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The top of the greenhouse

Climate change affecting Earth’s outermost atmosphere

by David Hosansky

Stan Solomon

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.

plasma map

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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