Upper atmosphere and greenhouse gases
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Subject: [CCG] Upper atmosphere and greenhouse gases
Source: Naval Research Laboratory Date: 2004-02-04
Increasing Greenhouse Gases Lead To Dramatic Thinning Of The Upper
Washington, D.C. (February 2, 2004) -- 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 that will appear in the Journal of Geophysical Research -
Space Physics, Dr. John Emmert and his colleagues, Drs. Michael
Picone, Judith Lean, and Stephen Knowles, report that the average
density of the thermosphere has decreased by about 10% during the
past 35 years. The thermosphere is the highest layer in the
atmosphere, and begins at an altitude of about 90 km (56 mi).
The study utilized orbital tracking data on 27 space objects that
have been aloft for over 30 years and whose closest approach to the
Earth ranges from 200-800 km (124-497 mi). The Space Shuttle
typically orbits at 300-450 km (186-279 mi), and the International
Space Station is at an altitude of about 400 km (248 mi). 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 km (7.5 mi)), 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 as one moves upwards. 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% carbon dioxide (compared to trace
amounts in the Earth's atmosphere), resulting in a very hot lower
atmosphere (8008F, 4278C) and a very cold and compact upper
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 NRL study utilizes more orbital data
over a longer period of time and employs more precise analysis
methods. By carefully examining all potential sources of error, the
NRL team has provided solid evidence that the trend is neither
artificial nor the result of physical processes other than internal
Based on the NRL 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.
This research was funded by the Office of Naval Research. 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.
These results appear in the Journal of Geophysical Research (In
NRL is the Department of the Navy's corporate laboratory and conducts
a broad program of scientific research, technology and advanced
development. The Laboratory, with a total complement of nearly 2,500
personnel, is located in southwest Washington, DC, with other major
sites at the Stennis Space Center, MS; and Monterey, CA.
Editor's Note: The original news release can be found here.
This story has been adapted from a news release issued by Naval
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