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Satellite temperature measurements

Satellites have been measuring the temperature of the troposphere since 1979; the usable ballon (radiosonde) record begins in 1958. Currently (July 2003) the trend in temperature according to Spencer and Christy version D is +0.074 oC/decade; the Mears et al version gives +0.097 oC/decade to 2001. 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 are data problems with both records.

Table of contents
1 The satellite temperature record
2 Discussion of the satellite temperature records
3 Satellite measurements of the stratospheric temperature
4 Weather balloons (radiosondes)
5 External references

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 [1]. 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 [1].

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 " class="external">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 [1]. 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 [1]. The data set is available at http://www.nsstc.uah.edu/data/msu/t2lt/tltglhmam_5.1 and the trend to date (July 2003) is 0.074 oC / decade.

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 oC / decade, compared with 0.17 +/- 0.06 oC / decade from surface measurements; however, by August 2003 the AUH trend was 0.074 oC / decade. The trend from the Mears et al satellite data set is 0.097 oC / decade [1].

Discussion of the satellite temperature records

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. [1].

A report by the National Research Council that reviewed upper air temperature trends states:
"Data collected by satellites and balloon-borne instruments since 1979 indicate little if any warming of the low- to mid- troposphere - the atmospheric layer extending up to about 5 miles from the Earth's surface. Climate models generally predict that temperatures should increase in the upper air as well as at the surface if increased concentrations of greenhouse gases are causing the warming." [1]

However, the same panel then concluded that

"the warming trend in global-mean surface temperature observations during the past 20 years is undoubtedly real and is substantially greater than the average rate of warming during the twentieth century. The disparity between surface and upper air trends in no way invalidates the conclusion that surface temperature has been rising."class="external">[1

The Spencer and Christy version D record from 1979 to 2002 show a warming trend of 0.04 oC/decade, compared to 0.06 to April 2002 or 0.074 to July 2003 (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 [1]. Other analyses of the same data produce different trends: Mears et al. find 0.097 oC/decade from 1979 to 2001 [1]. 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 [1].

Satellite measurements of the stratospheric temperature

The satellites also measure the stratospheric temperature [1] 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.

Weather balloons (radiosondes)

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 [1]. The radiosonde data set becomes usably global in about 1958.

A graph comparing of the surface, balloon and satellite records.

External references