atmospheric pressure. We see differences in latitudinal ice extent dependent upon day length (12 vs 18 hours) for otherwise similar parameters, and that the dynamics of the climate is similar to recent work by Feulner et al. (2022). This work, alongside others such as Charnay et al. (2017) and Feulner et al. (2023) provide a clear path to explaining why the Faint Young Sun paradox may finally be put to rest.References:Cawood et al. (2022) RG, 60, e2022RG000789; Charnay et al. (2017) EPSL 474, 97; Feulner et al. (2023) ESD 14, 533-547; Schmidt et al. (2013) JAMES, 6, 141-184; Som et al. (2016) NatGeo 9, 448; Way et al. (2017) ApJS. 231, 12.

We present the discovery of GJ 251 c, a candidate super-Earth orbiting in the habitable zone (HZ) of its M dwarf host star. Using high-precision Habitable-zone Planet Finder and NEID RVs, in conjunction with archival RVs from the Keck I High Resolution Echelle Spectrometer, the Calar Alto High-resolution Search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrograph, and the Spectropolarim & eacute;tre Infrarouge, we improve the measured parameters of the known planet, GJ 251 b (Pb = 14.2370 days; msin(i) = 3.85 -0.33+0.35 M circle plus), and we significantly constrain the minimum mass of GJ 251 c, placing it in a plausibly terrestrial regime (Pc = 53.647 +/- 0.044 days; msinic = 3.84 +/- 0.75 M circle plus). Using activity mitigation techniques that leverage chromatic information content, we perform a color-dependent analysis of the system and a detailed comparison of more than 50 models that describe the nature of the planets and stellar activity in the system. Due to GJ 251's proximity to Earth (5.5 pc), next generation, 30 meter class telescopes will likely be able to image terrestrial planets in GJ 251's HZ. In fact, GJ 251 c is currently the best candidate for terrestrial, HZ planet imaging in the northern sky.

Martian polar caps consist of both H2O and CO2 ice. While H2O ice is mainly passive on modern Mars, it may have not been the case in recent Martian history, when its obliquity was higher, or when it was changing rapidly. The distribution of ice species in the snowpack affects its physical and thermodynamic properties. In the upper layers, it determines its albedo and thermal emissivity. Thus understanding the mutual effect between these ices and their interaction with the atmosphere is crucial for understanding the evolution of Martian polar regions. In this study, we employ a newly-developed Exotic Ices snow model coupled to the NASA Goddard Institute for Space Studies (GISS) ROCKE-3D planetary General Circulation Model (GCM) [1] to study the behavior of Martian polar caps. ROCKE-3D is a planetary GCM developed at NASA GISS as an extension of its Earth climate model, modelE [2]. It has been extensively used to simulate climate of various planets, including Mars (e.g. [3,4]).The Exotic Ices snow model was specially developed for planetary applications which involve more than one condensable in the atmosphere, in which case snow can contain multiple species of ice (CO2 and H2O in the Mars case). For each species of ice, the model uses their proper physical properties and phase diagram, but otherwise it treats all species of ice on an equal footing. The combined effects on albedo, thermal inertia and mutual insulation are treated accordingly. The snowpack interacts with the atmospheric dust cycle, and can accumulate a prognostic amount of dust, though the effect of dust on snow properties is not currently treated explicitly, and is prescribed. In this study, we first validate our model against the modern Martial climate, for which we use mission results from Mars Climate Sounder (atmospheric temperature and dust optical depth), SPICAM on Mars Express (atmospheric water), and Viking 2 (surface pressure). We investigate the effect of snow radiative properties on CO2 and water cycles and the ability of our model to accurately reproduce those with minimal model tuning. We then perform simulations for several obliquities from a recent Martian past, and investigate the behavior of the Martian polar caps in such conditions.References: [1] Way, M. J. et al. (2017) ApJS, 231, 12. [2] Kelley, M. et al. (2020) J. Adv. Model. Earth Syst., 12, no. 8, e2019MS002025. [3] Schmidt, F. et al. (2022) Proc. Natl. Acad. Sci., 119, no. 4, e2112930118. [4] Guzewich, S.D. et al. (2021) J. Geophys. Res. Planets, 126, no. 7, e2021JE006825.

