Figure 12.4: (a) Observed
microwave sounding unit (MSU) global mean temperature in the lower strato sphere,
shown as dashed line, for channel 4 for the period 1979 to 97 compared with the
average of several atmosphere-ocean GCM simulations starting with different atmospheric
conditions in 1979 (solid line). The simulations have been forced with increasing
greenhouse gases, direct and indirect forcing by sulphate aerosols and tropospheric
ozone forcing, and Mt. Pinatubo volcanic aerosol and stratospheric ozone variations.
The model simula-tion does not include volcanic forcing due to El Chichon in 1982,
so it does not show stratospheric warming then. (b) As for (a), except for 2LT
temperature retrievals in the lower troposphere. Note the steady response in the
stratosphere, apart from the volcanic warm periods, and the large variability
in the lower troposphere (from Bengtsson et al., 1999).