The investigators will study a number of science questions related to global change effects on the mesosphere and thermosphere. The theoretical study entails comparisons with recent estimates of a thermospheric density decline from long-term satellite-drag observations over the last few decades. This will require extension of a three-dimensional Spectral Mesosphere/Lower Thermosphere Model (SMLTM) to the exobase and incorporation of relevant physical processes, including interactive diffusive separation of the main species. Preliminary work shows that the global thermospheric response is amenable to analytical analysis, which will provide a deeper insight into the workings of the upper atmosphere. The second thrust of this project will be investigation of robust universal spatial structures of the response found in previous numerical simulations and will be based on development of appropriate analytical models in conjunction with a global mean one-dimensional version of the SMLTM. Inconclusive and sometimes-contradictory analyses of available long-term observational records are generally consistent with the notion of greenhouse cooling in the mesosphere, thermosphere, and ionosphere (MTI). The cooling is caused by increasing amounts of greenhouse gases, such as carbon dioxide, transported from the lower atmosphere into the MTI region and resulting in more efficient emission of infrared radiation into space. It typically manifests in colder MTI temperatures, a noticeable thermospheric density decline, and a downward displacement of various ionospheric layers. The magnitude of these changes is expected to be substantially greater than in the lower atmosphere and they may well serve as predictors of global change in the whole terrestrial atmosphere system. Several modeling studies performed to date have elucidated the role of various mechanisms, provided uncertainty estimates, and explained some of the observed features, for example, the seasonal-latitudinal dependence of the cooling. Outstanding science issues still remain, however, to be addressed by suitable numerical models. Fuller understanding of the MTI response to thermal forcing may be achieved in combination of diagnostics of computer simulations with appropriate analytic models and their direct comparison with data analyses. Better estimates and forecasts, deeper understanding, and more adequate models of possible anthropogenic changes in the MTI will substantially enhance the infrastructure and provide new tools for research not only in the upper atmosphere but in the wider area of climate change in the whole Earth atmosphere system.