Eccentric planets may spend a significant portion of their orbits at large distances from their host stars, where low temperatures can cause atmospheric CO2 to condense out onto the surface, similar to the polar ice caps on Mars. The radiative effects on the climates of these planets throughout their orbits would depend on the wavelength-dependent albedo of surface CO2 ice that may accumulate at or near apoastron and vary according to the spectral energy distribution of the host star. To explore these possible effects, we incorporated a CO2 ice-albedo parameterization into a one-dimensional energy balance climate model. With the inclusion of this parameterization, our simulations demonstrated that F-dwarf planets require 29% more orbit-averaged flux to thaw out of global water ice cover compared with simulations that solely use a traditional pure water ice-albedo parameterization. When no eccentricity is assumed, and host stars are varied, F-dwarf planets with higher bond albedos relative to their M-dwarf planet counterparts require 30% more orbit-averaged flux to exit a water snowball state. Additionally, the intense heat experienced at periastron aids eccentric planets in exiting a snowball state with a smaller increase in instellation compared with planets on circular orbits; this enables eccentric planets to exhibit warmer conditions along a broad range of instellation. This study emphasizes the significance of incorporating an albedo parameterization for the formation of CO2 ice into climate models to accurately assess the habitability of eccentric planets, as we show that, even at moderate eccentricities, planets with Earth-like atmospheres can reach surface temperatures cold enough for the condensation of CO2 onto their surfaces, as can planets receiving low amounts of instellation on circular orbits.

