Satellite temperature measurements

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Satellite temperature measurements have been obtained from the troposphere since late 1978. By comparison, the usable balloon (radiosonde) record begins in 1958 but has less geographic coverage and is less uniform.

Note that the records commonly called the "satellite temperature record" are of the lower- or mid-tropospheric temperatures, not surface temperatures. Satellites can also measure surface (skin) temperatures; see below.

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.

To compare to the trend from the surface temperature record (approximately +0.07 °C/decade over the past century and +0.17 °C/decade since 1979) it is most appropriate to derive trends for the part of the atmosphere nearest the surface, i.e., the lower troposphere. Doing this, through July 2010:

  • RSS v3.2 finds a trend of +0.162 °C/decade.[3]
  • UAH v5.3 finds a trend of +0.138°C/decade.[4]

An alternative adjustment introduced by Fu et al. (2004)[5] finds trends (1979-2001) of +0.19 °C/decade when applied to the RSS data set[6].

Using the T2 channel (which include significant contributions from the stratosphere, which has cooled), Mears et al. of Remote Sensing Systems (RSS) find (through January 2010) a trend of +0.090 °C/decade.[3] Spencer and Christy of the University of Alabama in Huntsville (UAH), find a smaller trend of +0.047 °C/decade.[7] A less regularly updated analysis is that of Vinnikov and Grody with +0.20°C per decade (1978–2005).[8] Another satellite temperature analysis is provided by NOAA/NESDIS STAR Center for Satellite Application and Research and use simultaneous nadir overpasses (SNO)[9] to remove satellite intercalibration biases yielding more accurate temperature trends. The SNO analysis finds a 1979-2009 trend of +0.131°C/decade for T2 channel.[10]

The satellite records have the advantage of global coverage, whereas the radiosonde record is longer. There have been complaints of data problems with both records, and difficulty reconciling climate model predictions with the observed data.

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