Microwave brightness temperatures are based on the intensity of atmospheric oxygen emissions near the 60 GHz absorption band. However, the brightness temperature is also affected by variations in other factors besides the temperature of the atmospheric oxygen. In this study we have determined the non-thermometric contribution to the variability in brightness temperature due to several of these factors: sea ice, precipitable water, cloud liquid water, land surface emissivity and oceanic wind roughening.

The issue of the non-thermometric effects has been raised by the IPCC and CCSP as sources of possible misinterpretation of long-term changes in the global tropospheric temperature. For example, a decrease in sea ice extent would be interpreted as a spurious cooling since the surface emissions of sea ice are greater than those of open water. On the other hand, if ocean wind speeds increase and cause roughening of the surface over time, the effect on brightness temperatures would be perceived as a spurious warming.

Using several data sets, the individual effects of these non-thermometeric influences were calculated for 1988-2001 with the SSM/I data being prominent in their determination. The effects in all cases were very small in terms of the impact on the long term trend. In general, monthly values of the impacts were less than 0.03 °C, usually much smaller. The estimated effect when all factors were combined was equivalent to a spurious warming of approximately 0.01 °C/decade in the UAH lower tropospheric time series, whose unadjusted trend for Dec 1978 to Jun 2004 was +0.08 °C/decade.