Carbon dioxide (CO₂) is increasing throughout the atmosphere, but recent findings indicate that it is increasing faster near and above the mesopause than in the lower and middle atmosphere, and the mechanism behind this differential trend is unknown. We review this differential CO₂ trend measured by ACE-FTS onboard the SCISAT-1 satellite, and SABER onboard the NASA TIMED satellite. Increases in CO₂ cause the thermosphere to cool and contract, reducing its density, so faster increases should lead to greater cooling. Therefore, if CO₂ is actually increasing more rapidly in the lower thermosphere than in the fully mixed atmosphere, it could revise the conceptual basis for our understanding of global change in the thermosphere-ionosphere. We examine CO₂ trends in the lower and upper atmosphere using a combination of data analysis and model simulations, to understand whether this differential CO₂ trend is responsible for the observed changes in thermosphere and ionosphere climate, how the solar cycle and CO₂ trends contribute to those changes, how they vary with solar time, latitude, and season, and whether the differential CO­₂ trend could explain the larger changes reported in ion temperature.