Changes in climate are driven by natural and human-induced perturbations of the Earth's energy balance. These climate drivers or "forcings" include variations in greenhouse gases, aerosols, land use, and the amount of energy Earth receives from the Sun. Although climate throughout Earth's history has varied from "snowball" conditions with global ice cover to "hothouse" conditions when glaciers all but disappeared, the climate over the past 10,000 years has been remarkably stable and favorable to human civilization. Increasing evidence points to a large human impact on global climate over the past century. The report reviews current knowledge of climate forcings and recommends critical research needed to improve understanding. Whereas emphasis to date has been on how these climate forcings affect global mean temperature, the report finds that regional variation and climate impacts other than temperature deserve increased attention.

“The Early Evolution of the Atmospheres of Terrestrial Planets” presents the main processes participating in the atmospheric evolution of terrestrial planets. A group of experts in the different fields provide an update of our current knowledge on this topic. Several papers in this book discuss the key role of nitrogen in the atmospheric evolution of terrestrial planets. The earliest setting and evolution of planetary atmospheres of terrestrial planets is directly associated with accretion, chemical differentiation, outgassing, stochastic impacts, and extremely high energy fluxes from their host stars. This book provides an overview of the present knowledge of the initial atmospheric composition of the terrestrial planets. Additionally it includes some papers about the current exoplanet discoveries and provides additional clues to our understanding of Earth’s transition from a hot accretionary phase into a habitable world. All papers included were reviewed by experts in their respective fields. We are living in an epoch of important exoplanet discoveries, but current properties of these exoplanets do not match our scientific predictions using standard terrestrial planet models. This book deals with the main physio-chemical signatures and processes that could be useful to better understand the formation of rocky planets.

"Through the contributions of more than sixty leading experts in the field, Comparative Climatology of Terrestrial Planets sets forth the foundations for this emerging new science and brings the reader to the forefront of our current understanding of atmospheric formation and climate evolution"--Provided by publisher.

