Thermospheric resistance to “greenhouse cooling”: Effect of the collisional excitation rate by atomic oxygen on the thermal response to CO₂ forcing

Infrared radiation in the 15‐μm band of carbon dioxide is the major cooling mechanism of the middle and upper atmosphere. Increasing amounts of atmospheric CO 2 impose anthropogenic influence (“greenhouse cooling”) all the way through the mesosphere and thermosphere. Collisions with atomic oxygen are the primary excitation mechanism of CO 2 molecules in the thermosphere. Negative radiative forcing due to the CO 2 increase is roughly proportional to the rate of collisional excitation which in turn is proportional to the rate constant of collisional deactivation. The rate constant is still somewhat uncertain at present with various measurements and estimates varying within about a factor of 4. In light of recent laboratory measurements, two sets of numerical simulations have been performed to estimate the thermospheric response to doubling and a 15% increase of CO 2 for two values of the rate constant that differ by a factor of 2. Surprisingly, the temperature and density changes due to the CO 2 increases are practically independent of the rate constant. Simple diagnostics show that two physical mechanisms are primarily responsible: the strong temperature dependence of the radiative forcing itself in a combination with a temperature dependence of molecular heat conduction. Since the scenarios considered for the higher rate constant generally correspond to colder temperatures, the two physical mechanisms combined provide sufficiently strong negative feedbacks to entirely offset the initially stronger radiative forcing.