by David Hosansky
Stan Solomon. (Photo
by Carlye Calvin.) |
Carbon dioxide emissions from the burning of fossil fuels will produce
a
3% reduction in the density of Earth’s outermost atmosphere
by 2017, according to a team of scientists from NCAR and Pennsylvania
State University.
“We’re seeing climate change manifest itself in the upper
as well as lower atmosphere,” says NCAR scientist Stan Solomon,
a co-author of the study. “This shows the far-ranging impacts
of greenhouse gas emissions.”
The research team includes Liying Qian, Ray Roble and Solomon of
NCAR’s High Altitude Observatory and Tim Kane of PSU. The study
was supported by NASA’s Living With a Star program and NSF.
The findings appeared in the
6 December issue of Geophysical Research Letters and were presented
shortly thereafter at the annual meeting of the American Geophysical
Union.
Lower density in the thermosphere, which is the highest layer of
the atmosphere, reduces the drag on satellites in low Earth orbit,
allowing them to stay airborne longer. Forecasts of upper-level air
density could help NASA and other agencies plan the fuel needs and timing
of satellite launches more precisely.
Recent observations by scientists tracking satellite orbits have
shown that the thermosphere, which begins about 100 kilometers (60
miles) above Earth and extends up to 650 km (400 mi), is beginning
to become less dense. This confirms a prediction made at NCAR in
1989 by Roble and Robert Dickinson (now at the Georgia Institute
of Technology) that the thermosphere will cool and contract because
of increasing carbon dioxide levels. The new study is the first to
analyze whether the observed change will become more pronounced over
the next decade.
Why the cooling is a sign of global warming
Carbon dioxide cools the thermosphere, even though it acts to warm
the atmosphere near Earth’s surface (the troposphere). This
paradox occurs because the atmosphere thins with height. Near Earth’s
surface, carbon dioxide absorbs radiation escaping Earth, but before
the gas molecules can radiate the energy to space, frequent collisions
with other molecules in the dense lower atmosphere force the carbon
dioxide to release energy as heat, thus warming the air.
In the much thinner thermosphere, a carbon dioxide molecule absorbs
energy when it collides with an oxygen molecule, but there is ample
time for it to radiate energy to space before another collision occurs.
The result is a cooling effect. As it cools, the thermosphere settles,
so that the density at a given height is reduced.
Also affecting the thermosphere is the 11-year cycle of solar activity.
During the active phase of the cycle, ultraviolet light and energetic
particles from the Sun increase, producing a warming and expansion
of the upper atmosphere. When solar activity wanes, the thermosphere
settles and cools.
In order to analyze recent solar cycles and peer into the future,
the NCAR-PSU team used a computer model of the upper atmosphere that
incorporates the solar cycle as well as the gradual increase
of carbon dioxide due to human activities. The team also used a prediction
for the next solar cycle, issued by NCAR scientist Mausumi Dikpati
and colleagues, that calls for a stronger-than-usual solar cycle
over the next decade. The model showed a decrease in thermospheric
density from 1970 to 2000 of 1.7% per decade, or about 5% overall,
which agrees with observations. The team found that the decrease
was about three to four times more rapid during solar minimum than
solar maximum.
Many satellites, including the International Space Station and the
Hubble Space Telescope, follow a low Earth orbit at altitudes close
to 480 km (300 miles). Over time, the upper atmosphere drags the
satellites closer to Earth. The amount of drag depends on the density
of the thermosphere, which is why satellite planners need better
predictions of how the thermosphere changes.
“Satellite operators noticed the solar cycle changes in density
at the very beginning of the space age,” says Solomon. “We
are now able to reproduce the changes using the NCAR models and extend
them into the next solar cycle.”
Connecting
Earth and space weather
Researchers have discovered the first evidence that air
motions triggered by thunderstorms in the tropics affect
the structure of the ionosphere—the region of electrically
charged gas in Earth’s outer atmosphere, at the edge
of space. The findings were published last August in Geophysical
Research Letters.
“This discovery will help improve forecasts of turbulence
in the ionosphere, which can disrupt radio transmissions
and the reception of signals from the Global Positioning
System,” says Thomas Immel (University of California,
Berkeley), lead author of the GRL paper.
The team analyzed two known bands of plasma, or electrically
charged gas, that hover high above the tropics. They discovered
wave-like variations in the plasma bands by analyzing data
from NASA’s Imager for Magnetopause to Aurora Global
Exploration (IMAGE) satellite (see illustration). To envision
what might be causing the variations, they used simulations
by an NCAR computer model.
The research team included scientists from Japan’s
National Institute of Information and Communications Technology,
Utah State University, and Johns Hopkins University. NCAR’s
Maura
Hagan was on the team that made the thunderstorm connection.
Hagan is the lead developer of the Global Scale Wave Model,
which simulates the large air motions called atmospheric
tides.
The connection between surface and space is indirect, by
way of the atmospheric tides and an intermediate layer
of air that picks up electrical charge during daylight
hours. The NCAR model suggests that atmospheric tides excited
by thunderstorms modify this E-layer, which is part of
the ionosphere.
The Global Scale Wave Model is one ingredient nested within
a unique larger model built at NCAR that links the thermosphere,
ionosphere, and mesophere and makes studies of Earth’s
upper atmosphere possible. That larger model has recently
been merged with a global climate model, forming a hybrid
that stretches from ground level to more than 140 km (90
mi) high. The Whole Atmosphere Community Climate Model
(WACCM) will help scientists analyze how climate change
at lower altitudes will influence the highest reaches of
the atmosphere. Experts are already dreaming of connecting
WACCM and solar models so that one day the entire region
from Earth to Sun can be depicted.
This false-color image
shows ultraviolet light from two plasma bands
in the ionosphere that encircle Earth over the
equator. Bright, blue-white areas are where the
plasma is most dense. Solid white lines outline
the continents; Africa is on the left and North
and South America are on the right. Dotted white
lines mark regions where rising tides of hot
air indirectly create the bright, dense zones
in the bands. The picture is a composite built
up from 30 days of observations with NASA’s
IMAGE satellite (20 March to 20 April 2002).
(Image courtesy NASA/University of California,
Berkeley.) |
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