Background
Weather and climate are experienced in the troposphere, which extends upward to the tropopause at about 15 km. The Earth's middle atmosphere (the stratosphere and the mesosphere) extends from the tropopause to approximately 90 km (Figure 1.2). Understanding the middle atmosphere is crucial for global change studies for two primary reasons. Firstly, this is where approximately 90 percent of the Earth's ozone shield resides and, secondly, the region constitutes the upper boundary of the troposphere. Changes in the ozone layer modulate the amount of ultraviolet (UV) radiation reaching the biosphere and are thus of direct concern for life on Earth. A decrease in column ozone of 1 percent causes the UV dose in the spectral region damaging to deoxyribonucleic acid (DNA) to increase by about 2 percent. Changes in ozone, temperature, and other trace gases have been widely implicated in a variety of the mechanisms by which changes in the middle atmosphere might influence the biosphere by physically coupling to the troposphere and modifying the climate (NAS, 1982; Lacis et al., 1990; Schwarzkopf and Ramaswamy, 1993; Hauglustaine et al., 1994; Rind and Balachandran, 1994). Crucial to these mechanisms is ozone, which is highly responsive to, and also has a controlling influence on, the state of the middle atmosphere.
Ozone is directly influenced by changes in solar radiative and energetic particle inputs to this region.
All of the Sun's ultraviolet energy input at wavelengths between about 150 and 300 nm is deposited in the Earth's middle atmosphere. This energy plays an essential role in the chemistry, radiation, and dynamics of the region. Figure 1.2 shows the altitude at which solar radiation (from an overhead Sun) reaches 1/e of its original intensity and gives an approximate measure of the penetration depth for different wavelengths.
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