The highest layers of the Earth's atmosphere are cooling and contracting, most likely in response to increasing levels of greenhouse gases, according to a new study by scientists at the Naval Research Laboratory (NRL). This contraction could result in longer orbital lifetimes for both satellites and hazardous space debris.

Credit: NASA

In a paper to be published February 5 in the Journal of Geophysical Research - Space Physics, John Emmert, Michael Picone, Judith Lean, and Stephen Knowles report that the average density of the thermosphere has decreased by about 10 percent during the past 35 years. The thermosphere is the highest layer in the atmosphere, and begins at an altitude of about 90 kilometers [60 miles].

The study utilized orbital tracking data for 27 space objects that have been aloft for over 30 years and whose closest approach to the Earth ranges from 200-800 kilometers [100-500 miles]. The Space Shuttle typically orbits at 300-450 kilometers [200-280 miles], and the International Space Station is at an altitude of about 400 kilometers [200 miles]. Although the atmosphere is extremely thin in this region (the air at the Earth's surface is a trillion times thicker), it is enough to exert a drag force on satellites, causing their orbits to decay slowly and ultimately resulting in a fiery disintegration at lower altitudes. By analyzing changes in the orbits of the selected objects, the scientists derived the yearly average density encountered by each object. After adjusting for other factors, the data from every object indicated a long-term decline in the density of the thermosphere.

This decrease in density had been predicted by theoretical simulations of the upper atmosphere's response to increasing carbon dioxide and other 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, above about 12 kilometers [seven miles], however, these gases actually enhance the ability of the atmosphere to radiate heat out to space, thereby causing a cooling effect. As the amount of carbon dioxide increases, the upper atmosphere becomes cooler and contracts, bringing lower-density gas to lower heights. Consequently, at a given height, the average density will decrease. Because each layer of the atmosphere rests on the layers below it, small changes at lower altitudes become amplified at higher altitudes. The NRL study found that the observed decrease in density depends on height in the same way as predicted by the theoretical simulations, indicating that greenhouse gases are a likely source of the change.

An extreme example of the greenhouse gas effect can be found on Venus, whose atmosphere is 96 percent carbon dioxide (compared to trace amounts in the Earth's atmosphere), resulting in a very hot lower atmosphere 400 degrees Celsius [800 degrees Fahrenheit] and a very cold and compact upper atmosphere.

These new results verify and significantly expand a limited earlier investigation, by scientists at The George Washington University, which also used orbital data to derive a long-term decrease in thermospheric density. The new study utilizes more orbital data over a longer period of time and employs more precise methods of analysis. By carefully examining all potential sources of error, Emmert's team has provided solid evidence that the trend is neither artificial nor the result of physical processes other than internal atmospheric cooling.

Based on this analysis and projections of carbon dioxide levels in the atmosphere, the density at thermospheric heights could be cut in half by the year 2100. This change may present mixed blessings: while operational satellites will be able to stay aloft longer, using less fuel, so will damaging spacecraft debris, potentially increasing the frequency of collisions.

The research was funded by the Office of Naval Research.