Abstract We present simulations of the ancient Martian climate with the Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics (ROCKE-3D) general circulation model. We evaluate the efficacy of CO2-H2 and CO2-CH4 collision-induced absorption (CIA) in producing temperate conditions on Mars during the late Noachian-early Hesperian period near 3.7 Gya. We additionally study the fate of liquid water if it was on the surface during that time period in history. 1. Introduction Geologic evidence indicates that ancient Mars supported widespread surface liquid water approximately 3.5-4 Ga (e.g., [1], [2]). For nearly as long, climate modeling has attempted to find a self-consistent mixture of atmospheric gases with realistic atmospheric pressures that could support a hydrological cycle that is consistent with the geologic evidence (see [3] for an overview). Despite those efforts, substantial doubt remains about the feasibility of a “warm and wet” climate that could have existed for sufficient time to produce the geologic evidence during the period of the faint young Sun. Recently, [4] and [5] (among others) have shown that H2, in combination with CO2 and CH4, can produce efficient CIA that provides substantial warming at plausible surface pressures (i.e., <2 bar, see [6]) with modest H2 mixing ratios. Using the CIA tables provided by [5], we evaluate the ability to generate temperate climactic conditions on ancient Mars using the ROCKE-3D GCM. 1. Methodology We conduct a series of ROCKE-3D GCM [7] simulations in two broad groups that we term “dry” and “wet.” The “dry” group consists of simulations that are run without open liquid water initialized on the surface, while the “wet” group is initialized with some amount of surface liquid water as either lakes or fully-dynamic oceans. The dry simulations are conducted to evaluate what mix of pressure and gases can produce global surface temperatures above the freezing point of water. A range of pressures (from 0.5-2 bar) and H2 mixing ratio (0-10%) are evaluated. The wet simulations are all conducted with a surface pressure and gas mixture that is supportive of surface liquid water and is initialized with planetary water inventories from 10-500 m global equivalent layers. Both modern topography and a plausible paleotopography (following [8]) is used to evaluate the effect of the Tharsis emplacement and true polar wander on the climate state. We employ a stellar spectrum that is appropriate for 3.8 Ga. All simulations are run until radiative and hydrological equilibrium are reached. 1. Results and Conclusions We find that global mean surface air temperatures are only above freezing for high pressure (1.5-2 bar) and/or H2 mixing ratios of at least 3%. Using modern topography, the high elevations of Tharsis Montes remain below freezing, even with 2 bar surface pressure and 10% H2. At 1 bar surface pressures, only the lowest elevation areas (e.g., Hellas Planitia) experience any above-freezing temperatures during the year, but remain below freezing on an annual average basis (Figure 1). Including CH4 in the atmosphere (at 1%) produces a weak tropopause and distinct stratosphere (defined as warming temperatures with altitude), which also reduces cloud cover. Intriguingly, simulations without CH4 have increased cloud cover which serves as a more effective hygropause than the CH4-induced stratosphere, which may be relevant for ancient Martian water loss to space. The wet simulations that employ modern topography show that water is cold-trapped onto the Tharsis plateau, leaving comparatively little water (relative to the initial planetary inventory) in an active hydrological cycle. What water is available falls as both rain and snow onto Tharsis and near the planetary topographic dichotomy. The initial water inventory is not predictive of the location or amount of precipitation. However, planetary obliquity is important, with 0° obliquity showing increased amounts of precipitation, with some of it falling in locations congruent with valley network formations (e.g., [2]) (Figure 2). We will also present ongoing simulations with paleotopography and dynamic oceans. Figure 1: Percent of sols with above freezing daily average surface air temperatures for 10 ROCKE3D simulations with surface pressures and CO2 and H2 mixing ratios identified in the panel title. All simulations incorporate dry soil. The black line encloses the areas with 100%. Figure 2. Annual total liquid precipitation (mm) for 6 simulations initialized with surface liquid water as lakes as shown in the panel title. References [1] Masursky, H. (1973), An overview of geological results from Mariner 9, J. Geophys. Res., 78( 20), 4009– 4030, doi:10.1029/JB078i020p04009. [2] Hynek, B. M., Beach, M., and Hoke, M. R. T. (2010), Updated global map of Martian valley networks and implications for climate and hydrologic processes, J. Geophys. Res., 115, E09008, doi:10.1029/2009JE003548. [3] Wordsworth, R.D. (2016), The Climate of Early Mars, Annual Review of Earth and Planetary Science, 44, 381-408, https://doi.org/10.1146/annurev-earth-060115-012355. [4] Ramirez, R., Kopparapu, R., Zugger, M. et al. Warming early Mars with CO2 and H2.Nature Geosci 7, 59–63 (2014). https://doi.org/10.1038/ngeo2000. [5] Wordsworth, R.D., Kalugina, Y., Lokshtanov, S., Vigasin, A., Ehlmann, B., Head, J., Sanders, C., and Wang, H. (2017), Transient reducing greenhouse warming on early Mars, Geophys. Res. Lett., 44, 665– 671, doi:10.1002/2016GL071766. [6] Warren, A. O., Kite, E. S., Williams, J.‐P., & Horgan, B. ( 2019). Through the thick and thin: New constraints on Mars paleopressure history 3.8 ‐ 4 Ga from small exhumed craters. Journal of Geophysical Research: Planets, 124, 2793– 2818. https://doi.org/10.1029/2019JE006178. [7] Way, M.J., I. Aleinov, D.S. Amundsen, M.A. Chandler, T. Clune, A.D. Del Genio, Y. Fujii, M. Kelley, N.Y. Kiang, L. Sohl, and K. Tsigaridis, 2017: Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics 1.0: A general circulation model for simulating the climates of rocky planets. Astrophys. J. Supp. Series, 231, no. 1, 12, doi:10.3847/1538-4365/aa7a06. [8] Bouley, S., Baratoux, D., Matsuyama, I. et al (2016). Late Tharsis formation and implications for early Mars, Nature 531, 344–34, https://doi.org/10.1038/nature17171.

