April 16, 2004
Analyses of the orbits of near-Earth space objects reveal that the highest layers of the Earth's atmosphere are cooling and contracting, most likely in response to increasing concentrations of greenhouse gases. In the troposphere (the lowest layer of the atmosphere) greenhouse gases trap infrared radiation, causing the well-known "global warming" effect. Higher in the atmosphere, these gases convert thermal energy into infrared energy, which escapes out to space, thereby causing a cooling effect. The thermosphere is a very hot layer of the atmosphere above 90 km. Although the atmosphere is extremely thin in this region, it is enough to exert a drag force on satellites, causing their orbits to decay slowly. By analyzing changes in the orbits of near-Earth objects, the average density encountered by each object can be inferred. After adjusting for other factors, results suggest that the average density at heights of 200-800 km has decreased by 7-15% during the past 35 years, with the largest changes occurring at the highest altitudes. The observed changes in density are consistent with theoretical simulations of the upper atmosphere's response to increasing carbon dioxide and other greenhouse gases. As the amount of carbon dioxide increases, the upper atmosphere becomes cooler and contracts, bringing lower-density gas to lower heights. A careful examination of potential sources of error indicates that the trend is neither artificial nor the result of physical processes other than internal atmospheric cooling. Based on the experimental and theoretical results and projections of carbon dioxide levels in the atmosphere, thermospheric density could be cut in half by the year 2100. This change may present mixed blessings: while operational satellites will potentially be able to stay aloft longer or use less fuel, so will damaging spacecraft debris.
Dr. John T. Emmert is a Research Assistant Professor at George Mason University and a Research Associate at the Naval Research Laboratory. He received a bachelor's degree in Astronomy from the University of Arizona, served three years in the U.S. Navy, and then earned a Ph.D. in Physics from Utah State University. Dr. Emmert's research has focused on empirical analysis of thermospheric dynamics and composition, performing studies of thermospheric winds measured by a variety of ground- and space-based instruments. He has been studying long-term changes in thermospheric density related to global climate change, using a 35-year database of orbital element sets. He is currently refining the methodology in order to extract information on seasonal and solar cycle variations. He is also working on the climatology of winds in the thermosphere and the effect of geospace storms on global thermospheric dynamics, and is working on integrating these results into the empirical Horizontal Wind Model.