Is the polar mesosphere the miner's canary of global change?

The polar mesosphere is an atmospheric region located between latitude 50° and the pole, and between 50 and 90 km. During summer it becomes the coldest region on earth (<130K). This review focuses on past and future alterations of the temperature and water vapor content of this extremely cold region. These two influences are crucial for the formation of mesospheric ice particles in noctilucent clouds (NLC). A recent two-dimensional model study has been conducted of how long-term changes in carbon dioxide (CO2) and methane (CH4) concentrations may modify the temperature and water vapor concentration at mesopause heights. The model is a version of the well-known Garcia-Solomon model, modified to include accurate non-LTE cooling in the CO2 15 μm band. The existence region of NLC is defined as a domain where water-ice is supersaturated. Reduced levels of CO2 and CH4 are found to confine the model NLC existence region to within the perpetually-sunlit polar cap region, where the clouds would no longer be visible to a ground observer. A doubling of CO2 and CH4 could extend the NLC region to mid-latitudes, where they would be observable by a large fraction of the world's population.