Detection of Long-Term Climatological Variationsin the Earth's Upper Atmosphere ApparentlyAssociated with Anthropogenic Effects

From a study of long-term orbital decay of Earth satellites compared to standard empirical models (to remove known systematic variations) it has been discovered that thermospheric densities have declined substantially since at least 1976. Detection of this decline was first published by Keating et al (2000) in Geophysical Research Letters. According to theoretical 3D studies of Rishbeth and Roble (1992) maximum thermospheric response to anthropogenic increases of CO2 and CH4 should occur at solar minimum because of the colder thermosphere. We have therefore focused on detecting the trend at years of solar minimum and making small corrections for solar activity variability using the standard empirical models. We have now expanded our study to many more satellites. All the satellites investigated have shown a downward trend in densities over the period 1976-1996. Over this period we find densities near 380km have decreased 10.3 +/- 1.2 percent. It is estimated that CO2 will double before the end of the century. Exponential extrapolation of the observed trend indicates densities near 400km may decrease over this time interval by approximately 50 percent due to strong anthropogenic cooling of the thermosphere. A paper by Emmert et al. (2004) using a very similar approach, but looking at all levels of solar activity empirically show that maximum downward trends occur near solar minimum as we had hypothesized. Our most recent results show little difference between polar and equatorial trends indicating we have detected a global phenomenon. The results appear to be in general accord with the theoretical studies of Roble and Dickinson (1989), Rishbeth and Roble (1992),and Akmaev and Formichev (2000).