Climate change affecting Earth's outermost atmosphere

Dec 11, 2006

Carbon dioxide emissions from the burning of fossil fuels will produce a 3 percent reduction in the density of Earth's outermost atmosphere by 2017, according to a team of scientists from the National Center for Atmospheric Research (NCAR) and The Pennsylvania State University (PSU). The research, which appears in the latest issue of Geophysical Research Letters, will be presented today at the annual meeting of the American Geophysical Union.

"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 Solomon, Liying Qian, and Ray Roble of NCAR’s High Altitude Observatory; and Tim Kane of PSU. The study was supported by NASA’s Living With a Star program and by the National Science Foundation, NCAR's primary sponsor.

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, potentially saving millions of dollars.

Confirming and extending a prediction

Recent observations by scientists tracking satellite orbits have shown that the thermosphere, which begins about 60 miles above Earth and extends up to 400 miles, 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 the Earth's surface (the troposphere). This paradox occurs because the atmosphere thins with height. Near the 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.

The role of the solar cycle

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 percent per decade, or about 5 percent 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.

Impacts on satellites

Many satellites, including the International Space Station and the Hubble Space Telescope, follow a low Earth orbit at altitudes close to 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."

Citation: Geophysical Research Letters, 33, L23705, doi:10.1029/2006GL027185 (2006)

Source: University Corporation for Atmospheric Research

Explore further: A 3-D view of the Greenland Ice Sheet opens window on ice history

add to favorites email to friend print save as pdf

Related Stories

Obama recommends extended wilderness zone in Alaska

5 hours ago

US President Barack Obama said Sunday he would recommend a large swath of Alaska be designated as wilderness, the highest level of federal protection, in a move likely to anger oil proponents.

NASA craft set to beam home close-ups of Pluto

5 hours ago

Nine years after leaving Earth, the New Horizons spacecraft is at last drawing close to Pluto and on Sunday was expected to start shooting photographs of the dwarf planet.

Navy wants to increase use of sonar-emitting buoys

7 hours ago

The U.S. Navy is seeking permits to expand sonar and other training exercises off the Pacific Coast, a proposal raising concerns from animal advocates who say that more sonar-emitting buoys would harm whales and other creatures ...

Uganda seizes massive ivory and pangolin haul

7 hours ago

Ugandan wildlife officers have seized a huge haul of elephant ivory and pangolin scales, representing the deaths of hundreds of endangered animals, police said Sunday.

Recommended for you

Geologists solve mystery of Tibetan mountains

Jan 23, 2015

In the most comprehensive study of its kind, University of Kansas geologists have unraveled one of the geologic mysteries of Tibet. The research, recently published online in Nature Geoscience, shows that i ...

Image: Greenland's Leidy Glacier

Jan 23, 2015

Located in the northwest corner of Greenland, Leidy Glacier is fed by ice from the Academy Glacier (upstream and inland). As Leidy approaches the sea, it is diverted around the tip of an island that separates ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.