Temperature trends derived from Stratospheric Sounding Unit radiances: The effect of increasing CO₂ on the weighting function

Stratospheric Sounding Units (SSU) on the NOAA operational satellites have been the main source of near-global temperature trend data above the lower stratosphere. They have been used extensively for comparison with model-derived trends. The SSU senses in the 15 μm band of CO₂ and hence the weighting function is sensitive to changes in CO₂ concentrations. The impact of this change in weighting function has been ignored in all recent trend analyses. We show that the apparent trends in global-mean brightness temperature due to the change in weighting function vary from about -0.4 Kdecade^-1 to 0.4 Kdecade^-1, depending on the altitude sensed by the different SSU channels. For some channels, this apparent trend is of a similar size to the trend deduced from SSU data but ignoring the change in weighting function. In the mid-stratosphere, the revised trends are now significantly more negative and in better agreement with model-calculated trends.