from the lower stratosphere. This is problematical for detecting changes related to greenhouse warming, because the warming in the troposphere is expected to be accompanied by cooling in the lower stratosphere. Therefore, the blending of the radiation from these two layers seen by channel 2 partially or completely damps out any greenhouse warming signal.
There are two approaches to processing the observations to obtain a pure tropospheric temperature. One approach is to combine observations from the different channels of the MSU. The other approach is to exploit observations from the different scan angles using MSU channel 2 alone. The latter approach is the one discussed in this report because it is much more mature in terms of the extent of data-set development and validation. To obtain a temperature closer to the earth's surface in the latter approach, observations from different scan angles that the MSU uses to view the earth's atmosphere are arithmetically combined to reduce the influence of the upper troposphere and stratosphere (Spencer and Christy, 1992). The advantage of this technique is that the resulting lower tropospheric temperature (often referred to as "MSU 2LT") is closer to the earth's surface (centered about 4 km high). Because the central issue being examined in this study is the expectation by some that the lower troposphere should exhibit similar temperature trends as the surface, the panel exclusively discusses the MSU 2LT product in this report. The drawback of using the MSU 2LT product is that the retrieval method relies on a subtraction of adjacent view angles, which (a) increases measurement noise and (b) doubles the sensitivity of the measurements to surface emissions (to 10% over oceans, 20% over land) (Spencer and Christy, 1992). These effects more than double the error characteristics for MSU 2LT relative to MSU 2.
The geographical distribution of MSU lower to mid-tropospheric temperature trends is shown in Figure 7.1. It is evident that the regions of rapid surface warming apparent in Figure 6.2 (e.g., Western Europe, Eastern Russia) tend to be characterized by rapid warming aloft, and vice versa. In contrast to the surface data, which exhibit a warming trend at most locations, the satellite data show roughly equal areas of warming and cooling. Whereas it is obvious from a visual inspection of Figure 6.2continue