Potentially habitable exoplanets are targets of great interest for the James Webb Space Telescope and upcoming mission concepts such as the Habitable Worlds Observatory. Clouds strongly affect climate and habitability, but predicting their properties is difficult. In Global Climate Models (GCMs), especially those aiming at simulating Earth, cloud microphysics is often crudely approximated by assuming that all cloud particles have a single, constant size or a prescribed size distribution and that all clouds in a grid cell are identical. For exoplanets that range over a large phase space of planetary properties, this method could result in large errors. In this work, our goal is to determine how cloud microphysics on terrestrial exoplanets, whose condensable is mainly water vapor, depend on aerosol properties and planetary parameters such as surface pressure, surface gravity, and incident stellar radiation. We use the Community Aerosol and Radiation Model for Atmospheres as a 1D microphysical model to simulate the formation and evolution of clouds including the processes of nucleation, condensation, evaporation, coagulation, and vertical transfer. In these 1D idealized experiments, we find that the parameters that determine the macrophysical thermal structure of the atmospheres, including surface pressure and stellar flux, impact cloud radiative effect (CRE) most significantly. Parameters such as gravity and number density of aerosols working as cloud condensation nuclei affect the microphysical processes of cloud formation, including activation and vertical transfer. They also have a significant, though weaker effect on CRE. This work motivates the development of more accurate GCM cloud schemes and should aid in the interpretation of future observations.

This paper highlights methods from geostatistics that are relevant to the interpretation, intercomparison, and synthesis of atmospheric model data, with a specific application to exoplanet atmospheric modeling. Climate models are increasingly used to study theoretical and observational properties of exoplanets, which include a hierarchy of models ranging from fast and idealized models to those that are slower but more comprehensive. Exploring large parameter spaces with computationally expensive models can be accomplished with sparse sampling techniques, but analyzing such sparse samples can pose challenges for conventional interpolation functions. Ordinary kriging is a statistical method for describing the spatial distribution of a data set in terms of the variogram function, which can be used to interpolate sparse samples across any number of dimensions. Variograms themselves may also be useful diagnostic tools for describing the spatial distribution of model data in exoplanet atmospheric model intercomparison projects. Universal kriging is another method that can synthesize data calculated by models of different complexity, which can be used to combine sparse samples of data from slow models with larger samples of data from fast models. Ordinary and universal kriging can also provide a way to synthesize model predictions with sparse samples of exoplanet observations and may have other applications in exoplanet science.

Methane is thought to have been an important greenhouse gas during the Archean, although its potential warming has been found to be limited at high concentrations due to its high shortwave absorption. We use the Met Office Unified Model, a general circulation model, to further explore the climatic effect of different Archean methane concentrations. Surface warming peaks at a pressure ratio CH$_4$:CO$_2$ of approximately 0.1, reaching a maximum of up to 7 K before significant cooling above this ratio. Equator-to-pole temperature differences also tend to increase up to pCH$_4$ $\leq$300 Pa, which is driven by a difference in radiative forcing at the equator and poles by methane and a reduction in the latitudinal extend of the Hadley circulation. 3D models are important to fully capture the cooling effect of methane, due to these impacts of the circulation.

Rocky planets orbiting M-dwarf stars are among the most promising and abundant astronomical targets for detecting habitable climates. Planets in the M-dwarf habitable zone are likely synchronously rotating, such that we expect significant day-night temperature differences, and potentially limited fractional habitability. Previous studies have focused on scenarios where fractional habitability is confined to the substellar or "eye" region, but in this paper we explore the possibility of planets with terminator habitability, defined by the existence of a habitable band at the transition between a scorching dayside and a glacial nightside. Using a global climate model, we show that for water-limited planets it is possible to have scorching temperatures in the "eye" and freezing temperatures on the nightside, while maintaining a temperate climate in the terminator region, due to a reduced atmospheric energy transport. Whereas on water-rich planets, increasing stellar flux leads to increased atmospheric energy transport and a reduction in day-night temperature differences, such that the terminator does not remain habitable once the dayside temperatures approach runaway or moist greenhouse limits. We also show that, while water-abundant simulations may result in larger fractional habitability, they are vulnerable to water loss through cold-trapping on the nightside surface or atmospheric water vapor escape, suggesting that even if planets were formed with abundant water, their climates could become water-limited and subject to terminator habitability.

