Satellites do not measure temperature. They measure radiances in various wavelength bands, which must then be mathematically inverted to obtain indirect inferences of temperature.[1][2] The resulting temperature profiles depend on details of the methods that are used to obtain temperatures from radiances.
As a result, different groups that have analyzed the satellite data have obtained different temperature trends. Among these groups are Remote Sensing Systems (RSS) and the University of Alabama in Huntsville (UAH).
Furthermore the satellite series is not fully homogeneous - it is constructed from a series of satellites with similar but not identical instrumentation. The sensors deteriorate over time, and corrections are necessary for satellite drift in orbit.
Particularly large differences between reconstructed temperature series occur at the few times when there is little temporal overlap between successive satellites, making intercalibration difficult.[/b]
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As noted earlier, these temperature data, misinterpreted from the satellite data, are now known to have been too low.
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n important critique of the satellite record is its shortness—adding a few years on to the record or picking a particular time frame can change the trends considerably. The problems with the length of the MSU record is shown by the table below, which shows the UAH TLT (lower tropospheric) global trend (°C/decade) beginning with Dec 1978 and ending with December of the year shown.
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Year↓ UAH Trend↓ RSS Trend↓
1991 0.087
1992 0.024
1993 -0.013
1994 -0.003
1995 0.033
1996 0.036
1997 0.040
1998 0.112
1999 0.105
2000 0.095
2001 0.103
2002 0.121
2003 0.129
2004 0.130
2005 0.139
2006 0.140
2007 0.143
Likewise, even though they began with the same data, each of the major research groups has interpreted it with different results. Most notably, Mears et al. at RSS find 0.193 °C/decade for lower troposphere up to July 2005, compared to +0.123 °C/decade found by UAH for the same period.
There are ongoing efforts to resolve these differences. Some believe that much of the disparity may have been resolved by the three papers in Science, 11 August 2005, which pointed out errors in the UAH 5.1 record and the radiosonde record in the tropics.
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