GLOBAL CHANGE IN THE MESOSPHERE LOWER THERMOSPHERE REGION - HAS IT ALREADY ARRIVED

This tutorial review describes some possible future scenarios for chan ges in temperature and water vapor in the mesosphere-lower thermospher e (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, radiativ e transfer in non-local thermodynamic equilibrium and airglow cooling. The couplings between the various atmospheric properties are illustra ted by the use of a 2D zonally-symmetric model ranging from 16 to 120 lux. The importance of temperature and water vapor for the occurrence and scattered brightness of mesospheric clouds (at a height of about 8 3 km) is described in terms of their influence on nucleation, growth a nd sedimentation of ice particles. At the cold mesopause at high latit ude, IR effects would warm the region without dynamical feedbacks, whi ch in the 2D model to be described, cause a net cooling at all latitud es 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 mod el. All models predict a future lowering of temperature throughout muc h, if not ail of the MLT region, as a result of enhanced IR cooling. a nd dynamical feedbacks. The rise of methane will lead to an enhancemen t of water vapor concentrations throughout the upper atmosphere. The c loud existence region, defined in terms of water-ice saturation, is pr edicted to extend to lower-latitude, high population areas in the futu re. In a glacial-era scenario, the existence region is found to be con fined to a small region near the summertime polar mesopause. Over the past century, with a doubling of methane methane and a 30% increase in carbon dioxide, the mesospheric cloud existence region may, have adva nced from near the pole to its current location inside the 50 degrees- 90 degrees latitude zone. The uncertainties in current models and need for further studies are discussed. Copyright (C) 1996 Elsevier Scienc e Ltd