Comparative Climatology of Terrestrial Planets

1. INTRODUCTION The greenhouse effect occurs when short-wavelength solar radiation penetrates an atmosphere more readily than the long-wavelength infrared radiation (IR) emanating from the surface and from the atmosphere itself. Fourier (1827) pointed out that this effect would warm Earth’s surface by trapping heat supplied by the Sun. He also noted that freely rising warm air — natural convection — would counteract the effect to some extent (so the greenhouse analogy is somewhat misleading). Subsequent nineteenth- and twentieth-century work beginning with laboratory measurements by Tyndall (1873) quantified IR absorption by water vapor, carbon dioxide (CO2), and other atmospheric constituents. Arrhenius (1896) and Calendar (1938) then computed surface temperature increases due to atmospheric CO2 produced by fossil fuel burning. These were only hypothetical increases because the ocean could potentially absorb nearly all the CO2. But when Revelle and Suess (1957) demonstrated that a substantial fraction of human-produced CO2 remains in the atmosphere, longstanding theory then implied that global warming would follow. Although human-induced global warming (a change of temperature) is logically distinct from the natural greenhouse effect and its influence on equilibrium steady-state temperature, the two processes are related. Indeed, as observations of human-produced atmosphere CO2 accumulated (Keeling et al., 1976), spacecraft observations of Venus found both a massive CO2 atmosphere and an extremely hot surface (Hunten and Goody, 1969; Pollack et al., 1980). Modern observations have confirmed natural greenhouse effects on Earth and other worlds (Houghton, 2002) as well as “very likely” anthropogenic global warming on Earth during recent decades (Solomon et al., 2007). The greenhouse effect is only one of many factors that determine climate. Solar luminosity is the ultimate energy source of a world unless its internal heat sources are significant . Atmospheric gases, cloud particles, aerosols (other suspended particles), and the surface absorb and scatter solar radiation as well as IR; at the temperatures considered in this chapter they also emit significant amounts of IR. Therefore any change of solar behavior, atmospheric constituents, or surface properties can potentially change weather and climate. Traditionally such changes are divided into “forcing factors” external to the system under consideration (e.g., solar luminosity variations, asteroid and comet impacts, volcanic eruptions) and “feedback processes” that result from the initial climate change due to the forcing factors . Feedback can either be positive or negative, i.e., either reinforcing or counteracting the initial climate change. This chapter applies the above concepts to the four known terrestrial worlds possessing substantial atmospheres : Venus, Earth, Mars, and Titan. In this chapter a “terrestrial” world is defined as mainly rocky in composi163 The Greenhouse Effect and Climate Feedbacks Curt Covey Lawrence Livermore National Laboratory Robert M. Haberle and Christopher P. McKay NASA Ames Research Center Dmitri V. Titov European Space Agency This chapter reviews the theory of the greenhouse effect and climate feedback. It also compares the theory with observations, using examples taken from all four known terrestrial worlds with substantial atmospheres: Venus, Earth, Mars, and Titan. The greenhouse effect traps infrared radiation in the atmosphere, thereby increasing surface temperature. It is one of many factors that affect a world’s climate. (Others include solar luminosity and the atmospheric scattering and absorption of solar radiation.) A change in these factors — defined as climate forcing — may change the climate in a way that brings other processes — defined as feedbacks — into play. For example, when Earth’s atmospheric carbon dioxide increases, warming the surface, the water vapor content of the atmosphere increases. This is a positive feedback on global warming because water vapor is itself a potent greenhouse gas. Many positive and negative feedback processes are significant in determining Earth’s climate, and probably the climates of our terrestrial neighbors. Covey C., Haberle R. M., McKay C. P., and Titov D. V. (2013) The greenhouse effect and climate feedbacks. In Comparative Climatology of Terrestrial Planets (S. J. Mackwell et al., eds.), pp. 163–179. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816530595-ch007. 164 Comparative Climatology of Terrestrial Planets tion, thus excluding the large gaseous and watery planets, and “substantial atmosphere” means (rather arbitrarily) a surface pressure ≥1 mbar, thus excluding Triton. It so happens that none of the four selected worlds possesses internal heat sources large enough to directly affect its global weather and climate. 2. RADIATIVE BALANCE AND GLOBAL TEMPERATURE 2.1. The Greenhouse Effect and Global Energy Balance 2.1.1. Wavelength dependence of infrared emission. Spacecraft observing Earth and other terrestrial worlds detect atmospheric IR absorption at the wavelengths and...