Questions of long-term climate change and the influence of humans on the earth's environment are of concern to citizens and governments. Much of the research on which the discussion is based originated in attempts to explain the climates of other planets, including Venus with its runaway greenhouse effect, Mars with its thin atmosphere, and Jupiter, Saturn, and Neptune with their dramatic storm systems (Plate 8.1). Astronomers and atmospheric scientists have developed models to help understand and eventually predict the dynamics of planetary atmospheres and the physical conditions that result in environments hospitable to life. Some of the models work tolerably well for the simplest planetary atmosphere, that of the planet Mars, but fail for more complex planets, including our own. These models can be improved by comparison with the observations of other planets to make reliable predictions for our own environment.
Weather satellites are one of the practical benefits of the space age. The tropospheric temperature sounders used in national security applications and soon to be used in civilian weather satellites are direct descendants of the planetary radio astronomy instruments used to probe the atmosphere of Venus. Remote sensing from satellites is one of the best methods for monitoring the earth's ecosystem.
Radio astronomers, for example, have adapted the techniques of millimeter wave astronomy to studies of ozone depletion. In 1977 astronomers initiated a program to measure the stratospheric concentration of chlorine oxide (ClO), the most important tracer of the destruction of ozone by chlorofluorocarbons. Measurements of the diurnal variation of ClO in the middle stratosphere provided a critical test of the proposed photochemical models (Solomon et al., 1984). Subsequent measurements of the high concentration of ClO in the lower stratosphere during early spring and its subsequent disappearance in October (de Zafra et al., 1987), when ozone levels returned to normal, demonstrated that chlorine chemistry was responsible for the Antarctic ozone hole (Plate 8.2). Automatic millimeter wave instruments built by a commercial company founded by astronomers will measure both ozone and ClO as part of a worldwide network.
Atmospheric ozone also varies significantly due to natural causes. The solar activity cycle produces an 11-year variation in the sun 's ultraviolet radiation, and this in turn affects the terrestrial ozone abundance. Solar variability in the ultraviolet must be known in sufficient detail to delimit the natural causes of ozone change before one can confidently extract the man-made component of that change. Ultraviolet solar variability is thus of practical as well as astronomical interest.