RADIATIVE FORCING AND TEMPERATURE TRENDS FROM STRATOSPHERIC OZONE CHANGES

Detailed shortwave and longwave radiative transfer models are used to calculate the radiative forcing and temperature trends due to stratosp heric ozone depletion. These were calculated using the fixed dynamical heating approximation to adjust the stratospheric temperatures. Recen t estimates of stratospheric ozone loss between 1979 and 1991 (from so lar backscattered ultraviolet (SBUV) and stratospheric aerosol and gas experiment (SAGE) instruments) and updated radiative transfer schemes are used to obtain improved estimates of the radiative forcings. An a nnually and globally averaged radiative forcing of -0.13 +/- 0.02 W m( -2) decade(-1) (-0.22 +/- 0.03 W m(-2) for the 1979-1996 period) was f ound from SBUV total column ozone trends, applying a constant percenta ge ozone depletion to a 7 km thick layer directly above the tropopause SAGE ozone trends gave forcing estimates of -0.10 +/- 0.02 W m(-2) de cade(-1) (-0.17 +/- 0.03 W m for the 1979-1996 period), although assum ptions needed to be made about the choice of the vertical profile of t he ozone depletion below 17 km. Using Dobson instrument trends from 19 64 to 1996, the total ozone forcing could be as negative as -0.26 +/- 0.05 W m(-2). The quoted error bars derive from uncertainties in the t otal ozone trends. Using these values, the stratospheric ozone change may have offset about 30% of the forcing due to increases in well-mixe d greenhouse gases since 1979, and about 15% of the forcing since 1964 , at least on a global and annual mean. The ozone forcings are shown t o be nearly a linear function of the ozone amount depleted from the at mospheric column, provided the vertical profile of the depletion remai ns constant. As in previous studies, it was found that stratospheric a djustment altered the sign of the ozone forcing by strongly cooling th e lower stratosphere. It is shown that (depending on the vertical stru cture of ozone depletion) coolings of up to 0.4 K decade(-1) can be fo und at altitudes of 35 km; this provides a mechanism for cooling the s tratosphere nearly as large as that from well-mixed greenhouse gas inc reases, at altitudes where ozone changes were previously thought not t o strongly affect stratospheric temperature trends. Previous studies h ave examined the effect of ozone trends by looking at the response of the surface temperature to a fixed absolute ozone change at different heights in the atmosphere. We argue that using absolute ozone perturba tions places an unrealistically large emphasis on ozone changes near t he tropopause. As the vertical profile of ozone change is more often r eported as a percentage change, we present the sensitivity of the surf ace temperature to constant percentage, rather than absolute, changes in ozone.