Work in Progress, just started:
Temperature measurements used to eventually compute a global average are many and various,
Firstly, the locations are many – near earth surface, sea surface, ocean depths (various), atmosphere (various altitudes,
Secondly, the methods are various – thermometers, IR detection, PROXIES – tree rings, ice characteristics etc
This is an extract from http://www.omega.com/temperature/z/pdf/z063-066.pdf
INFRA-RED TEMPERATURE MEASUREMENT – Theory and Application.
As previously stated IR energy is emitted by all materials above 0°K. Infrared radiation is part of the Electromagnetic Spectrum and occupies frequencies between visible light and radio waves.
The IR part of the spectrum spans wavelengths from 0.7 micrometers to 1000 micrometers (microns). Figure 1.
Within this wave band, only frequencies of 0.7 microns to 20 microns are used for practical, everyday temperature measurement. This is because the IR detectors currently available to industry are not sensitive enough to detect the very small amounts of energy available at wavelengths beyond 20 microns.
This explanation seems to be the basis for atmospheric temperature measurements related to IR back radiation causing planet warming. Yet, it is claimed that the CO2 AND H2O wavelengths don’t even get included.
Not being an atmospheric properties scientist, I leave this hypothesis to them, BUT IF………………………………..