7.2.4.2 Radiative processes in the stratosphere

The stratosphere lies immediately above the troposphere, with the height of the bounding tropopause varying from about 15 km in the tropics to about 7 km at high latitudes. The mass of the stratosphere represents only about 10 to 20% of the total atmospheric mass, but changes in stratospheric climate are important because of their effect on stratospheric chemistry, and because they enter into the climate change detection problem (Randel and Wu, 1999; Shine and Forster, 1999). In addition there is a growing realisation that stratospheric effects can have a detectable and perhaps significant influence on tropospheric climate.

Solar radiative heating of the stratosphere is mainly from absorption of ultraviolet (UV) and visible radiation by ozone, along with contributions due to the near-infrared absorption by carbon dioxide and water vapour. Depletion of the direct and diffuse solar beams arises from scattering by molecules, aerosols, clouds and surface (Lacis and Hansen, 1974).

The long-wave process consists of absorption and emission of infrared radiation, principally by carbon dioxide, methane, nitrous oxide, ozone, water vapour and halocarbons (CFCs, HFCs, HCFCs, PFCs etc.). The time-scales for the radiative adjustment of stratospheric temperatures is less than about 50 to 100 days.

For CO₂, part of the main 15 micron band is saturated over quite short vertical distances, so that some of the upwelling radiation reaching the lower stratosphere originates from the cold upper troposphere. When the CO₂ concentration is increased, the increase in absorbed radiation is quite small and increased emission leads to a cooling at all heights in the stratosphere. But for gases such as the CFCs, whose absorption bands are generally in the 8 to 13 micron �atmospheric window�, much of the upwelling radiation originates from the warm lower troposphere, and a warming of the lower stratosphere results, although there are exceptions (see Pinnock et al., 1995). Methane and nitrous oxide are in between. In the upper stratosphere, increases in all well-mixed gases lead to a cooling as the increased emission becomes greater than the increased absorption. Equivalent CO₂ is the amount of CO₂ used in a model calculation that results in the same radiative forcing of the surface-troposphere system as a mixture of greenhouse gases (see e.g., IPCC, 1996) but does not work well for stratospheric temperature changes (Wang et al., 1991; Shine, 1993; WMO, 1999).

An ozone loss leads to a reduction in the solar heating, while the major long-wave radiative effects from the 9.6 and 14 micron bands (Shine et al., 1995) produce a cooling tendency in the lower stratosphere and a positive radiative change above (Ramaswamy et al., 1996; Forster et al., 1997). Large transient loadings of aerosols in the stratosphere follow volcanic eruptions (IPCC, 1996) which leads to an increase of the heating in the long-wave. For the solar beam, aerosols enhance the planetary albedo while the interactions in the near-infrared spectrum yield a heating which is about one third of the total solar plus long-wave heating (IPCC, 1996; WMO, 1999). In addition, ozone losses can result from heterogeneous chemistry occurring on or within sulphate aerosols, and those changes produce a radiative cooling (Solomon et al., 1996).

The Antarctic ozone hole is a stratospheric phenomenon with a documented impact on temperature and, during the period 1979 to 1994, ozone decreases very likely contributed a negative radiative forcing of the troposphere-surface that offset perhaps as much as one half of the positive radiative forcing attributable to the increases in CO₂ and other greenhouse gases (Hansen et al., 1997; Shine and Forster, 1999). It appears that most of the observed decreases in upper-tropospheric and lower-stratospheric temperatures were due to ozone decreases rather than increased CO₂ (Ramaswamy et al., 1996; Tett et al., 1996; Bengtsson et al., 1999).

The subject of solar effects on climate and weather (see Section 6.10) has enjoyed a recent resurgence, in part because of observational studies (Labitzke and van Loon, 1997), but more so because of modelling studies that suggest viable mechanisms involving the stratosphere. As solar irradiance changes, proportionally much greater changes are found in the ultraviolet which leads to photochemically induced ozone changes, and the altered UV radiation changes the stratospheric heating rates per amount of ozone present (Haigh, 1996; Shindell et al., 1999a). Including the altered ozone concentrations gave an enhanced tropospheric response provided the stratosphere was adequately resolved.

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