In the next few years, the launch of the James Webb Space Telescope (JWST), along with the construction of new ground-based observatories, will provide the opportunity to attempt atmospheric characterization of terrestrial planets in the habitable zones of nearby M dwarf stars. For the first time, the assessment of habitability and the possibility of detecting biosignatures from planets around other stars will be within the capabilities of astronomical observatories. Truly Earth-like planets (i.e. orbiting a Sun-like, G-type star) are not yet accessible, and may not be until the selection, construction, and launch of a next-generation space telescope, such as LUVOIR or HabEx, which are under consideration for potential prioritization by the 2020 Decadal Survey on Astronomy and Astrophysics. In the immediate future, it will only be possible to characterize the atmospheres of Earth-sized planets that orbit M dwarf hosts, because the methods of observation for imminent observatories favor shorter-period planets and stronger signal can be achieved with smaller star-planet size ratios. However, planets orbiting M dwarfs face an evolutionary history very different than a planet like Earth, orbiting a Sun-like, G-type star. Additionally, these stars go through a much longer superluminous pre-main-sequence phase than G dwarfs, which can drive ocean loss via the runaway greenhouse effect, subsequent photolysis from stellar UV radiation, and, finally, permanent loss of hydrogen to space. As a result, M dwarf habitable zone planets can be stripped of their volatiles before life could originate and proliferate. Even if life did originate, M dwarf stars generally exhibit intense levels of high-energy activity throughout their main-sequence lifetimes, so the planetary surface can experience much more extreme irradiation than the early Earth environment. Additionally, because M dwarfs are small and dim, planets must orbit much closer to the star than Earth does to the Sun to allow for the possibility of liquid water on their surfaces. This proximity increases the probability for such planets to be synchronously rotating with their host star, which may result in large temperature differences between the permanent day and permanent night sides, raising the possibility of atmospheric collapse on the night side of the planet. Despite these challenges, the observational advantages of M dwarf stars mean that they will be the first place to search for habitability and life outside the Solar System. Several small planets have recently been discovered in the habitable zones around nearby M dwarf stars during ground-based surveys (e.g. TRAPPIST, MEarth, and HARPS). Of these, I focus on the TRAPPIST-1 planetary system, whose seven Earth-sized planets provide an unprecedented opportunity to study planetary evolution and habitability in a single system, which includes three planets in the traditional habitable zone. As TRAPPIST-1 is an ultra-cool dwarf star (spectral type M8V), its planets are more easily amenable to near-term observations compared to other terrestrial-sized planet discoveries around earlier-type stars (e.g. LHS 1140 b and c, Ross 128 b), because of the exceptionally diminutive size of the TRAPPIST-1 star (barely larger than Jupiter), maximizing the planet-to-star signal. To support upcoming observations of nearby M dwarf planetary systems, I provide foundational modeling efforts to understand the range of likely environmental states of the TRAPPIST-1 planets and how to spectrally discriminate them. I developed a versatile, coupled climate-photochemical model for terrestrial exoplanets. Using this model, I present self-consistent climate-photochemical model atmospheres of a wide range of potential TRAPPIST-1 planetary states, and generate and analyze synthetic spectra of these planets to identify observational features that can be used to distinguish between these planetary environmental outcomes. The modeled planetary states span evolved, post-runaway, desiccated planets with thick atmospheres, to a variety of water worlds. To assess a variety of environments that could be possible using a robust radiative transfer model, but also consider the day-night differences these planets may experience, I develop a two-column, day-night mode for an advanced 1D radiative-convective climate model, VPL Climate. The diversity of possible environments modeled here supports the habitable zone as probabilistic: encompassing a range of possible states for each planet, which may or may not be habitable. Planets within the habitable zone could be either freezing, temperate, or hot, depending on their atmospheric composition. Planets beyond the outer edge, such as TRAPPIST-1 h, could also have temperate or hot atmospheres, if they have a Venus-like greenhouse effect. Potential observational discriminants for these atmospheres in transmission and emission spectra are influenced by photochemical processes and aerosol formation. The atmospheric states simulated here include collision-induced oxygen absorption (O2-O2), and O3, CO, SO2, and H2O absorption features, with transit signals of up to 200 ppm, well above the 20-30 ppm putative noise floor of JWST in the NIR. These simulated transmission spectra are consistent with K2, Hubble Space Telescope, and Spitzer Space Telescope observations of the TRAPPIST-1 planets. To help discriminate ambiguous observations, including the detection of water vapor, I assess the possibility of detecting isotopic evidence for ocean loss in transit transmission spectra. In the Solar System, differences in isotopic abundances between the Solar abundance and planetary atmospheres have been used to infer the history of ocean loss and atmospheric escape (e.g. Venus, Mars). I show that H2O and CO2 isotopologues could similarly be used as indicators of past ocean loss and atmospheric escape of terrestrial planets around M dwarfs. These measurements may be possible with JWST if the escape mechanisms and resulting isotopic fractionation were similar to Venus, but exist in a more transparent atmosphere, such as N2-dominated, or an O2-dominated atmosphere that may result from extreme water loss. In these atmospheres, isotopologue bands are detectable throughout the near-infrared (1-8 [micro]m), especially 3-4 [micro]m. These are not likely detectable in CO2-dominated atmospheres because the saturated CO2 bands obscure key HDO features, and at the high temperatures exhibited by a Venus-like atmosphere, the ro-vibrational quantum states of the rare isotopologues are not occupied. The results of spectral modeling suggest that the detection of O2-O2 along with increased fractionation in HDO relative to Earth would be strong evidence that a planet is not habitable, despite detections of atmospheric oxygen and water, which would normally be considered evidence of an inhabited Earth-like world. The results of this dissertation have demonstrated a small but diverse selection of plausible planetary conditions given current knowledge of planetary processes that may exist on other worlds, which nonetheless have provided a broad exploration of environmental states for the TRAPPIST-1 planets. The combined studies point to multiple spectral discriminants to identify past ocean loss and to differentiate between different environmental states. Although spatially-resolved models (from two columns to full 3D GCMs) can assess the climate distribution on a planet, transit transmission spectra are most sensitive to regions of the atmosphere where temperature gradients are usually small, and where the primary processes are radiation and photochemistry, a regime ideally suited to 1D coupled climate-photochemical models. The spectral discriminants presented here and in future work will help guide and interpret upcoming observations of planets in and around the habitable zones of M dwarf stars, particularly the TRAPPIST-1 system, which is already scheduled for observation with JWST.

Given the universal interest in whether extraterrestrial life has developed or could eventually develop, it is vital that an examination of planetary habitability go beyond simple assumptions. This book has resulted from a workshop at the International Space Science Institute (ISSI) which brought together experts to discuss the multi-faceted problem of how the habitability of a planet co-evolves with the geology of the surface and interior, the atmosphere, and the magnetosphere.

The warming of the Earth has been the subject of intense debate and concern for many scientists, policy-makers, and citizens for at least the past decade. Climate Change Science: An Analysis of Some Key Questions, a new report by a committee of the National Research Council, characterizes the global warming trend over the last 100 years, and examines what may be in store for the 21st century and the extent to which warming may be attributable to human activity.