[en] The present comparative evaluation of the long-term evolution of the Venus, earth, and Mars climates suggests that the earth's climate has remained temperate over most of its history despite a secular solar luminosity increase in virtue of a negative-feedback cycle based on atmospheric CO₂ levels and climate. The examination of planetary climate histories suggests that an earth-sized planet should be able to maintain liquid water on its surface at orbital distances in the 0.9-1.5 AU range, comparable to the orbit of Mars; this, in turn, implies that there may be many other habitable planets within the Galaxy

[en] At high obliquity, Martian polar ground temperatures could exceed the melting point of ice for considerable periods of time (approximately 90 Earth days). Under special conditions ice itself might melt. Carbon dioxide adsorbed on the Martian regolith is not expected to buffer the seasonal pressure wave except in the unlikely event that the soil pore size is very large (50 micrometers). For a basaltic soil composition the maximum CO₂ that could be desorbed over obliquity time scales due to thermal forces is a few millibars. At low obliquities the atmospheric pressures may drop, desorbing the soil. The only means to achieve higher CO₂ pressures is to have much higher planet-wide temperatures due to some greenhouse effect, or to be at an epoch before the regolith or carbonates formed. The water ice budget between north and south polar caps was considered, and summer sublimation rates imply that the ice could be exchanged between the poles during obliquity cycles. A critical factor in the polar cap water budget is the interaction of water and dust. The origin of the Martian polar laminae is probably due to variations in this interaction

[en] Concern has been raised over the short- and long-term consequences of the dust, smoke, radioactivity, and toxic vapors that would be generated by a nuclear war. The discovery that dense clouds of soil particles may have played a major role in past mass extinctions of life on Earth has encouraged the reconsideration of nuclear war effects. These developments have led the authors to calculate, using new data and improved models, the potential global environmental effects of dust and smoke clouds (henceforth referred to as nuclear dust and smoke) generated in a nuclear war. They neglect the short-term effects of blast, fire, and radiation. Most of the world's population could probably survive the initial nuclear exchange and would inherit the postwar environment. Accordingly, the longer-term and global-scale aftereffects of nuclear war might prove to be as important as the immediate consequences of the war

[en] The authors present a study of methane moist convection on Neptune. They examine the stability conditions to initiate moist convection of methane clouds. Temperature lapse rates that are unstable to moist convection are subadiabatic by a factor of two or more while adiabatic and superadiabatic lapse rates are stable. In the observed Neptune temperature structure, vertical velocities ≥ 40 m s^-1 are required to lift moist air above the stable region and initiate moist convection. Alternatively, moist convection could be initiated by air that is slightly drier than its surroundings. Moist convective clouds achieve altitudes up to 100 mb, thereby exceeding the temperature minimum and depositing methane in the Neptune stratosphere. They predict that at least 2000 such clouds penetrate the Neptune stratosphere per hour

[en] A number of studies have suggested that anthropogenic increases in aerosol production will alter the earth's climate. The change is due not only to the direct effect of the aerosols on the earth's radiation budget, but also to the indirect effects of aerosols on clouds. During the past few years we have been systematically examining the relations between changes in aerosol abundance and cloud properties with the goal of quantifying these interactions. Findings indicate that not all types of clouds respond in the same manner to changes in aerosol concentrations and that some hypotheses may be incorrect. By coupling a radiative transfer model to a microphysical cloud model and developing a one-dimensional turbulent transport model to represent boundary-layer dynamical processes, different cloud type responses can be simulated. The sensitivity of marine stratus, cirrus, and continental stratus cloud types are discussed

