Satellite temperature measurements

Satellites have been measuring the temperature of the troposphere since 1979; the usable balloon (radiosonde) record begins in 1958.

Currently (through March 2005) the trend in satellite data from the Mears et al version is +0.132 °C/decade [1] (http://www.ssmi.com/msu/msu_data_description.html#msu_decadal_trends) and from Spencer and Christy version 5.1, +0.085 °C/decade [2] (http://www.nsstc.uah.edu/data/msu/t2lt/tltglhmam_5.1). Less regularly updated analyses include Fu et al, with 0.2 °C/decade (May 04) [3] (http://www.spaceref.com/news/viewpr.html?pid=14166) and Vinnikov and Grody, with +0.22°C to 0.26°C per decade (Oct. 03) [4] (http://www.sciencemag.org/cgi/doi/10.1126/science.1087910), [5] (http://stephenschneider.stanford.edu/Publications/PDF_Papers/VinnikovGrody2003.pdf). This can be compared to the increase from the surface record of approximately 0.06 °C/decade over the past century and 0.15 °C/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 (http://www.grida.no/climate/ipcc_tar/wg1/060.htm).

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. Only the Christy et al, data have been independently confirmed by weather-balloon data.

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] (http://www.ssmi.com/msu/img/msu_weighting_functions.png). 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] (http://www.usgcrp.gov/usgcrp/seminars/960521SM.html) [8] (http://www.ghcc.msfc.nasa.gov/MSU/hl_measuretemp.htm).

The process of constructing a temperature record from a radiance record is difficult. The best-known record, from Roy Spencer and John Christy at the University of Alabama at Huntsville (UAH), is currently on version 5.1, which incorporates corrections for orbital drift and other factors. [9] (http://www.geog.ox.ac.uk/~mnew/teaching/Online_Articles/Christy_etal_MSUv5_JAOT2003.pdf) The record comes from a succession of different satellites and problems with inter-calibration between the satellites are important, especially NOAA-9 [10] (http://www.ssmi.com/msu/msu_data_description.html).

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, their satellite temperature trend was only 0.04 °C / decade, compared with 0.17 +/- 0.06 °C / decade from surface measurements; however, in the years since the UAH trend has roughly doubled to come more in line with other trends.

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. [11] (http://www.globalwarming.org/article.php?uid=167). A report by the National Research Council that reviewed upper air temperature trends stated:

"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." [12] (http://www4.nas.edu/news.nsf/isbn/0309068916?OpenDocument)

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."[13] (http://books.nap.edu/books/0309068916/html/2.html#pagetop)[14] (http://www.msnbc.com/news/356875.asp?cp1=1)

An 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 S+C MSU TLT global trend (°C/decade) beginning with Jan 1979 and ending with December of the year shown.

   1992    -0.003372
   1993    -0.044424
   1994    -0.043102
   1995    -0.012106
   1996    -0.007443
   1997    0.0001289
   1998    0.0702235
   1999    0.0579218
   2000    0.0466267
   2001    0.0551461
   2002    0.0724646
   2003    0.081553

Likewise, even though they began with the same data, each of the major research groups has interpreted it with different results. In addition to the UAH trends, Mears et al. find 0.097 °C/decade from 1979 to 2001 [15] (http://www.ssmi.com/msu/msu_data_description.html), while Fu et al find for the same period twice the increase, or 0.2 oC/decade.[16] (http://www.spaceref.com/news/viewpr.html?pid=14166) 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 [17] (http://www.sciencemag.org/cgi/doi/10.1126/science.1087910). Towards solving these differences, the groups met in 2003 (http://www.ncdc.noaa.gov/oa/rvtt.html).

Satellite measurements of the stratospheric temperature

The satellites also measure the stratospheric temperature [18] (http://www.ghcc.msfc.nasa.gov/MSU/msusci.html) 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 [19] (http://www.grida.no/climate/ipcc_tar/wg1/059.htm). The radiosonde data set becomes usably global in about 1958.

A graph comparing of the surface, balloon and satellite records (http://www.met-office.gov.uk/research/hadleycentre/CR_data/Monthly/upper_air_temps.gif).

External references

• Measuring the Temperature of Earth From Space (http://science.nasa.gov/newhome/headlines/notebook/essd13aug98_1.htm) NASA news

• Scientists Present 1998 Earth-Temperature Trends (http://science.nasa.gov/newhome/headlines/essd12jan99_1.htm) NASA: Updated 20-year temperature record unveiled at 1999 AMS Meeting

• Global Hydrology and Climate Center (http://www.ghcc.msfc.nasa.gov/) at NASA

• What Microwaves Teach Us About the Atmosphere (http://www.ghcc.msfc.nasa.gov/overview/microwave.html)

• Globally-Averaged Atmospheric Temperatures (http://wwwghcc.msfc.nasa.gov/MSU/msusci.html)