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Currently (July 2004) the trend in satellite data from the Mears et al version is +0.129 °C/decade [1], from the Spencer and Christy version D +0.083 °C/decade [2], from Fu et al 0.2 oC/decade [3] and from Vinnikov and Grody, +0.22°C to 0.26°C per decade [4], [5]. This can be compared to the increase from the surface record of approximately 0.06 oC/decade over the past century and 0.15 oC/decade since 1979.

An extensive comparison and discussion of trends from different data sources and periods is given in the IPCC TAR section 2.2.4.

The satellite record has the advantage of global coverage, whereas the radiosonde record is longer. There have been complaints of data problems with both records. Climate models predict that the troposphere should warm faster than the surface, so only the Fu et al or Vinnikov and Grody versions of the satellite record are compatible with this and the surface records.

1 The satellite temperature record

Since 1979, Microwave Sounding Units (MSUs) on NOAA polar orbiting satellites have measured the intensity of upwelling microwave radiation from atmospheric oxygen. The intensity is proportional to the temperature of broad vertical layers of the atmosphere, as demonstrated by theory and direct comparisons with atmospheric temperatures from radiosonde (balloon) profiles. Upwelling radiance is measured at different frequencies; these different frequency bands sample a different weighted range of the atmosphere [6]. Channel 2 is broadly representative of the troposphere.

Records have been created by merging data from nine different MSUs, each with peculiarities (e.g., time drift of the spacecraft relative to the local solar time) that must be calculated and removed because they can have substantial impacts on the resulting trend [7] [8].

The process of constructing a temperature record from a radiance record is difficult. The best-known record, from Spencer and Christy, is currently on version D, which incorporates numerous corrections over version C (what are they...). The record comes from a succession of different satellites and problems with inter-calibration between the satellites are important, especially NOAA-9. A description of the Mears et al. MSU data set is available from http://www.ssmi.com/msu/msu_data_description.html

John Christy and Roy Spencer at the University of Alabama-Huntsville (UAH) maintain a record of satellite-based temperature measurements that begins in January 1979 and is updated monthly [9]. Christy et al (2003) show the global temperature trend in the lower atmosphere from earth's surface to about 25,000 feet is 0.06ºC (± 0.05ºC per decade; 90% confidence limits) from January 1979 through April 2002 [10]. The data set is available at http://www.nsstc.uah.edu/data/msu/t2lt/tltglhmam_5.1 and the trend to date (end of September 2004) is 0.077 °C / decade globally, 0.147 °C / decade in the northern hemisphere and 0.008 °C / decade in the southern hemisphere.

For some time, the UAH satellite data's chief significance is that they appeared to contradict the United Nations' IPCC predictions about global warming. In April 2002, for example, the satellite temperature trend was only 0.04 °C / decade, compared with 0.17 +/- 0.06 °C / decade from surface measurements; however, by the end of September 2004 the AUH trend was 0.077 °C / decade. The trend from the Mears et al satellite data set is 0.131 °C / decade [11].