Pollution Is Blamed for Thinner Air at Edge of Atmosphere
February 10, 2004
By ANDREW C. REVKIN
Scientists say they have found strong new evidence that
carbon dioxide, the main smokestack and tailpipe emission
linked to global warming, is cooling and shrinking the
atmosphere's outermost layers in ways that could aid as
well as endanger space activities.
The average density of the air in the region more than 60
miles up - just a trillionth of that near the surface - has
dropped 10 percent over the last 36 years, and it could
decline by a total of 50 percent by the end of the century,
scientists at the Naval Research Laboratory in Washington
The effect had long been predicted, but observations
showing that it is occurring have been scant.
Satellites, less affected by friction with the outer
atmosphere, should be able to stay aloft longer but may be
in more danger from collisions with orbiting debris, which
would also last longer, the scientists said.
The only logical cause for the contraction and cooling is
the rising level of carbon dioxide, the researchers said.
The other possible influences, including variations in the
sun, have been accounted for, said Dr. John T. Emmert, a
George Mason University researcher working at the Navy lab,
who is the lead author of a paper in the current Journal of
Geophysical Research - Space Physics.
The link to carbon dioxide is very likely, Dr. Emmert said,
adding, "There are not that many other options left."
The findings provide some of the clearest evidence that
emissions of carbon dioxide and other so-called greenhouse
gases can significantly change the dynamics and even the
dimensions of the atmosphere, Dr. Emmert said. "We're
impacting the environment all the way out to space," he
Earlier studies by American and European researchers had
found spotty evidence of the cooling and contraction, but
the new analysis appears to cement the case, several
experts not involved in the latest study said.
"This is pretty compelling evidence for the effects of
carbon dioxide," said Dr. Gerald M. Keating, a research
scientist at George Washington University and the Langley
Research Center of NASA, who made some of the original
measurements of the phenomenon. "The whole structure of the
upper atmosphere will change as this effect becomes
stronger and stronger."
The changes in density were measured in the thermosphere, a
layer of hot, rarefied air 60 to 400 miles above the
surface, including the region where many satellites and
spacecraft orbit. The anticipated effects on spacecraft and
debris come through the relationship between the density of
air and the drag it exerts.
The lower the density of the thermosphere, the less drag on
everything that orbits there, from the International Space
Station to old rocket casings. Dr. Emmert said a 50 percent
decline in density, by reducing friction, could halve the
rate of descent of satellites and debris, extending the
life of orbiting equipment but also hazards.
The thermosphere changes were detected by sifting precise
Air Force records on the orbits of thousands of objects
like satellites, meteors and spent rocket sections. The
researchers focused on 27 objects that had records dating
from the 1960's and crisscrossed the thermosphere in a
variety of orbits, assessing orbital changes compared with
changes in the sun and other factors.
Dr. Keating's study in 2000 looked at five objects. British
researchers in the late 1990's used measurements of the
ionosphere, the blanket of charged particles in the
thermosphere, to estimate that that part of the atmosphere
was contracting, probably from building greenhouse gases.
But those measurements sample only a small portion of the
atmosphere, experts said.
The link between carbon dioxide and high-altitude cooling
is much clearer than the gas's link to warming near the
surface, scientists said, because the highest atmospheric
layer, barely more than a scattering of ions and molecules,
sees no confounding influence from clouds, weather systems
"When the composition of gases changes, their radiation
effects show up quickly," said Dr. John R. Christy,
director of the Earth System Science Center at the
University of Alabama.
Near the surface, the influences driving temperature shifts
are far harder to discern.
"Down here," Dr. Christy said, "energy transfers are
affected by a lot of other things."