9.4.4 Free Atmosphere Temperature

9.4.4.1 Observed Changes

Observed free atmosphere temperature changes are discussed in Section 3.4.1 and Karl et al. (2006) provide a comprehensive review. Radiosonde-based observations (with near global coverage since 1958) and satellite-based temperature measurements (beginning in late 1978) show warming trends in the troposphere and cooling trends in the stratosphere. All data sets show that the global mean and tropical troposphere has warmed from 1958 to the present, with the warming trend in the troposphere slightly greater than at the surface. Since 1979, it is likely that there is slightly greater warming in the troposphere than at the surface, although uncertainties remain in observed tropospheric warming trends and whether these are greater or less than the surface trend. The range (due to different data sets) of the global mean tropospheric temperature trend since 1979 is 0.12°C to 0.19°C per decade based on satellite-based estimates (Chapter 3) compared to a range of 0.16°C to 0.18°C per decade for the global surface warming. While all data sets show that the stratosphere has cooled considerably from 1958 and from 1979 to present, there are large differences in the linear trends estimated from different data sets. However, a linear trend is a poor fit to the data in the stratosphere and the tropics at all levels (Section 3.4.1). The uncertainties in the observational records are discussed in detail in Section 3.4.1 and by Karl et al. (2006). Uncertainties remain in homogenised radiosonde data sets which could result in a spurious inference of net cooling in the tropical troposphere. Differences between temperature trends measured from different versions of tropospheric satellite data result primarily from differences in how data from different satellites are merged.