We present the discovery of an Earth-mass planet (M-b sin i = 1.26 +/- 0.21 M-circle plus) on a 15.6 d orbit of a relatively nearby (d similar to 9.6 pc) and low-mass (0.167 +/- 0.011 M-circle dot) M5.0 V star, Wolf 1069. Sitting at a separation of 0.0672 +/- 0.0014 au away from the host star puts Wolf 1069 b in the habitable zone (HZ), receiving an incident flux of S = 0.652 +/- 0.029 S-circle plus. The planetary signal was detected using telluric-corrected radial-velocity (RV) data from the CARMENES spectrograph, amounting to a total of 262 spectroscopic observations covering almost four years. There are additional long-period signals in the RVs, one of which we attribute to the stellar rotation period. This is possible thanks to our photometric analysis including new, well-sampled monitoring campaigns undergone with the OSN and TJO facilities that supplement archival photometry (i.e., from MEarth and SuperWASP), and this yielded an updated rotational period range of P-rot = 150-170 d, with a likely value at 169.3(-3.6)(+3.7)d. The stellar activity indicators provided by the CARMENES spectra likewise demonstrate evidence for the slow rotation period, though not as accurately due to possible factors such as signal aliasing or spot evolution. Our detectability limits indicate that additional planets more massive than one Earth mass with orbital periods of less than 10 days can be ruled out, suggesting that perhaps Wolf 1069 b had a violent formation history. This planet is also the sixth closest Earth-mass planet situated in the conservative HZ, after Proxima Centauri b, GJ 1061 d, Teegarden's Star c, and GJ 1002 b and c. Despite not transiting, Wolf 1069 b is nonetheless a very promising target for future three-dimensional climate models to investigate various habitability cases as well as for sub-m s(-1) RV campaigns to search for potential inner sub-Earth-mass planets in order to test planet formation theories.

Planets in synchronous rotation around low-mass stars are the most salient targets for current ground- and space-based missions to observe and characterize. Such model calculations can help to prioritize targets for observation with current and future missions; however, intrinsic differences in the complexity and physical parameterizations of various models can lead to different predictions of a planet's climate state. Understanding model differences is necessary if such models are to guide target selection and aid in the analysis of observations. This paper presents a protocol to intercompare models of a hypothetical planet with a 15-day synchronous rotation period around a 3000 K blackbody star across a parameter space of surface pressure and incident instellation. We conduct a sparse sample of 16 cases from a previously published exploration of this parameter space with the ExoPlaSim model. By selecting particular cases across this broad parameter space, the SAMOSA intercomparison will identify areas where simpler models are sufficient, as well as areas where more complex GCMs are required. Our preliminary comparison using ExoCAM shows general consistency between the climate state predicted by ExoCAM and ExoPlaSim except in regions of the parameter space most likely to be in a steam atmosphere or incipient runaway greenhouse state. We use this preliminary analysis to define several options for participation in the intercomparison by models of all levels of complexity. The participation of other GCMs is crucial to understand how the atmospheric states across this parameter space differ with model capabilities.

