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.

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.

Notes for Journalists:

Journalists (only) may obtain a pdf copy of this paper upon request
to Kara LeBeau: klebeau@agu.org. Please provide your name,
name of publication, phone, and email address. The paper and this
press release are not under embargo.

Title: “Global change in the thermosphere: Compelling evidence
of a secular decrease in density”

Citation: Emmert, J. T., J. M. Picone, J. L. Lean, and S. H.
Knowles (2004), Global change in the thermosphere: Compelling
evidence of a secular increase in density, J. Geophys. Res., 109,
A02301, doi:10.1029/2003JA010176.

Contact information for the authors:

The Naval Research Laboratory requests that media contact with
the authors be initiated through the NRL public affairs office:
Janice Schultz: schultz@ccs.nrl.navy.mil or +1 202-767-2856, or
Dick Thompson: rthompso@ccs.nrl.navy.mil or +1 202-767-2541

Dr. Emmert conducted the study as a National Research Council
Postdoctoral Research Associate at NRL. Drs. Picone and Lean are
members of NRL’s Space Science Division, and Dr. Knowles is a
former Navy employee, now with SAIC.