On the secular trend of CO_x and CO₂ in the lower thermosphere

An analysis of recent observations (2004-2013) made by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) instrument indicate that total carbon (CO_x=CO+CO₂) has been increasing rapidly in the lower thermosphere, above 10^-3 hPa (90 km). The estimated trend (~9% per decade) is about a factor of 2 larger than the rate of increase that can be ascribed to anthropogenic emissions of CO₂ (~5% per decade). Here we investigate whether the observed trends of CO₂ and CO_x can be reproduced using the Whole Atmosphere Community Climate Model (WACCM), a comprehensive global model with interactive chemistry, wherein vertical eddy diffusion is estimated from a parameterization of gravity wave breaking that can respond to changes in the model climate. We find that the modeled trends of CO₂ and CO_x do not differ significantly at any altitude from the value expected from anthropogenic increases of CO₂ and that WACCM does not produce significant changes in eddy diffusivity. We show that the discrepancy between model and observations cannot be attributed to uncertainties associated with geophysical noise and instrumental effects, to difficulties separating a linear trend from the 11 year solar signal, or to sparse sampling by ACE-FTS. Estimates of the impact of vertical diffusion on CO₂ in the model indicate that a large increase in K_zz (~30% per decade) would be necessary to reconcile WACCM results with observations. It might be possible to ascertain whether such a large change in vertical mixing has in fact taken place by examining the trend of water vapor in the upper mesosphere.