Global change in the mesosphere-lower thermosphere region: has it already arrived?

Abstract

This tutorial review describes some possible future scenarios for changes in temperature and water vapor in the mesosphere-lower thermosphere (MLT) region (50–100 km). The structure and dynamics of this region are controlled by physical processes, some of which are very different than in the lower atmosphere, such as gravity-wave breaking, radiative transfer in non-local thermodynamic equilibrium and airglow cooling. The couplings between the various atmospheric properties are illustrated by the use of a 2D zonally-symmetric model ranging from 16 to 120 km. The importance of temperature and water vapor for the occurrence and scattered brightness of mesospheric clouds (at a height of about 83 km) is described in terms of their influence on nucleation, growth and sedimentation of ice particles. At the cold mesopause at high latitude, IR effects would warm the region without dynamical feedbacks, which in the 2D model to be described, cause a net cooling at all latitudes and seasons. The effects of a future doubling of carbon dioxide and methane (and a past halving) are examined by means of the same 2D model. All models predict a future lowering of temperature throughout much, if not all of the MLT region, as a result of enhanced IR cooling and dynamical feedbacks. The rise of methane will lead to an enhancement of water vapor concentrations throughout the upper atmosphere. The cloud existence region, defined in terms of water-ice saturation, is predicted to extend to lower-latitude, high population areas in the future. In a glacial-era scenario, the existence region is found to be confined to a small region near the summertime polar mesopause. Over the past century, with a doubling of methane and a 30% increase in carbon dioxide, the mesospheric cloud existence region may, have advanced from near the pole to its current location inside the 50°–90° latitude zone. The uncertainties in current models and need for further studies are discussed.