[en] We review current observational and theoretical knowledge of the stratospheric aerosols. These particles, which are composed primarily of sulfuric acid and other sulfates, are concentrated in a layer extending 20 km or more above the tropopause. The aerosols affect the chemistry of the stratosphere and the climatology of the earth. A number of important chemical and physical roles for the aerosols are discussed. We describe the properties of stratospheric aerosols as revealed by experimental data. Remote-sensing optical instruments, both active (lidars) and passive (satellite), allow routine mapping of the global aerosol distribution. Extensive in situ measurements obtained by mechanical collection (filters and impactors) and scattered-light detection yield the overall size dispersion of the aerosols. Laboratory analyses of preserved aerosol samples define the bulk composition (and possible origins) of the particles. Quantitative studies of aerosol precursor gases (SO₂, OCS, and CS₂) by wet chemical, cryogenic, and spectroscopic techniques reveal the photochemical sources of particulate mass. Theoretical aspects of the stratospheric aerosols are also reviewed. We discuss aerosol chemical reactions including those of gaseous precursors, those in aqueous solution, and those on particle surfaces. We also describe aerosol microphysical processes including neculeation, condensation/evaporation, coagulation, and sedimentation. Existing models of aerosols which incorporate these chemical and physical processes are outlined. Aerosol model predictions are appraised vis a vis observations. The simulations are shown to agree with measurments in many important respects. Areas requiring further investigation include the identification of the nucleation mechanisms for the aerosols and the characterization of the tenuous upper extent of the aerosol layer above approx.25 km

[en] The atmosphere of Titan is characterized by means of model computations based on Voyager IRIS IR spectra and published data from laboratory determinations of absorption coefficients and cloud refractive indices. The results are presented in tables and graphs, and it is pointed out that the presence of Ar is not required in the model. Particular attention is given to the role of CH4, which is found to form patchy clouds (with particle radii of 50 microns or greater and visible/IR optical depths of 2-5) at altitudes up to about 30 km. The mechanisms by which such rain-sized particles could form are discussed, and it is suggested that the observed 500-600/cm spectrum is affected much less by the CH4 clouds than by H2 or variations in the temperature of the high-altitude haze. 42 references

[en] Recent measurements of the single-scattering albedo omega-tilde₀ of tropospheric aerosols indicate the presence of a strongly absorbing material which has tentatively been identified as graphitic carbon (soot). Theoretical calculations, based on several different models of the way in which soot might be mixed with other aerosol materials, show that a minimum of 20% soot by volume is necessary to achieve the observed urban value of omega-tilde₀ = 0.6. Rural values of the order of 0.8 can be accounted for with 1--5% soot by volume. These same values of omega-tilde₀ can be produced by similar amounts of the iron oxide magnetite, which is shown to be virtually indistinguishable from soot by optical measurements performed on bulk samples. Calculations of phase functions for various mixtures of soot also indicate the diffuculty of determining aerosol composition by optical scattering techniques. The climatic effects of these absorbing aerosols are computed using a simple one-layer model, and the results suggest that heating rates in urban pollution layers may be of the order of 4 K/day

[en] An outline of the most recent and complete reviews on the history of Martian climate is presented and compared to the history of terrestrial climate. Suggestions are made on how solar luminosity changes may have affected both planets. Observations of the albedo of Titan occurring over a solar cycle are discussed, and the solar mechanisms that might be responsible for the observed albedo changes are considered

[en] Recent findings by this group confirmed by workers in Europe, the US and the USSR, suggest that the long-term climatic effects of a major nuclear war are likely to be much severer and farther-reaching than had been supposed. In the aftermath of such a war vast areas of the earth could be subjected to prolonged darkness, abnormally low temperatures, violent windstorms, toxic smog and persistent radioactive fallout - in short, the combination of conditions that has come to be known as nuclear winter. In brief, the authors' initial results, published in Science in December, 1983, showed that the potential global atmospheric and climatic consequences of nuclear war...are serious. Significant hemispherical attenuation of the solar radiation flux and subfreezing land temperatures may be caused by fine dust raised in high-yield nuclear surface bursts and by smoke from city and forest fires ignited by airbursts of all yields. Subsequent studies, based on more powerful models of the general circulation of the earth's atmosphere, have tended to confirm both the validity of the authors' investgative approach and the main thrust of their findings. Most of this article is devoted to reviewing the current state of knowledge on this vital issue