We present simulation results of the dust cycle on Mars using the NASA Goddard Institute for Space Studies (GISS) ROCKE-3D [1] general circulation model with radiatively active dust aerosol tracers. Dust aerosols are represented by a sectional scheme that partitions the simulated dust mass into eight size classes, covering a total size range from 0.1 to 32 μm particle diameter. The model simulates emission from sources, advection, and turbulent, gravitational, and wet deposition of dust. The strength of the dust cycle can be calibrated with a global factor for the dust emission. ROCKE-3D is coupled to the Suite of Community Radiative Transfer codes based on Edwards and Slingo (SOCRATES) [2,3], which applies Mie theory to calculate scattering and absorption of radiation by aerosols. We carried out a series of experiments over 11 Mars years, for which we varied the strength of the dust cycle, and for radiatively active and inactive dust. The simulated dust aerosol optical depths were compared to gridded retrievals of the dust AOD from measurements over 11 years [4, 5]. We find that the dust cycle displays nonlinear behavior with the strength of emission, when the dust is radiatively active, which is absent for radiatively inactive dust. When the dust cycle strength exceeds a certain threshold the simulated mean annual cycle of dust starts to exhibit features that are similar to the observed mean annual cycle. We hypothesize that feedbacks involving the dust radiative effect introduce important non-linearities, which are essential for reproducing and understanding the observed dust cycle on Mars. References: [1] Way, M. J. et al. (2017) ApJS, 231, 12. [2] Edwards, J. M. (1996), JAtS, 53, 1921. [3] Edwards, J. M., & Slingo, A. (1996), QJRMS, 122, 689. [4] Montabone, L. et al. (2015) Icarus, 251, 65. [5] Montabone, L. et al. (2020) JGR Planets, 2019JE006111.

To identify promising exoplanets for atmospheric characterization and to make the best use of observational data, a thorough understanding of their atmospheres is needed. Three-dimensional general circulation models (GCMs) are one of the most comprehensive tools available for this task and will be used to interpret observations of temperate rocky exoplanets. Due to parameterization choices made in GCMs, they can produce different results, even for the same planet. Employing four widely used exoplanetary GCMs-ExoCAM, LMD-G, ROCKE-3D, and the UM-we continue the TRAPPIST-1 Habitable Atmosphere Intercomparison by modeling aquaplanet climates of TRAPPIST-1e with a moist atmosphere dominated by either nitrogen or carbon dioxide. Although the GCMs disagree on the details of the simulated regimes, they all predict a temperate climate with neither of the two cases pushed out of the habitable state. Nevertheless, the intermodel spread in the global mean surface temperature is nonnegligible: 14 K and 24 K in the nitrogen- and carbon dioxide-dominated case, respectively. We find substantial intermodel differences in moist variables, with the smallest amount of clouds in LMD-Generic and the largest in ROCKE-3D. ExoCAM predicts the warmest climate for both cases and thus has the highest water vapor content and the largest amount and variability of cloud condensate. The UM tends to produce colder conditions, especially in the nitrogen-dominated case due to a strong negative cloud radiative effect on the day side of TRAPPIST-1e. Our study highlights various biases of GCMs and emphasizes the importance of not relying solely on one model to understand exoplanet climates.

The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) is a community project that aims to quantify how differences in general circulation models (GCMs) could impact the climate prediction for TRAPPIST-1e and, subsequently, its atmospheric characterization in transit. Four GCMs have participated in THAI: ExoCAM, LMD-Generic, ROCKE-3D, and the UM. This paper, focused on the simulated observations, is the third part of a trilogy, following the analysis of two land planet scenarios (Part I) and two aquaplanet scenarios (Part II). Here we show a robust agreement between the simulated spectra and the number of transits estimated to detect the land planet atmospheres. For the cloudy aquaplanet ones, a 5 σ detection of CO 2 could be achieved in about 10 transits if the atmosphere contains at least 1 bar of CO 2 . That number can vary by 41%–56% depending on the GCM used to predict the terminator profiles, principally due to differences in the cloud deck altitude, with ExoCAM and LMD-G producing higher clouds than ROCKE-3D and UM. Therefore, for the first time, this work provides “GCM uncertainty error bars” of ∼50% that need to be considered in future analyses of transmission spectra. We also analyzed the intertransit spectral variability. Its magnitude differs significantly between the GCMs, but its impact on the transmission spectra is within the measurement uncertainties. THAI has demonstrated the importance of model intercomparison for exoplanets and also paved the way for a larger project to develop an intercomparison meta-framework, namely, the Climates Using Interactive Suites of Intercomparisons Nested for Exoplanet Studies.