The Atmospheric Infrared Sounder (AIRS) and the Advanced Microwave Sounding Unit (AMSU) were launched on the EOS Aqua satellite into polar orbit in May 2002. Analysis of the AIRS sea surface temperature measurements relative to the NCEP provided Real Time Global sst (RTG.SST) shows better than 3mK per year radiometric stability. The radiometric stability for all AIRS channels is established through the common onboard full aperture blackbody and space view. The atmospheric temperature sounding channels of AIRS make use of the opacity of co2, while the sounding channels for AMSU depend on the oxygen opacity. Comparison of the raw brightness temperatures of AMSU channels 5 and 6 with matching AIRS channels at 2388 and 2386 cm-1, respectively, for the tropical oceans show a steady cooling of the AIRS relative to AMSU at a rate of about 100mK/year, which is consistent with the global increase in co2 at a rate of 2.6 ppmv/year. This finding using global ocean data confirms the potential use of AIRS data to identify regional sources of co2.