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Satellites have been measuring the temperature of the troposphere since 1979; the usable balloon (radiosonde) record begins in 1958.
Currently (July 2004) the trend in satellite data from the Mears et al version is +0.129 °C/decade , from the Spencer and Christy version D +0.083 °C/decade and from Vinnikov and Grody, +0.22°C to 0.26°C per decade , . 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 .
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, but when corrected the trends still show much less warming than predicted by IPCC climate models.
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 . 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 .
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
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 . 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 . The data set is available at http://www.nsstc.uah.edu/data/msu/t2lt/tltglhmam_5.1 and the trend to date (May 2004) is 0.083 °C / decade globally, 0.154 °C / decade in the northern hemisphere and 0.013 °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 May 2004 the AUH trend was 0.083 °C / decade. The trend from the Mears et al satellite data set is 0.131 °C / decade .
In the late 1990s the disagreement between the surface temperature record and the satellite records was a subject of research and debate: the lack of warming then seen in the records was noted, e.g. .
A report by the National Research Council that reviewed upper air temperature trends states:
However, the same panel then concluded that
The Spencer and Christy version D record from 1979 to 2002 show a warming trend of 0.04 °C/decade, compared to 0.06 to April 2002 or 0.074 to July 2003; or 0.082 to Feb 2004 (see above). This illustrates a severe problem with the satellite record, its shortness - adding a few years on to the record can change the trends considerably. These satellite data are controversial and incorporate a number of corrections for, amongst other effects, orbital drift of the satellites . Other analyses of the same data produce different trends: Mears et al. find 0.097 °C/decade from 1979 to 2001 . An even more recent but still controversial analysis (Vinnikov and Grody, Science, 2003) finds a trend of +0.22°C to 0.26°C per decade .
The problems with the shortness of the MSU record is shown by the table below, which shows S+C MSU TLT global trend (oC/decade) beginning with Jan 1979 and ending with Dec of the year shown.
The satellites also measure the stratospheric temperature and show a decline in stratospheric temperatures, interspersed by "noise" from volcanic eruptions. This is what is expected from Global Warming theory: the troposphere should warm, whilst the stratosphere should cool. However, this simple picture is complicated by ozone depletion, which also causes a cooling of the stratosphere.
The longest data sets of upper air temperature are derived from instruments carried aloft by balloons (radiosondes). Changes in balloon instrumentation and data processing over the years have been pervasive, however, resulting in discontinuities in these temperature records . The radiosonde data set becomes usably global in about 1958.
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