Carbon dioxide trends in the mesosphere and lower thermosphere

We investigated trends of carbon dioxide (CO 2 ) in the upper atmosphere, using data from the Atmosphere Chemistry Experiment Fourier Transform Spectrometer and from the Sounding of the Atmosphere using Broadband Emission Radiometry. Recent analyses of these measurements had indicated that CO 2 above approximately 90 km appeared to be increasing about twice as fast as it was in the lower atmosphere. Models could not reproduce this differential CO 2 trend, calculating instead that the proportional CO 2 increase is approximately constant with altitude. We found three issues with the methodologies used to derive trends from CO 2 profiles: the way that seasonal changes and sampling are accounted for in the analysis, referred to as deseasonalizing; the registration of profiles in pressure versus altitude coordinates; and data quality indicators. Each of these can have significant effects on the derivation of trends. We applied several deseasonalizing procedures, using both pressure and altitude coordinates, also used a time series fit without deseasonalizing, and applied data quality filters. The derived trends were approximately constant with pressure or altitude, about 5.5% per decade, consistent with lower atmosphere CO 2 trends, and consistent with model calculations. We conclude that the difference between the trend of CO 2 above the CO 2 homopause and the trend in the lower, well‐mixed atmosphere is not statistically significant.