Trends in 20 years of AIRS data: Indications of climate change?

The Atmospheric Infrared Sounder (AIRS) on the EOS Aqua Spacecraft was launched on May 4, 2002, with a 95% confidence design lifetime of 5 years. Since 2002 AIRS has performed exceptionally well. We now have 20 years of AIRS data. We show that key channels from AIRS are more accurate and close to one order of magnitude more stable than the 100 mK absolute and 10 mK/yr required to measure climate trends. However, the real limitation on the usable absolute accuracy and stability at the 100 mK and 10 mK/yr are time varying residual cloud contamination at all wavelength, and surface contamination effects, particularly in the SW channels. Large interannual variabilities due to ENSO type effects further inflate trend uncertainties. The 20 years of AIRS data contain at least two changes of potential climate significance: The change of the daily count of Deep Convective Cloud (DCC) in the tropical oceans, but it is opposite in sign to the expected trend, and a decrease in the count of clear cases, as determined by the Spatial Coherence Test (SCT) . Both indicate unexpected changes in the spatial distribution of clouds on a 45x45 km scale.